C++ Coding Stand
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C++ Coding Standard
Last Modified: 2007-01-09
tmh@possibility.com / http://www.possibility.com/TmhUsing this Standard. If you want to make a local copy of this standard and use it as your own you are perfectly free to do so. That's why I made it! If you find any errors or make any improvements please email me the changes so I can merge them in. I also have a programming blog at http://radio.weblogs.com/0103955/categories/stupidHumanProgramming/ that is accidently interesting at times, as is my wiki at http://www.possibility.com/epowiki/Wiki.jsp.
Contents
- Introduction
- Standardization is Important
- Standards Enforcement
- Accepting an Idea
- 6 Phases of a Project (joke)
- Flow Chart of Project Decision Making (joke)
- On Leadership
- Resources- Take a Look!
- General
- Book Recommendations
- Names
- Make Names Fit
- Include Units in Names
- No All Upper Case Abbreviations
- Class Names
- Class Library Names
- Method Names
- Class Attribute Names
- Method Argument Names
- Variable Names on the Stack
- Pointer Variables
- Reference Variables and Functions Returning References
- Global Variables
- Global Constants
- Static Variables
- Type Names
- Enum Names
- #define and Macro Names
- C Function Names
- C++ File Extensions
- Documentation
- Comments Should Tell a Story
- Document Decisions
- Use Extractable Headers
- Comment All Questions a Programmer May Have When Looking at Your Code
- Make Your Code Discoverable by Browsing
- Write Comments as You Code
- Make Gotchas Explicit
- Interface and Implementation Documentation
- Directory Documentation
- Include Statement Documentation
- Block Comments
- Complexity Management
- Layering
- Minimize Dependencies with Abstract Base Classes
- Liskov's Substitution Prinicple
- Open/Closed Principle
- Register/Dispatch Idiom
- Delegation
- Follow the Law of Demeter
- Design by Contract
- Classes
- Naming Class Files
- Class Layout
- What should go in public/protected/private?
- Prototype Source File
- Use Header File Guards
- Required Methods for a Class
- Method Layout
- Formating Methods with Multiple Arguments
- Different Accessor Styles
- Init Idiom for Initializing Objects
- Initialize all Variables
- Minimize Inlines
- Think About What Work to do in Constructors
- Don't Over Use Operators
- Thin vs. Thick Class Interfaces
- Short Methods
- In a Source file Indicate if a Method is Static or Virtual
- Process
- Use a Design Notation and Process
- Using Use Cases
- Using Stories
- Unified Modeling Language
- Code Reviews
- Create a Source Code Control System Early and Not Often
- Create a Bug Tracking System Early and Not Often
- Create a Wiki System Early and Not Often
- RCS Keyword, Change Log, and History Policy
- Honor Responsibilities
- Process Automation
- Tools Agreement
- Non-Blocking Scheduling
- Using Personas
- Use a Continuous Build System
- Code in the Dominant Style
- Run Unit Tests Before Every Check-in
- Formatting
- Brace {} Policy
- Indentation/Tabs/Space Policy
- Parens () with Key Words and Functions Policy
- A Line Should Not Exceed 78 Characters
- If Then Else Formatting
- switch Formatting
- Use of goto,continue,break and ?:
- One Statement Per Line
- Alignment of Declaration Blocks
- Document Null Statements
- Include static and virtual Key Words in Source File
- Exceptions
- Create One Exception for Each Library
- Selecting Between Exceptions And Asserts
- Be Careful Throwing Exceptions in Destructors
- Working With Libaries Using All Sorts of Different Policies
- Templates
- Namespaces
- Create Unique Name Space Names
- Don't Globally Define using
- Create Shortcut Names
- Miscellaneous
- Be Const Correct
- Use Streams
- No Magic Numbers
- Error Return Check Policy
- To Use Enums or Not to Use Enums
- Macros
- Do Not Default If Test to Non-Zero
- The Bull of Boolean Types
- Usually Avoid Embedded Assignments
- Reusing Your Hard Work and the Hard Work of Others
- Commenting Out Large Code Blocks
- Use #if Not #ifdef
- Creating a C Function in C++
- Mixing C and C++
- No Data Definitions in Header Files
- Make Functions Reentrant
- Use the Resource Acquisition is Initialization (RAII) Idiom
- Portability
- Use Typedefs for Types
- Alignment of Class Members
- Compiler Dependent Exceptions
- Compiler Dependent RTTI
- Popular Myths
- Promise of OO
- You can't use OO and C++ on Embedded Systems
Introduction
Standardization is Important
It helps if the standard annoys everyone in some way so everyonefeels they are on the same playing field. The proposal herehas evolved over many projects, many companies, and literallya total of many weeks spent arguing. It is no particular person's styleand is certainly open to local amendments.
Good Points
When a project tries to adhere to common standards a fewgood things happen:
- Programmers can go into any code and figure out what's going on.
- New people can get up to speed quickly.
- People new to C++ are spared the need to develop a personal style and defend it to the death.
- People new to C++ are spared making the same mistakes over and over again.
- People make fewer mistakes in consistent environments.
- Programmers have a common enemy :-)
Bad Points
Now the bad:
- The standard is usually stupid because it was made by someone who doesn't understand C++.
- The standard is usually stupid because it's not what I do.
- Standards reduce creativity.
- Standards are unnecessary as long as people are consistent.
- Standards enforce too much structure.
- People ignore standards anyway.
- Standards can be used as a reason for NIH (not invented here) because the new/borrowed code won't follow the standard.
Discussion
The experience of many projects leads to the conclusion that usingcoding standards makes the project go smoother. Are standardsnecessary for success? Of course not. But they help, and we need all thehelp we can get! Be honest, most arguments against a particularstandard come from the ego. Few decisions in a reasonable standard reallycan be said to be technically deficient, just matters of taste.So be flexible, control the ego a bit, and remember any projectis fundamentally a team effort.
Standards Enforcement
First, any serious concerns about the standard should be broughtup and worked out within the group. Maybe the standard is notquite appropriate for your situation. It may have overlookedimportant issues or maybe someone in power vehementlydisagrees with certain issues :-)
In any case, once finalized hopefully people will play the adult and understand that this standard is reasonable, and has been found reasonable by many other programmers, and therefore is worthy of being followedeven with personal reservations.
Failing willing cooperation it can be made a requirement thatthis standard must be followed to pass a code inspection.
Failing that the only solution is a massive tickling party on the offending party.
Accepting an Idea
- It's impossible.
- Maybe it's possible, but it's weak and uninteresting.
- It is true and I told you so.
- I thought of it first.
- How could it be otherwise.
If you come to objects with a negative preconception pleasekeep an open mind. You may still conclude objects are bunk,but there's a road you must follow to accept something different. Allow yourself to travel it for a while.
6 Phases of a Project
- Enthusiasm
- Disillusionment
- Panic
- A Search for the Guilty
- The Punishment of the Innocent
- Praise and Honor for the Non-Participants
Flow Chart for Project Decision Making
+---------+
| START |
+---------+
|
V
YES +------------+ NO
+---------------| DOES THE |---------------+
| | DAMN THING | |
V | WORK? | V
+------------+ +------------+ +--------------+ NO
| DON'T FUCK | | DID YOU FUCK |-----+
| WITH IT | | WITH IT? | |
+------------+ +--------------+ |
| | |
| | YES |
| V |
| +------+ +-------------+ +---------------+ |
| | HIDE | NO | DOES ANYONE |<------| YOU DUMBSHIT! | |
| | IT |<----| KNOW? | +---------------+ |
| +------+ +-------------+ |
| | | |
| | V |
| | +-------------+ +-------------+ |
| | | YOU POOR | YES | WILL YOU | |
| | | BASTARD |<------| CATCH HELL? |<-----+
| | +-------------+ +-------------+
| | | |
| | | | NO
| | V V
| V +-------------+ +------------+
+-------------->| STOP |<------| SHITCAN IT |
+-------------+ +------------+
Leadership
I wish i had said this, but it was said by asd@asd.com in comp.software-eng.
Leaders:
- lead by example
- don't ask anything of anyone they wouldn't do themselves
- are called on to make difficult and unpopular decisions
- keep the team focused
- reward/support their team in whatever they do
- keep/clear unnecessary crap out of the way of the team
Consensus is great. If it lasts for the project lifecycle,consider yourself blessed. I've been on a couple projectswhere two engineers just blantantly disagreed!They were always:
Programmer #1 says " x = 1"
Programmer #2 says " x != 1"
That's when a Project Leader is required. Unless youwant to flip a coin.
Oh yah - one more thing. Project leaders: TAKE the blamewhen things go wrong and SHARE the credit when thingsgo right.
Ain't easy - but it's the way I try to run my life.
Resources- Take a Look!
General
- Code Reviews. If you are interested in coding standards you may also be interested in Code Review Standards I have created at http://www.possibility.com/epowiki/Wiki.jsp?page=CodeReviews.
- Design Stories
- OO Info Sources
- Designing Qt-Style C++ APIs - A good paper on API I design for C++. I don't agree with everything, but it is good.
- Unified Modeling Language (UML)
- OO FAQ - All You Wanted to Know About OO
- C++ FAQ LITE
- C++ Source Libraries
- C++ Tutorials
- ACE C++ Library
- Collection of Other Standards
- Design by Contract from Eiffle
- C++ isn't Perfect, Here are Some Reasons Why
- Doxygen - is a 'javadoc' like utility that extracts comments and relevant information from your C++/C programs and generates HTML pages from it.
- Const Correctness - A very nice article on const correctness by Chad Loder.
- Abraxis Code Check - A program for checking code for coding standard violations and other problems.
Book Recommendations
What are some good C++ books you can buy for you and your team?
- Koenig/Moo's "Accelerated C++"
- Lippman/Moo's "C++ Primer" 4th Edition
- Bruce Eckel's "Thinking In C++"
- Scott Meyers "Effective C++"
- Dewhurst's "C++ Gotchas"
- Meyers' "Effective STL"
- Josuttis' "The C++ Standard Library"
- Vandevoorde/Josuttis' "C++ Templates"
- Langer/Kreft's "Standard C++ IOStreams and Locales"
- Sutter's "Exceptional C++"
- Sutter's "More Exceptional C++ and Exceptional C++ Style"
- Martin's "Agile Software Development: Principles, Patterns, and Practices"
Names
Make Names Fit
Names are the heart of programming. In thepast people believed knowing someone's true name gave them magicalpower over that person. If you can think up the true name for something,you give yourself and the people coming after power over the code.Don't laugh!
A name is the result of a long deep thought process aboutthe ecology it lives in. Only a programmer who understands the system as a wholecan create a name that "fits" with the system. If the name is appropriate everything fits together naturally, relationships are clear, meaning is derivable,and reasoning from common human expectations works as expected.
If you find all your names could be Thing and DoIt then you should probablyrevisit your design.
Class Names
- Name the class after what it is. If you can't think of what it is that is a clue you have not thought through the design well enough.
- Compound names of over three words are a clue your design may be confusing various entities in your system. Revisit your design. Try a CRC card session to see if your objects have more responsibilities than they should.
- Avoid the temptation of bringing the name of the class a class derives from into the derived class's name. A class should stand on its own. It doesn't matter what it derives from.
- Suffixes are sometimes helpful. For example, if your system uses agents then naming something DownloadAgent conveys real information.
Method and Function Names
- Usually every method and function performs an action, so the nameshould make clear what it does: CheckForErrors()instead of ErrorCheck(), DumpDataToFile() instead of DataFile().This will also make functions and data objects more distinguishable.
Classes are often nouns. By making function names verbs and following other naming conventions programs can be read more naturally.
- Suffixes are sometimes useful:
- Max - to mean the maximum value something can have.
- Cnt - the current count of a running count variable.
- Key - key value.
For example: RetryMax to mean the maximum number of retries, RetryCnt to mean the current retry count.
- Prefixes are sometimes useful:
- Is - to ask a question about something. Whenever someone sees Is they will know it's a question.
- Get - get a value.
- Set - set a value.
For example: IsHitRetryLimit.
Include Units in Names
If a variable represents time, weight, or some other unit then include the unit in the name so developers can more easily spot problems.For example:
uint32 mTimeoutMsecs;
uint32 mMyWeightLbs;
Better yet is to make a variable into a class so bad conversionscan be caught.
No All Upper Case Abbreviations
- When confronted with a situation where you could use an all upper case abbreviation instead use an initial upper case letter followed by all lower case letters. No matter what.
Justification
- People seem to have very different intuitions when making names containing abbreviations. It's best to settle on one strategy so the names are absolutely predictable.
Take for example NetworkABCKey. Notice how the C from ABC and K from key are confused. Some people don't mind this and others just hate it so you'll find different policies in different code so you never know what to call something.
Example
class FluidOz // NOT FluidOZ
class NetworkAbcKey // NOT NetworkABCKey
Class Names
- Use upper case letters as word separators, lower case for the rest of a word
- First character in a name is upper case
- No underbars ('_')
Justification
- Of all the different naming strategies many people found this one the best compromise.
Example
class NameOneTwo
class Name
Class Library Names
- Now that name spaces are becoming more widely implemented, name spaces should be used to prevent class name conflicts among libraries from different vendors and groups.
- When not using name spaces, it's common to prevent class name clashes by prefixing class names with a unique string. Two characters is sufficient, but a longer length is fine.
Example
John Johnson's complete data structure library could use JJas a prefix, so classes would be:
class JjLinkList
{
}
Method Names
- Use the same rule as for class names.
Justification
- Of all the different naming strategies many people found this one the best compromise.
Example
class NameOneTwo
{
public:
int DoIt();
void HandleError();
}
Class Attribute Names
- Attribute names should be prepended with the character 'm'.
- After the 'm' use the same rules as for class names.
- 'm' always precedes other name modifiers like 'p' for pointer.
Justification
- Prepending 'm' prevents any conflict with method names. Often your methods and attribute names will be similar, especially for accessors.
Example
class NameOneTwo
{
public:
int VarAbc();
int ErrorNumber();
private:
int mVarAbc;
int mErrorNumber;
String* mpName;
}
Method Argument Names
- The first character should be lower case.
- All word beginnings after the first letter should be upper case as with class names.
Justification
- You can always tell which variables are passed in variables.
- You can use names similar to class names without conflicting with class names.
Example
class NameOneTwo
{
public:
int StartYourEngines(
Engine& rSomeEngine,
Engine& rAnotherEngine);
}
Variable Names on the Stack
- use all lower case letters
- use '_' as the word separator.
Justification
- With this approach the scope of the variable is clear in the code.
- Now all variables look different and are identifiable in the code.
Example
int
NameOneTwo::HandleError(int errorNumber)
{
int error= OsErr();
Time time_of_error;
ErrorProcessor error_processor;
Time* p_out_of_time= 0;
}
The standard pointer notation is not entirely satisfactory because it doesn't look quiteright, but it is consistent.
How do you handle statics? There's never a reason to have a static local toa function so there's no reason to invent a syntax for it.
Pointer Variables
- pointers should be prepended by a 'p' in most cases
- place the * close to the pointer type not the variable name
Justification
- The idea is that the difference between a pointer, object, and a reference to an object is important for understanding the code, especially in C++ where -> can be overloaded, and casting and copy semantics are important.
- Pointers really are a change of type so the * belongs near the type.One reservation with this policy relates to declaring multiple variableswith the same type on the same line. In C++ the pointer modifier only applies to the closest variable, not all of them, which can be very confusing, especially for newbies. You want to have one declaration per line anywayso you can document each variable.
Example
String* pName= new String;
String* pName, name, address; // note, only pName is a pointer.
Reference Variables and Functions Returning References
- References should be prepended with 'r'.
Justification
- The difference between variable types is clarified.
- It establishes the difference between a method returning a modifiable object and the same method name returning a non-modifiable object.
Example
class Test
{
public:
void DoSomething(StatusInfo& rStatus);
StatusInfo& rStatus();
const StatusInfo& Status() const;
private:
StatusInfo& mrStatus;
}
Global Variables
- Global variables should be prepended with a 'g'.
Justification
- It's important to know the scope of a variable.
Example
Logger gLog;
Logger* gpLog;
Global Constants
- Global constants should be all caps with '_' separators.
Justification
It's tradition for global constants to named this way.You must be careful to not conflict with other global#defines and enum labels.
Example
const int A_GLOBAL_CONSTANT= 5;
Static Variables
- Static variables may be prepended with 's'.
Justification
- It's important to know the scope of a variable.
Example
class Test
{
public:
private:
static StatusInfo msStatus;
}
Type Names
- When possible for types based on native types make a typedef.
- Typedef names should use the same naming policy as for a class with the word Type appended.
Justification
- Of all the different naming strategies many people found this one the best compromise.
- Types are things so should use upper case letters. Type is appended to make it clear this is not a class.
Example
typedef uint16 ModuleType;
typedef uint32 SystemType;
Enum Names
Labels All Upper Case with '_' Word Separators
This is the standard rule for enum labels.
Example
enum PinStateType
{
PIN_OFF,
PIN_ON
};
Enums as Constants without Class Scoping
Sometimes people use enums as constants. When an enum is notembedded in a class make sure you use some sort of differentiatingname before the label so as to prevent name clashes.
Example
enum PinStateType If PIN was not prepended a conflict
{ would occur as OFF and ON are probably
PIN_OFF, already defined.
PIN_ON
};
Enums with Class Scoping
Just name the enum items what you wish and always qualifywith the class name: Aclass::PIN_OFF.
Make a Label for an Error State
It's often useful to be able to say an enum is notin any of its valid states. Make a label foran uninitialized or error state. Make it the first labelif possible.
Example
enum { STATE_ERR, STATE_OPEN, STATE_RUNNING, STATE_DYING};
#define and Macro Names
- Put #defines and macros in all upper using '_' separators.
Justification
This makes it very clear that the value is not alterable and in the case of macros, makes it clear that you are using a construct that requires care.
Some subtle errors can occur when macro names and enum labels use thesame name.
Example
#define MAX(a,b) blah
#define IS_ERR(err) blah
C Function Names
- In a C++ project there should be very few C functions.
- For C functions use the GNU convention of all lower case letters with '_' as the word delimiter.
Justification
- It makes C functions very different from any C++ related names.
Example
int
some_bloody_function()
{
}
C++ File Extensions
In short: Use the .h extension for header files and .cc for source files.
For some reason an odd split occurred in early C++ compilers around whatC++ source files should be called. C header files always use the .h and C source files always use the .c extension. What should we usefor C++?
The short answer is as long as everyone on your project agrees it doesn't really matter. The build environment should be able to invoke the right compiler forany extension. Historically speaking here have been the options:
- Header Files: .h, .hh, .hpp
- Source Files: .C, .cpp, .cc
Header File Extension Discussion
Using .hh extension is not widely popular but makes a certain kindof sense. C header files use .h file extension and C++ basedheader files use .hh file extension. The problem is if we consider a header file an interface to a service then we can have a C interface to a service and C++ interface to the service in the same file. Using preprocessor directives this is possible and common. The recommendation is to stick with using the .h extension.
Source File Extension Discussion
The problem with the .C extension is that it is indistinguishable fromthe .c extensions in operating systems that aren't case sensitive. Yes,this is a UNIX vs. windows issue. Since it is a simple step aiding portabilitywe won't use the .C extension. The .cpp extension is a little wordy.So the .cc extension wins by default.
Documentation
Comments Should Tell a Story
Consider your comments a story describing the system. Expectyour comments to be extracted by a robot and formed intoa man page. Class comments are one part of the story,method signature comments are another part of the story,method arguments another part, and method implementationyet another part. All these parts should weave togetherand inform someone else at another point of time justexactly what you did and why.
Document Decisions
Comments should document decisions. At every pointwhere you had a choice of what to do place a commentdescribing which choice you made and why. Archeologistswill find this the most useful information.
Use Extractable Headers
Use a document extraction system like Doxygenwhen documenting your code.
These headers are structured in such a way as theycan be parsed and extracted. They are not uselesslike normal headers. So take time to fill them out.If you do it right once no more documentation may benecessary.
As part of your nighlty build system have a step the generates the documentation from thesource. Then index the source using a tool like Lucene. Have a front end to the searchso developers can do full text searches on nightly builds and for release builds.This is a wonderfully useful feature.
The next step in automation is to front the repository with a web server documentation can directly refer to a source file with a URL.
Comment All Questions a Programmer May Have When Looking at Your Code
At every point in your code think about what questions a programmer may have aboutthe code. It's crucial you answer all those questions somehow, someway. If youdon't, as the code writer, answer those questions, who will?
If you think your code is so clear and wonderful that nobody will have any questionsthen you are lying to yourself. I have never seen a large system with this wonderfulself-documenting code feature. I've seen very few small libraries are even a singleclass that are so wonderfully self-documented.
You have a lot of tools at your disposal to answer questions:
- A brain to think up the questions you should be answering. Why? Who? When? How?What?
- Variable names.
- Class names.
- Class decomposition.
- Method decomposition.
- File names.
- Documentation at all levels: package, class, method, attribute, inline.
The better you are at orchestrating all these elements together the clearer yourcode will be to everyone else.
I don't really consider unit tests a question answering device because if you can'tunderstand the code by reading it, reading something else about the code you don'tunderstand won't help you understand it better.
Make Your Code Discoverable by Browsing
Programmers should be able to navigate your code by looking at markers in the code, namely the names and the directory structure. Nothing is more frustrating to thanto have to look at pile of code and have no idea what it's organizing principles are.
Have a logicanl directory structure. Have directories called doc, lib, src, bin, test, pkg, install, etc and whatever, so I at least have some idea where stuff is. People use the weirdestnames and lump everything together so that it it can be detangles. Clear thought is evidencedfrom the beginning by a directory stucture.
Don't put more than one class in a file. Otherwise, how will I know its there when I browse yourcode? Should I really need to use search to find every last thing? Can't I just poke around thecode and find it? I can if you organize your code.
Name your files after your classes. I didn't believe this one until I saw it. Why you namea file different than the class? How I am possibly supposed to know what's in the file otherwise?
Write Comments as You Code
You won't every go back later and document your code. You just won't. Don't lie to yourself, the world,and your mother by saying that you will.
So when you do something document it right then and there. When you create a class- document it. Whenyou create a method- document it. And so on. That way when you finish coding you will also be finisheddocumenting.
I advocate simultaneously writing code, writing tests, and writing documentaiton.Which comes first depends on you and the problem. I don't think there is any rule that sayswhich should come first. On the path to getting stuff done I'll take the entrance that seems easiest to me at the time. Once on the path it's easy to follow the entire trail.
Won't this break the flow? No, I think it improves flow because it keeps you mindful of what you are doing, why you are doing, and how it fits in the big picture. My take on TDD (test drivendevelopment) is that it's not the tests that are really important, it's that the tests keep youmindful while programming. A test means you are keeping everything in you mind at once youneed to remember to successfully code something up. As you can't keep large chunks in your mindthen smaller chunks are better. Writing a test forces you to remember what your code is supposedto accomplish. It's forcing you to also think about the use case/story/intent behind whyyou are writing the code.
The result is a pointed mind that has focussed all its powers on doing one thing. When you canbring that focus to you programming you can be successful. The tests are really secondary. Ifyour system/acceptance tests can't find bugs you are screwed anyway. And I find code writtenmindfully, one step at a time, has very few bugs. Unit tests are just one definition of a "step." You can use the orignial story you are implementingas a step as well. I use unit tests more as a mental focussing device while developing, like ZenArchery, than for the actual tests. After development unit tests are very useful in making surecode doesn't break. So I am not saying unit tests aren't useful. I just don't think they are thereal reason behind why TDD generates working code. With a clear well functioning focussed mindwe generate working code. But getting into that state is hard.
Writing comments simultaneously with all other aspects of development deepens your mindfulnessbecause you are thinking about everything at once. All the interconnections are present in yourbrain because you are explaining the intent behind what you are doing.
There's a saying that you don't know something until you teach it. Comments are teaching what you are doing to someone else. When you are writing comments you must generate the thoughts toteach, to explain to someone else the intent behing what you are doing. It's very difficultto make a coding error when all this context is hot in your mind.
I'll go back and forth between documenting, testing, and coding. I'll let the problem dictatewhat happens when as I am working my way through solving the problem. Saying testing shouldalways come first is too simple a rule and I think misses the larger point about softwaredevelopment.
Software is ultimately mind stuff. Using our minds better is the real methodology.
Make Gotchas Explicit
Explicitly comment variables changed out of the normal controlflow or other code likely to break during maintenance. Embeddedkeywords are used to point out issues and potential problems. Consider a robot will parse your comments looking for keywords, stripping them out, and makinga report so people can make a special effort where needed.
Gotcha Keywords
- :TODO: topic
Means there's more to do here, don't forget. - :BUG: [bugid] topic
means there's a Known bug here, explain it and optionally give a bug ID. - :KLUDGE:
When you've done something ugly say so and explain how you would do it differently next time if you had more time. - :TRICKY:
Tells somebody that the following code is very tricky so don't go changing it without thinking. - :WARNING:
Beware of something. - :COMPILER:
Sometimes you need to work around a compiler problem. Document it. The problem may go away eventually. - :ATTRIBUTE: value
The general form of an attribute embedded in a comment. You can make up your own attributes and they'll be extracted.
Gotcha Formatting
- Make the gotcha keyword the first symbol in the comment.
- Comments may consist of multiple lines, but the first line should be a self-containing, meaningful summary.
- The writer's name and the date of the remark should be part of the comment. This information is in the source repository, but it can take a quite a while to find out when and by whom it was added. Often gotchas stick around longer than they should. Embedding date information allows other programmer to make this decision. Embedding who information lets us know who to ask.
Example
// :TODO: tmh 960810: possible performance problem
// We should really use a hash table here but for now we'll
// use a linear search.
// :KLUDGE: tmh 960810: possible unsafe type cast
// We need a cast here to recover the derived type. It should
// probably use a virtual method or template.
See Also
See Interface and Implementation Documentation for more details on how documentation should be laid out.
Interface and Implementation Documentation
There are two main audiences for documentation:
- Class Users
- Class Implementors
With a little forethought we can extract both types ofdocumentation directly from source code.
Class Users
Class users need class interface information which when structuredcorrectly can be extracted directly from a header file. Whenfilling out the header comment blocks for a class, onlyinclude information needed by programmers who use the class.Don't delve into algorithm implementation details unless thedetails are needed by a user of the class. Consider commentsin a header file a man page in waiting.
Class Implementors
Class implementors require in-depth knowledge of how a classis implemented. This comment type is found inthe source file(s) implementing a class. Don't worry aboutinterface issues. Header comment blocks in a source file should cover algorithm issues and other design decisions. Comment blocks within a method's implementation should explain even more.
Directory Documentation
Every directory should have a README file that covers:
- the purpose of the directory and what it contains
- a one line comment on each file. A comment can usually be extracted from the NAME attribute of the file header.
- cover build and install directions
- direct people to related resources:
- directories of source
- online documentation
- paper documentation
- design documentation
- anything else that might help someone
Consider a new person coming in 6 months after everyoriginal person on a project has gone. That lone scaredexplorer should be able to piece together a picture of thewhole project by traversing a source directory tree andreading README files, Makefiles, and source file headers.
Include Statement Documentation
Include statements should be documented, telling the user why aparticular file was included. If the file includes a classused by the class then it's useful to specify a class relationship:
- ISA - this class inherits from the class in the include file.
- HASA - this class contains, that is has as a member attribute, the class in the include file. This class owns the memory and is responsible for deleting it.
- USES - this class uses something from the include file.
- HASA-USES - this class keeps a pointer or reference to the class in the include file, but this class does not own the memory.
Example
#ifndef XX_h
#define XX_h
// SYSTEM INCLUDES
//
#include // standard IO interface
#include // HASA string interface
#include // USES auto_ptr
Notice how just by reading the include directives the code is startingto tell you a story of why and how it was built.
Block Comments
Use comments on starting and ending a Block:
{
// Block1 (meaningful comment about Block1)
... some code
{
// Block2 (meaningful comment about Block2)
... some code
} // End Block2
} // End Block1
This may make block matching much easier to spot when youdon't have an intelligent editor.
Complexity Management
Layering
Layering is the primary technique for reducing complexity ina system. A system should be divided into layers. Layersshould communicate between adjacent layers using well defined interfaces. When a layer uses a non-adjacent layer then a layering violation has occurred.
A layering violation simply means we have dependency betweenlayers that is not controlled by a well defined interface.When one of the layers changes code could break. We don'twant code to break so we want layers to work only withother adjacent layers.
Sometimes we need to jump layers for performance reasons.This is fine, but we should know we are doing it and documentappropriately.
Minimize Dependencies with Abstract Base Classes
One of the most important strategies in C++ is to remove dependenciesamong different subsystems. Abstract base classes (ABCs) area solid technique for dependency removal.
An ABC is an abstraction of a common form such that it can beused to build more specific forms. An ABC is a common interfacethat is reusable across a broad range of similar classes.By specifying a common interface as long as a class conformingto that interface is used it doesn't really matter what is the typeof the derived type. This breaks code dependencies. New classes,conforming to the interface, can be substituted in at will without breaking code. In C++ interfaces are specified by using base classeswith virtual methods.
The above is a bit rambling because it's a hard idea toconvey. So let's use an example: We are doing a GUIwhere things jump around on the screen. One approachis to do something like:
class Frog
{
public:
void Jump();
}
class Bean
{
public:
void Jump();
}
The GUI folks could instantiate each object and call the Jumpmethod of each object. The Jump method of each object containsthe implementation of jumping behavior for that type of object.Obviously frogs and beans jump differently even though bothcan jump.
Unfortunately the owner of Bean didn't like the wordJump so they changed the method name to Leap. This broke thecode in the GUI and one whole week was lost.
Then someone wanted to see a horse jump so a Horse class was added:
class Horse
{
public:
void Jump();
}
The GUI people had to change their code again to add Horse.
Then someone updated Horse so that its Jump behavior was slightly different.Unfortunately this caused a total recompile of the GUI code and theywere pissed.
Someone got the bright idea of trying to remove all the abovedependencies using abstract base classes. They made one baseclass that specified an interface for jumping things:
class Jumpable
{
public:
virtual void Jump() = 0;
}
Jumpable is a base class because other classes need to derivefrom it so they can get Jumpable's interface. It's anabstract base class because one or more of its methods has the = 0 notation which means the method is a pure virtual method. Pure virtual methodsmust be implemented by derived classes. The compilerchecks.
Not all methods in an ABC must be pure virtual, some may have animplementation. This is especially true when creating a base class encapsulating a process common to a lot of objects.For example, devices that must be opened, diagnostics run, booted,executed, and then closed on a certain event may create an ABCcalled Device that has a method called LifeCycle which calls all othermethods in turn thus running through all phases of a device's life. Each device phase would have a pure virtual method in the base classrequiring implementation by more specific devices. This waythe process of using a device is made common but the specificsof a device are hidden behind a common interface.
Back to Jumpable. All the classes were changed to derivefrom Jumpable:
class Frog : public Jumpable
{
public:
virtual void Jump() { ... }
}
etc ...
We see an immediate benefit: we know all classes derived fromJumpable must have a Jump method. No one can go changingthe name to Leap without the compiler complaining. One dependencybroken.
Another benefit is that we can pass Jumpable objectsto the GUI, not specific objects like Horse or Frog:
class Gui
{
public:
void MakeJump(Jumpable*);
}
Gui gui;
Frog* pFrog= new Frog;
gui.MakeJump(pFrog);
Notice Gui doesn't even know it's making a frog jump, itjust has a jumpable thing, that's all it cares about. WhenGui calls the Jump method it will get the implementationfor Frog's Jump method. Another dependency down. Guidoesn't have to know what kind of objects are jumping.
We also removed the recompile dependency.Because Gui doesn't contain any Frog objects it will notbe recompiled when Frog changes.
Downside
Wow! Great stuff! Yes but there are a few downsides:
Overhead for Virtual Methods
Virtual methods have a space and time penalty. It's not huge,but should be considered in design.
Make Everything an ABC!
Sometimes people overdo it, making everything an ABC.The rule is make an ABC when you need one not when youmight need one. It takes effort to design a good ABC,throwing in a virtual method doesn't an ABC make. Pickand choose your spots. When some process orsome interface can be reused and people will actuallymake use of the reuse then make an ABC and don't look back.
Liskov's Substitution Principle (LSP)
This principle states:
All classes derived from a base class should be interchangeable
when used as a base class.
The idea is users of a class should be able to count on similarbehavior from all classes that derive from a base class. Nospecial code should be necessary to qualify an object beforeusing it. If you think about it violating LSP is also violatingthe Open/Closed principle because the codewould have to be modified every time a derived class was added.It's also related to dependency management using abstract base classes.
For example, if the Jump method of aFrog object implementing the Jumpable interface actually makes a call and orders pizza we can say its implementation is not in the spirit of Jump and probably all other objects implementing Jump.Before calling a Jump method a programmer would now have to check for the Frog type so it wouldn't screw up the system. We don'twant this in programs. We want to use base classes and feelcomfortable we will get consistent behaviour.
LSP is a very restrictive idea. It constrains implementors quitea bit. In general people support LSP and have LSP as a goal.
Open/Closed Principle
The Open/Closed principle states a class must be open andclosed where:
- open means a class has the ability to be extended.
- closed means a class is closed for modifications other than extension. The idea is once a class has been approved for use having gone through code reviews, unit tests, and other qualifying procedures, you don't want to change the class very much, just extend it.
The Open/Closed principle is a pitch for stability. A system is extended by addingnew code not by changing already working code. Programmers often don't feelcomfortable changing old code because it works! This principle just givesyou an academic sounding justification for your fears :-)
In practice the Open/Closed principle simply means making good use of ourold friends abstraction and polymorphism. Abstraction to factor out commonprocesses and ideas. Inheritance to create an interface that must be adheredto by derived classes. In C++ we are talking about using abstract base classes . A lot.
Register/Dispatch Idiom
Another strategy for reducing dependencies in a system isthe Register/Dispatch Idiom (RDI). RDI treats largegrained occurrences in a system as events. Events are identifiedby some unique identifier. Objects in the system register witha dispatch system for events or classes of events it is interestedin. Objects that are event sources send events into the dispatchsystem so the dispatch system can route events to consumers.
RDI separates producers and consumers on a distributed scale.Event producers and consumers don't have to know about eachother at all. Consumers can drop out of the event streamby deregistering for events. New consumers can registerfor events at anytime. Event producers can drop out withno ill effect to event consumers, the consumer just won'tget any more events. It is a good idea for producers to havean "I'm going down event" so consumers can react intelligently.
Logically the dispatch system is a central entity. The implementationhowever can be quite different. For a highly distributed systema truly centralized event dispatcher would be a performance bottleneck and a single point of failure. Think of event dispatchers as beinga lot of different processes cast about on various machines for redundancypurposes. Event processors communicate amongst each other to distributeknowledge about event consumers and producers. Much like a routingprotocol distributes routing information to its peers.
RDI works equally well in the small, in processes and single workstations.Parts of the system can register as event consumers and event producersmaking for a very flexible system. Complex decisions in a system are expressedas event registrations and deregistrations. No further level of cooperationrequired.
More expressive event filters can also be used. The above proposal filtersevents on some unique ID. Often you want events filtered on more complexcriteria, much like a database query. For this to work the system has tounderstand all data formats. This is easy if you use a common format likeattribute value pairs. Otherwise each filter needs code understandingpacket formats. Compiling in filter code to each dispatcher is one approach.Creating a downloadable generic stack based filter language has been used withsuccess on other projects, being both simple and efficient.
Delegation
Delegation is the idea of a method using another object'smethod to do the real work. In some sense the top layer methodis a front for the other method. Delegation is a formof dependency breaking. The top layer method never has tochange while it's implementation can change at will.
Delegation is an alternative to using inheritance for implementation purposes. One can use inheritanceto define an interface and delegation to implementthe interface.
Some people feel delegation is a more robust formof OO than using implementation inheritance. Delegationencourages the formation of abstract class interfacesand HASA relationships. Both of which encourage reuseand dependency breaking.
Example
class TestTaker
{
public:
void WriteDownAnswer() { mPaidTestTaker.WriteDownAnswer(); }
private:
PaidTestTaker mPaidTestTaker;
}
In this example a test taker delegates actually answering thequestion to a paid test taker. Not ethical but a definiteexample of delegation!
Follow the Law of Demeter
The Law of Demeter states that you shouldn'taccess a contained object directly from the containingobject, you should use a method of the containing objectthat does what you want and accesses any of its objectsas needed.
Justification
The purpose of this law is to break dependenciesso implementations can change without breaking code.If an object wishes to remove one of its contained objectsit won't be able to do so because some other object is using it.If instead the service was through an interface the objectcould change its implementation anytime without ill effect.
Caveat
As for most laws the Law of Demeter should be ignored incertain cases. If you have a really high level objectthat contains a lot of subobjects, like a car containsthousands of parts, it can get absurd to created a methodin car for every access to a subobject.
Example
class SunWorkstation
{
public:
void UpVolume(int amount) { mSound.Up(amount); }
SoundCard mSound;
private:
GraphicsCard mGraphics;
}
SunWorksation sun;
Do : sun.UpVolume(1);
Don't: sun.mSound.Up(1);
Design by Contract
The idea of design by contract is strongly related to LSP .A contract is a formal statement of what to expect from another party. In this casethe contract is between pieces of code. An object and/or method states that it doesX and you are supposed to believe it. For example, when you ask an object for itsvolume that's what you should get. And because volume is a verifiable attributeof a thing you could run a series of checks to verify volume is correct, that is,it satisfies its contract.
The contract is enforced in languages likeEiffel by pre and post condition statements that are actually part of the language.In other languages a bit of faith is needed.
Design by contract when coupled with language based verification mechanismsis a very powerful idea. It makes programming more like assembling spec'd parts.
Using Design by Contract
- DO NOT PUT "REAL" CODE IN DBC CALLS.. Dbc calls shouldonly test conditions. No code that can't be compiled out shouldbe included in Dbc calls.
- Every method should define its pre and post conditions.
- Every class should define its invariants.
- Callers are responsible for checking preconditions.An object may not and is not required to test for assertion violations.
- Method pre-conditions should be documented in a method'sinterface documentation.
- Pre-conditions can be weakened by derived classes.
- Post-conditions can be strengthened by derived classes.
- Every Class Should:
- Develop its class invariants.
- Code its invariants and call them in its operations.
- Document its invariants in the class documentation.
- Every Method Should:
- Develop a list of exceptions.
- Develop operation pre-conditions.
- Develop operation post-conditions.
- Code the exceptions, pre, and post conditions.
- Document the exceptions, pre, and post conditions.
Classes
Naming Class Files
Class Definition in One File
Each class definition should be in its own file where each file is named directly after the class's name:
ClassName.h
Implementation in One File
In general each class should be implemented in one source file:
ClassName.cc // or whatever the extension is: cpp, c++
But When it Gets Really Big...
If the source file gets too large or you want to avoid compilingtemplates all the time then add additional files named accordingto the following rule:
ClassName_section.C
section is some name that identifies why the code is chunkedtogether. The class name and section name are separated by '_'.
Class Layout
A common class layout is critical from a code comprehension point of view andfor automatically generating documentation. C++ programmers, through a newset of tools, can enjoy the same level generated documentation Java programmerstake for granted.
Class and Method Documentation
It is recommended a program likeDoxygenbe used to document C++ classes, method, variables, functions,and macros. The documentation can be extracted and put in placesin a common area for all programmers to access. This saves programmershaving to read through class headers. Documentation generationshould be integrated with the build system where possible.
Template
Please use the following template when creating a new class.
/** A one line description of the class.
*
* #include "XX.h" <BR>
* -llib
*
* A longer description.
*
* @see something
*/
#ifndef XX_h
#define XX_h
// SYSTEM INCLUDES
//
// PROJECT INCLUDES
//
// LOCAL INCLUDES
//
// FORWARD REFERENCES
//
class XX
{
public:
// LIFECYCLE
/** Default constructor.
*/
XX(void);
/** Copy constructor.
*
* @param from The value to copy to this object.
*/
XX(const XX& from);
/** Destructor.
*/
~XX(void);
// OPERATORS
/** Assignment operator.
*
* @param from THe value to assign to this object.
*
* @return A reference to this object.
*/
XX& operator=(XX& from);
// OPERATIONS
// ACCESS
// INQUIRY
protected:
private:
};
// INLINE METHODS
//
// EXTERNAL REFERENCES
//
#endif // _XX_h_
Required Methods Placeholders
The template has placeholders for required methods .You can delete them or implement them.
Ordering is: public, protected, private
Notice that the public interface is placed first in the class, protected next,and private last. The reasons are:
- programmers should care about a class's interface more than implementation
- when programmers need to use a class they need the interface not the implementation
It makes sense then to have the interface first. Placing implementation, the private section, first is a historical accident as the first examples used the private first layout. Over time emphasis has switched deemphasizing a class's interface over implementation details.
LIFECYCLE
The life cycle section is for methods that control the life cycle of an object. Typically these methods include constructors, destructors, and state machine methods.
OPERATORS
Place all operators in this section.
OPERATIONS
Place the bulk of a class's non access and inquiry method methods here. A programmer will look here for the meat of a class's interface.
ACCESS
Place attribute accessors here.
INQUIRY
These are the Is* methods. Whenever you have a question to ask aboutan object it can be asked via in Is method. For example: IsOpen()will indicate if the object is open. A good strategy is instead of makinga lot of access methods you can turn them around to be questions about theobject thus reducing the exposure of internal structure. Without the IsOpen()method we might have had to do: if (STATE_OPEN == State()) which is much uglier.
What should go in public/protected/private?
Public Section
Only put an object's interface in the public section. DO NOT exposeany private data items in the public section. At least encapsulateaccess via access methods. Ideally your method interface should make mostaccess methods unnecessary. Do not put data in the public interface.
Protected and Private Section
What should go into the protected section versus the private section isalways a matter of debate.
All Protected
Some say there should be no private sectionand everything not in the public section should go in the protected section.After all, we should allow all our children to change anything they wish.
All Private
Another camp says by making the public interface virtual any derivedclass can change behavior without mucking with internals.
Wishy Washy
Rationally decide where elements should go and put them there. Notvery helpful.
And the Winner Is...
Keeping everything all private seems the easiest approach. Bymaking the public methods virtual flexibility is preserved.
Prototype Source File
#include "XX.h" // class implemented
/////////////////////////////// PUBLIC ///////////////////////////////////////
//============================= LIFECYCLE ====================================
XX::XX()
{
}// XX
XX::XX(const XX&)
{
}// XX
XX::~XX()
{
}// ~XX
//============================= OPERATORS ====================================
XX&
XX::operator=(XX&);
{
return *this;
}// =
//============================= OPERATIONS ===================================
//============================= ACESS ===================================
//============================= INQUIRY ===================================
/////////////////////////////// PROTECTED ///////////////////////////////////
/////////////////////////////// PRIVATE ///////////////////////////////////
Use Header File Guards
Include files should protect against multiple inclusion throughthe use of macros that "guard" the files.
When Not Using Namespces
#ifndef filename_h
#define filename_h
#endif
The new line after the endif if is required by some compilers.
When Using Namespaces
If namespaces are used then to be completely safe:
#ifndef namespace_filename_h
#define namespace_filename_h
#endif
- Replace filename with the name of the file being guarded.This should usually be the name of class containedin the file. Use the exact class name. Some standards say use all upper case. This is a mistake because someone could actuallyname a class the same as yours but using all upper letters.If the files end up be included together one file will preventthe other from being included and you will be one very confusedpuppy. It has happened!
- Most standards put a leading _ and trailing _.This is no longer valid as the C++ standard reserves leading _ tocompiler writers.
- When the include file is not for a class then the file name should be used as the guard name.
- Compilers differ on how comments are handled on preprocessor directives.Historically many compilers have not accepted comments on preprocessordirectives.
- Historically many compilers require a new line after last endif.
Required Methods for a Class
To be good citizens almost all classes should implementthe following methods. If you don't have to define and implement any of the "required" methods they should still be represented in your class definition as comments.
- Default Constructor
If your class needs a constructor, make sure to provide one.You need one if during the operation of the class it createssomething or does something that needs to be undone when theobject dies. This includes creating memory, opening file descriptors,opening transactions etc.
If the default constructor is sufficient add a comment indicating that the compiler-generated version will be used.
If your default constructor has one or more optional arguments, add acomment indicating that it still functions as the default constructor.
- Virtual Destructor
If your class is intended to be derived from by other classes then make the destructor virtual.
- Copy Constructor
If your class is copyable, either define a copy constructor andassignment operator or add a comment indicating that thecompiler-generated versions will be used.
If your class objects should not be copied, make the copy constructorand assignment operator private and don't define bodies for them. Ifyou don't know whether the class objects should be copyable, then assumenot unless and until the copy operations are needed.
- Assignment Operator
If your class is assignable, either define a assignment operator or add a comment indicating that the compiler-generated versions will be used.
If your objects should not be assigned, make the assignment operator private and don't define bodies for them. Ifyou don't know whether the class objects should be assignable, then assumenot.
Justification
- Virtual destructors ensure objects will be completely destructedregardless of inheritance depth. You don't have to use avirtual destructor when:
- You don't expect a class to have descendants.
- The overhead of virtualness would be too much.
- An object must have a certain data layout and size.
- A default constructor allows an object to be used in an array.
- The copy constructor and assignment operator ensure an objectis always properly constructed.
The Law of The Big Three
A class with any of (destructor, assignment operator, copy constructor) generally needs all 3. For more information see http://www.parashift.com/c++-faq-lite/coding-standards.html#[25.9].
Example
The default class template with all requiredmethods. An example using default values:
class Planet
{
public:
/** The following is the default constructor if no arguments are supplied.
*/
Planet(int radius= 5);
// Use compiler-generated copy constructor, assignment, and destructor.
// Planet(const Planet&);
// Planet& operator=(const Planet&);
// ~Planet();
};
Method Layout
The approach used is to place a comment block before each method thatcan be extracted by a tool and be made part of the class documentation.Here we'll use Doxygen which supports the Javadoc format. See the Doxygen documentation fora list of attributes supported by the document generator.
Method Header
Every parameter should be documented. Every return code shouldbe documented. All exceptions should be documented.Use complete sentences when describing attributes.Make sure to think about what other resources developersmay need and encode them in with the @see attributes.
/** Assignment operator.
*
*
* @param val The value to assign to this object.
* @exception LibaryException The explanation for the exception.
* @return A reference to this object.
*/
XX& operator=(XX& val);
Additional Sections
In addition to the standard attribute set, the followingsections can be included in the documentation:
- PRECONDITION
Document what must have happened for the object to be in a statewhere the method can be called. - WARNING
Document anything unusual users should know about this method. - LOCK REQUIRED
Some methods require a semaphore be acquired before using the method. Whenthis is the case use lock required and specify the name of the lock. - EXAMPLES
Include exampes of how to use a method. A picture says a 1000 words,a good example answers a 1000 questions.
For example:
/** Copy one string to another.
*
* PRECONDITION
* REQUIRE(from != 0)
* REQUIRE(to != 0)
*
* WARNING
* The to buffer must be long enough to hold
* the entire from buffer.
*
* EXAMPLES
** strcpy(somebuf, "test")
*
*
* @param from The string to copy.
* @param to The buffer to copy the string to.
*
* @return void
*/
void strcpy(const char* from, char* to);
Common Exception Sections
If the same exceptions are being used in a number ofmethods, then the exceptions can be documented oncein the class header and referred to from the methoddocumentation.
Formatting Methods with Multiple Arguments
We should try and make methods have as few parameters as possible. If you find yourselfpassing the same variables to every method then that variable should probably bepart of the class. When a method does have a lot of parameters format it like this:
int AnyMethod(
int arg1,
int arg2,
int arg3,
int arg4);
Different Accessor Styles
Why Accessors?
Access methods provide access to the physical or logical attributes of an object. Accessing an object's attributes directly as we do for C structures is greatly discouraged in C++. We disallow direct access to attributes to break dependencies, the reason we do most things. Directly accessing an attribute exposes implementation details about the object.
To see why ask yourself:
- What if the object decided to provide the attribute in a way other than physical containment?
- What if it had to do a database lookup for the attribute?
- What if a different object now contained the attribute?
If any of the above changed code would break. An object makesa contract with the user to provide access to a particular attribute; it should not promise how it gets those attributes.Accessing a physical attribute makes such a promise.
Accessors Considered Somewhat Harmful
At least in the public interface having accessors many times is an admissionof failure, a failure to make an object's interface complete.At the protected or private level accessors are fine as theseare the implementation levels of a class.
Implementing Accessors
There are three major idioms for creating accessors.
Get/Set
class X
{
public:
int GetAge() const { return mAge; }
void SetAge(int age) { mAge= age; }
private:
int mAge;
}
The problem with Get/Set is twofold:
- It's ugly. Get and Set are strewn throughout the code cluttering it up.
- It doesn't treat attributes as objects in their own right. An object will have an assignment operator. Why shouldn't age be an object and have its own assignment operator?
One benefit, that it shares with the One Method Name, iswhen used with messages the set method can transparently transformfrom native machine representations to network byte order.
One Method Name
class X
{
public:
int Age() const { return mAge; }
void Age(int age) { mAge= age; }
private:
int mAge;
}
Similar to Get/Set but cleaner. Use this approach when notusing the Attributes as Objects approach.
Attributes as Objects
class X
{
public:
int Age() const { return mAge; }
int& rAge() { return mAge; }
const String& Name() const { return mName; }
String& rName() { return mName; }
private:
int mAge;
String mName;
}
The above two attribute examples shows the strength and weaknessof the Attributes as Objects approach.
When using an int type, which is not a real object, the int is set directly because rAge() returns a reference. The object can do no checking of the value or do any representation reformatting. For manysimple attributes, however, these are not horrible restrictions. A wayaround this problem is to use a class wrapper around base types likeint.
When an object is returned as reference its = operator isinvoked to complete the assignment. For example:
X x;
x.rName()= "test";
This approach is also more consistent with the object philosophy:the object should do it. An object's = operator can doall the checks for the assignment and it's done once in oneplace, in the object, where it belongs. It's also clean froma name perspective.
When possible use this approach to attribute access.
Init Idiom for Initializing Objects
- Objects with multiple constructors and/or multiple attributes should define a private Init() method to initialize all attributes. If the number of different member variables is small then this idiom may not be a big win and C++'s constructor initialization syntax can/should be used.
Justification
- When using C++'s ability to initialize variables in the constructor it's difficult with multiple constructors and/or multiple attributes to make sure all attributes are initialized. When an attribute is added or changed almost invariably we'll miss changing a constructor.
It's better to define one method, Init(), that initializes all possible attributes. Init() should be called first from every constructor.
- The Init() idiom cannot be used in two cases where initialization from a constructor is required:
- constructing a member object
- initializing a member attribute that is a reference
Example
class Test
{
public:
Test()
{
Init(); // Call to common object initializer
}
Test(int val)
{
Init(); // Call to common object initializer
mVal= val;
}
private:
int mVal;
String* mpName;
void Init()
{
mVal = 0;
mpName= 0;
}
}
Since the number of member variables is small, this might be better
written as:
class Test
{
public:
Test(int val = 0, String* name = 0)
: mVal(val), mpName(name) {}
private:
int mVal;
String* mpName;
};
Initialize all Variables
- You shall always initialize variables. Always. Every time.
Justification
- More problems than you can believe are eventually traced back to a pointer or variable left uninitialized. C++ tends to encourage this by spreading initialization to each constructor. See Init Idiom for Initializing Objects .
Minimize Inlines
Minimize inlining in declarations or inlining in general. As soon as you put your C++ code in a shared library which you want to maintain compatibility with in the future, inlined code is a major pain in the butt. It's not worth it, for most cases.
Think About What Work to do in Constructors
Should you do work that can fail in constructors? If youhave a compiler that does not support exceptions (or threadsafe exceptions if it matters to you) then the answer is definitelyno. Go directly to Do Work in Open.If your compiler supports exception then go to Do Work in Constructor. There are still reasons to use an Open method even with exceptions.
Use Open Reasons
- It is difficult to write exception safe code in constructor. It's possible to throw an exception and not destruct objects allocated in the constructor. Use of auto_ptr can help prevent this problem.
- Some compilers do not support thread safe exceptions on all platforms.
- Virtual methods are not available in base classes. If the base class is expecting a virtual method implemented by derived classes to be available during construction then initialization must follow construction. This is common in frameworks.
- Larger scale state machines may dictate when initialization should occur. An object may contain numerous other objects that may have complex initialization conditions. In this case we could wait to construct objects but then we always have to worry about null pointers.
- If deletion is needed to free resources we still may want to keep the state around for debugging or statistics or as a supplier of information for other objects.
Do Work in Constructor
With exceptions work done in the constructor can signal failureso it is fine to perform real work in the constructor. This isthe guru endorced approach as a matter of fact. But there are reasons to still use an open style approach.
The constructor code must still be very careful not to leak resources in the constructor. It's possible to throw an exception and not destruct objects allocated in the constructor.
There is a pattern called Resource Acquisition as Initializationthat says all initialization is performed in the constructor andreleased in the destructor. The idea is that this is a safer approachbecause it should reduce resource leaks.
Do Work in Open
Do not do any real work in an object's constructor. Inside aconstructor initialize variables only and/or do onlyactions that can't fail.
Create an Open() method for an object which completes construction. Open() should be called after object instantiation.
Example
class Device
{
public:
Device() { /* initialize and other stuff */ }
int Open() { return FAIL; }
};
Device dev;
if (FAIL == dev.Open()) exit(1);
Don't Over Use Operators
C++ allows the overloading of all kinds of weird operators.Unless you are building a class directly related to maththere are very few operators you should override. Onlyoverride an operator when the semantics will be clearto users.
Justification
- Very few people will have the same intuition as you about what a particular operator will do.
Thin vs. Thick Class Interfaces
How many methods should an object have? The right answer of course is just the right amount, we'll callthis the Goldilocks level. But what is the Goldilocks level? It doesn'texist. You need to make the right judgment for your situation, which is reallywhat programmers are for :-)
The two extremes are thin classes versus thick classes. Thin classes are minimalist classes. Thin classes have as few methods as possible.The expectation is users will derive their own class from the thin class addingany needed methods.
While thin classes may seem "clean" they really aren't. You can't do much witha thin class. Its main purpose is setting up a type. Since thin classes have solittle functionality many programmers in a project will create derived classeswith everyone adding basically the same methods. This leads to code duplicationand maintenance problems which is part of the reason we use objects in the first place. The obvious solution is to push methods up to the base class. Push enough methods up to the base class and you get thick classes.
Thick classes have a lot of methods. If you can think of it a thick classwill have it. Why is this a problem? It may not be. If the methods are directlyrelated to the class then there's no real problem with the class containingthem. The problem is people get lazy and start adding methods to a class thatare related to the class in some willow wispy way, but would be better factoredout into another class. Judgment comes into play again.
Thick classes have other problems. As classes get largerthey may become harder to understand. They also become harder to debugas interactions become less predictable. And when a method is changed thatyou don't use or care about your code will still have to be recompiled, possiblyretested, and rereleased.
Short Methods
- Methods should limit themselves to a single page of code.
Justification
- The idea is that the each method represents a technique forachieving a single objective.
- Most arguments of inefficiency turn out to be false in the long run.
- True function calls are slower than not, but there needs to athought out decision (see premature optimization).
In a Source file Indicate if a Method is Static or Virtual
In a source file you can't tell a method is static or virtual becausethis information is in the header file. Knowing this information ina source file is useful and can be communicated using comments:
/*virtual*/ void
Class::method()
{
}
/*static*/ void
Class::method()
{
}
I've only seen this format once in source code, but it is interestingenough that I thought I would include here for your consideration.Notice how the method name sits alone on its own line. This looks more like C codeand looks very clean for some reason.
Process
Use a Design Notation and Process
Programmers need to have a common language for talkingabout coding, designs, and the software process in general.This is critical to project success.
Any project brings together people of widely varying skills,knowledge, and experience. Even if everyone on a projectis a genius you will still fail because people willendlessly talk past each other because there isno common language and processes binding the project together.All you'll get is massive fights, burnout, and little progress.If you send your group to training they may not come back seasoned experts but at least your group will all be on the same page;a team.
There are many popular methodologies out there. Thepoint is to do some research, pick a method, trainyour people on it, and use it. Take a look at the top ofthis page for links to various methodologies.
You may find an Agile methodology to your liking. For more informationsee http://www.possibility.com/epowiki/Wiki.jsp?page=agile.
You may find the CRC (class responsibility cards) approach toteasing out a design useful. Many others have. It is an informalapproach encouraging team cooperation and focusing onobjects doing things rather than objects having attributes. There'seven a whole book on it: Using CRC Cards by Nancy M. Wilkinson.
Using Use Cases
A use case is a generic description of an entire transactioninvolving several objects. A use case can also describe the behaviourof a set of objects, such as an organization. A use case model thuspresents a collection of use cases and is typically used to specifythe behavior of a whole application system together with one or moreexternal actors that interact with the system.
An individual use case may have a name (although it is typicallynot a simple name). Its meaning is often written as an informal textdescription of the external actors and the sequences of eventsbetween objects that make up the transaction. Use cases caninclude other use cases as part of their behaviour.
Requirements Capture
Use cases attempt to capture the requirements for a system inan understandable form. The idea is by running through a setof use case we can verify that the system is doing what it should be doing.
Have as many use cases as needed to describe what a system needs to accomplish.
The Process
- Start by understanding the system you are trying to build.
- Create a set of use cases describing how the system is to be used by all its different audiences.
- Create a class and object model for the system.
- Run through all the use cases to make sure your model can handle all the cases. Update your model and create new use cases as necessary.
Using Stories
A user story consists of just a phrase or a few sentences written down, announcing something the user wants to do. The user story is not used as a requirements specification, but as a marker for a future conversation.
- User stories are used as tokens in the planning process after assessment of business value and risk. The customer prioritizes the stories and schedules them for implementation.
- User Story is a story about how the system is supposed to solve a problem. Each UserStory represents a chunk of functionality that is coherent in some way to the customer.
- User Story is a token that stands in for a conversation whose purpose is roughly equivalent to that of a UseCase. The content of a UserStory is often an abbreviated version of the main success scenario of a UseCase, but it doesn't have to be.
- User Story is a marker that reminds the customer and developers what to talk about, and also inspires them in exploring some particular area of business value. A UserStory, then, contains whatever the customer thinks is necessary to jog the memories and inspirations of those who will later explore the story. Expertise in the business area will help the customer decide what are the essential "jogs" to record in the initial UserStory marker.
- Developers should be able to implement a story in 2-4 weeks. The iteration length depends on you project.
A Story Is
- Testable -You can write automatic tests to detect the presence of the story.
- Progress - The customers side of the team is willing to accept the story as a sign of progress toward their larger goal.
- Bite-sized - The story should be completable within the iteration.
- Estimatable - The technical side of the team must be able to guess how much of the team's time the story will require to get working.
How It All Fits Together
There are a lot of entrants in this part of the development process.We have user stories, use cases, task analysis, task cases, usagescenarios, etc and etc.
It seems we need something more detailed than requirements as requirements tend to be fairly high level and are usually not directly useful fordevelopment. A requirement will cut across many subsystems and each of the cross-cuts must be identified. Thus requirements need to go through refinement and elaboration to be used in development.
If we get too detailed then we get bogged down in detailed designbefore we are ready. If we aren't detailed enough then we can't know if our requirements are met. Nor can we convince ourselves that weknow how to solve the problem.
User Stories Need to be Fleshed Out
User Stories are symbolic of the stuff that needs to get done. Theyare like the reminder notes you make to yourself. In the detaileddesign these can be expanded out to uses cases, to more detailednotes, state machines, etc. Or TestDrivenDesign practices couldbe used to flesh out the detailed implementation.
For more information see http://www.possibility.com/epowiki/Edit.jsp?page=UserStory.
Unified Modeling Language
The Unified Modeling Language is too large to present here. Fortunatelyyou can see it at Rational's web site. Since you do need a modeling language UML is a safe choice. Itcombines features from several methods into one unified language. Rememberall languages and methods are open to local customization. If their languageis too complex then use the parts you and your project feel they need andjunk the rest.
Code Reviews
If you can make a formal code review work then my hat is offto you. Code reviews can be very useful. Unfortunately theyoften degrade into nit picking sessions and endless argumentsabout silly things. They also tend to take a lot of people'stime for a questionable payback.
My god he's questioning code reviews, he's not an engineer!
Not really, it's the form of code reviews and how they fit intonormally late chaotic projects is what is being questioned.
First, code reviews are way too late to do much ofanything useful. What needs reviewing are requirements anddesign. This is where you will get more bang for the buck.
Get all relevant people in a room. Lock them in. Go over the class designand requirements until the former is good and the latter is being met. Having all the relevant people in the room makes this processa deep fruitful one as questions can be immediately answered andissues immediately explored. Usually only a couple of such meetingsare necessary.
If the above process is done well coding will takecare of itself. If you find problems in the codereview the best you can usually do is a rewrite after someone has sunka ton of time and effort into making the code "work."
You will still want to do a code review, just do it offline. Have a couple people you trust read the code in question and simply makecomments to the programmer. Then the programmer and reviewerscan discuss issues and work them out. Email and quick pointed discussions work well. This approach meets the goalsand doesn't take the time of 6 people to do it.
Create a Source Code Control System Early and Not Often
A common build system and source code control system should be put in placeas early as possible in a project's lifecycle, preferably before anyone startscoding. Source code control is the structural glue binding a project together.If programmers can't easily use each other's products then you'll never beable to make a good reproducible build and people will piss away a lot of time.It's also hell converting rogue build environments to a standard system.But it seems the right of passage for every project to build their own custom environment that never quite works right.
Some issues to keep in mind:
- Appoint a buildmaster who is in charge of the build environment and making builds. If you have a tools group so much the better as build environments generally generate a lot of tools.
- Programmers should generally leave tagging and other advanced operations to the buildmaster.
- Shared source environments like CVS usually work best in largish projects.
- Makefiles should be completely templated so developers need no makefile expertise.
- You will need multiple builds of the same code meaning programmers will want to have the option of building against optimized code versions, debugger code versions, code with the -DDEBUG macro defined and versions without, etc. One way to do this is have everyone check out all code and do a total rebuild. The other better way is for the buildmaster to build all necessary versions of code so programmers can check out only the code which they are modifying. The idea is that each combination of compile flags and macros defines a build.
- Dependency checking should work!
- Maintain a cycle of builds. Create a nightly build so problems can be detected early. Create a valid build every week or for every feature set that is guaranteed to be good. Every release to QA or to customers should be tagged and available in the tree. Official builds should be official because they contain a certain set of features that assumably meet a schedule, not because it's a certain date.
- If you use CVS use a reference tree approach. With this approach a master build tree is kept of various builds. Programmers checkout source against the build they are working on. They only checkout what they need because the make system uses the build for anything not found locally. Using the -I and -L flags makes this system easy to setup. Search locally for any files and libraries then search in the reference build. This approach saves on disk space and build time.
- Get a lot of disk space. With disk space as cheap it is there is no reason not to keep plenty of builds around and have enough space for programmers to compile.
- Make simple things simple. It should be dead simple and well documented on how to:
- check out modules to build
- how to change files
- how to setup makefiles
- how to add new modules into the system
- how to delete modules and files
- how to check in changes
- what are the available libraries and include files
- how to get the build environment including all compilers and other tools
Make a web page or document or whatever. New programmers shouldn't have to go around begging for build secrets from the old timers.
- Use the idea of services. If using a feature requires linking to 3 libraries think about making a service specifiable in the makefile programmers can use instead of having to know the exact libraries needed. With this approach the libraries making up a service can be changed in a based makefile without requiring changes in all makefiles.
- On checkins log comments should be useful. These comments should be collected every night and sent to interested parties.
Source Code Control Systems
Some options for source code control systems are:
- Subversion - a free, popular, and well featured system.
- Perforce - a not free, popular, and very solid system that scales beautifully and just works.
- Clear Case - a not free, popular in larger corporations, and requires a dedicated admin team, yet you can accomplish a lot with it.
There are many other options out there. It doesn't matter which one you pick asmuch as it does that you pick one and use it.
Create a Bug Tracking System Early and Not Often
The earlier people get used to using a bug tracking system the better. If youare 3/4 through a project and then install a bug tracking system it won'tbe used. You need to install a bug tracking system early so people will use it.
Programmers generally resist bug tracking, yet when used correctly it can really help a project:
- Problems aren't dropped on the floor.
- Problems are automatically routed to responsible individuals.
- The lifecycle of a problem is tracked so people can argue back and forth with good information.
- Managers can make the big schedule and staffing decisions based on the number of and types of bugs in the system.
- Configuration management has a hope of matching patches back to the problems they fix.
- QA and technical support have a communication medium with developers.
Not sexy things, just good solid project improvements.
FYI, it's not a good idea to reward people by the number of bugs they fix :-)
Source code control should be linked to the bug tracking system. During the partof a project where source is frozen before a release only checkins accompaniedby a valid bug ID should be accepted. And when code is changed to fix a bugthe bug ID should be included in the checkin comments.
Bug Tracking Systems
Some options for bug tracking systems are:
- Jira - a full featured and reliable bug tracking system. It works best when combined with Confluence, their wiki product.
- Bugzilla - a free product that is functionaland widely used.
As for source code control systems there are many available bug tracking systems.It's more important that you use one than which one you use.
RCS Keywords, Change Log, and History Policy
When using RCS directly this policy must change, but whenusing other source code control systems like CVS thatsupport RCS style keywords:
- Do not use RCS keywords within files.
- Do not keep a change history in files.
- Do not keep author information in files.
Justification
- The reasoning is your source control system already keeps all this information. There is no reason to clutter up source files with duplicate information that:
- makes the files larger
- makes doing diffs difficult as non source code lines change
- makes the entry into the file dozens of lines lower in the file which makes a search or jump necessary for each file
- is easily available from the source code control system and does not need embedding in the file
- When files must be sent to other organizations the comments may contain internal details that should not be exposed to outsiders.
Create a Wiki System Early and Not Often
The heart of software development is information. Where do you put all your documentation? Where do we put your coding guidelines? Where you putall your process documents? Where do you put your design documents? Wheredo you put all the 1000s of bits of information that hold a project together?
With a strategy information is usually spread like seeds in the wind.Much is in people's heads, in email, in IM, in files on people's filesystems, or files on different servers. Nobody can ever find anythingthey need.
A solution to this mess is to store all your information in a wiki. AWiki is a web site written by many users. It's very easy for a community of people to create and edit content. The idea is that people can add a page for any topic they want and add to any existing page by simply editing it. Over time we can create a site addressing most of the issues people have about projects, products, development, process, and more.
A wiki is perfect support for software development.
I've written up my hard one advice for using wikis atGetting Your Wiki Adopted.
Honor Responsibilities
Responsibility for software modules is scoped. Modules are either the responsibility of a particular person or are common. Honor this division of responsibility. Don'tgo changing things that aren't your responsibility to change. Only mistakesand hard feelings will result.
Face it, if you don't own a piece of code you can't possibly be in a position to change it. There's too much context. Assumptions seemingly reasonable to youmay be totally wrong. If you need a change simply ask the responsible personto change it. Or ask them if it is OK to make such-n-such a change. If they say OKthen go ahead, otherwise holster your editor.
Every rule has exceptions. If it's 3 in the morning and you need to make a changeto make a deliverable then you have to do it. If someone is on vacation and no onehas been assigned their module then you have to do it. If you make changes in otherpeople's code try and use the same style they have adopted.
Programmers need to mark with comments code that is particularly sensitive tochange. If code in one area requires changes to code in an another area thensay so. If changing data formats will cause conflicts with persistent storesor remote message sending then say so. If you are trying to minimize memoryusage or achieve some other end then say so. Not everyone is as brilliant as you.
The worst sin is to flit through the system changing bits of code to match yourcoding style. If someone isn't coding to the standards then ask them or askyour manager to ask them to code to the standards. Use common courtesy.
Code with common responsibility should be treated with care. Resist making radical changes as the conflicts will be hard to resolve. Put comments in the file on howthe file should be extended so everyone will follow the same rules. Try and usea common structure in all common files so people don't have to guess on whereto find things and how to make changes. Checkin changes as soon as possible soconflicts don't build up.
As an aside, module responsibilities must also be assigned for bug tracking purposes.
Process Automation
It's a sad fact of human nature that if you don't measure it or check forit: it won't happen. The implication is you must automate as much ofthe development process as possible and provide direct feedbackto developers on specific issues that they can fix.
Process automation also frees up developers to do real work because theydon't have to babysit builds and other project time sinks.
Automated Builds and Error Assignment
Create an automated build system that can create nightly builds, parse the build errors, assign the errors to developers, and email developerstheir particular errors so they can fix them.
This is the best way to maintain a clean build. Make sure the listof all errors for a build is available for everyone to see soeveryone can see everyone elses errors. The goal is replacea blaim culture with a culture that tries to get things rightand fixes them when they are wrong. Immediate feedback makes thispossible.
Automated Code Checking
As part of the automated build process you can check for coding standardviolations and for other problems. If you don't check for it people will naturally do their own thing. Code reviews aren't good enoughto keep the code correct. With a tool likeAbraxis Code Check you can check the code for a lot of potential problems.
This feature like the automated error assignment makes problems immediatelyvisible and immediately correctable, all without a lot of blameand shame.
Documentation Extraction
Related to this principle is the need to automatically extract documentationfrom the source code and make it available on line for everyone to use.If you don't do this documentation will be seen as generally useless anddevelopers won't put as much effort into it. Making the documentationvisible encourages people to do a better job.
Connect Source Code Control System and Bug Tracking System
- When a check-in of source code fixes a bug then have the check-inautomatically tell the bug tracking system that the bug was fixed.
- When a bug fix is built in a build change the state of the bug toBUILT.
- Have a submit tool where people ask judges of they can submita bug fix on the branch.
There are lots of things you can do depending on how complicatedyour environment. The more complicated the environment the moreyou should think about connecting all systems together.
Tools Agreement
The reality of different tool preferences is something to deal withexplicitly and openly. Tools include IDEs, languages, editors, make program,source code control, bug system, debuggers, test framework, etc. Some tooldecisions by their nature must be project wide, other decisionscan be customized per developer.
A split might also be done by who is performing the build. For example, an IDE should be able to used in local builds, but the make program would be used for nightly and release builds.
Certain things are easy/trivial/useful with one tool, but hard/complicated/stupid with another tool. Unstated tool assumptions can be the source of a lot of confusion. "Get a better editor" is not always a workable response, though sometimes that's all there is to it!
Non-Blocking Scheduling
Schedules are lies. Schedules suck. Yes, yes, yes. But we still need them.
The most effective scheduling rule i've used is to schedule so as to unblock others. The idea is to complete the portions of a feature that will unblock those dependent on you. This way development moves along smoothly because more lines of development can be active at a time. For example, instead of implementing the entire database, implement the simple interface and stub it out. People can work for a very long time this way using that portion of the feature that caused others not to block. Plus it's a form of rapid prototyping because you get immediate feedback on these parts. Don't worry about the quality of implementation because it doesn't matter yet.
Using Personas
Personas are a powerful design tool, especially when combined with responsibility driven design.Cooper's personas are:.
simply pretend users of the system you're building. You describe
them, in a surprising amount of detail, and then design your
system for them.
I have a standard set of personas that i consider when creatinga design/architecture that don't seem to be common. When you writecode their are a lot of personas looking over your shoulder:
- other programmers using the code
- maintenance
- extension
- documentation group
- training group
- code review
- test and validation
- manufacturing
- field support
- first and second line technical support
- live debugging
- post crash debugging
- build system (documentation generation and automatic testing)
- unit testing
- system testing
- source code control
- code readers
- legal
You are much more careful and more thorough when you really thing aboutall the personas, all the different people and all their different roles and purposes.
Use a Continuous Build System
Setup a continuous build system for your group. It should build all new check-ins andrun the unit tests for each change. If you have the resources run a smoke testfor each build as well.
If there's a failure email should go out to the developer's who made the check-ins.
This policy will allow you to catch errors as early as possible which will makeyour system more stable over time.
Code in the Dominant Style
Let's say someone invites you into your home as a guest. How do you act?Do you act as you do at home or do you try and fit in with the householdin which you are staying? You try to fit in of course.
So when you have to write code for a new codebase do you 1) write codehowever you want or 2) write code like the code that already exists?
When writing code in a new codebase write code how the code is alreadywritten. Don't use your own style. Take some time, look around, andsee how the code is written. To come in like a bull in a china shopis the mark of someone who is not a professional developer.
Run Unit Tests Before Every Check-in
Before every check-in developers must run unit tests to verify all unit tests should pass. This prevents letting any errors creep in.
Finding, debugging, and handling errors is a tremendous drain on thedevelopment process so we want have a solid working system from thestart. The start means your code that you check-in should always workand we know it works by running unit tests and smoke tests if yourorganization has them.
Formatting
Braces {} Policy
Brace Placement
Of the three major brace placement strategies two are acceptable,with the first one listed being preferable:
- Place brace under and inline with keywords:
if (condition) while (condition)
{ {
... ...
} } - Traditional Unix policy of placing the initial brace on the same line as the keyword and the trailing brace inline on its own line with the keyword:
if (condition) { while (condition) {
... ...
} }
Justification
- Another religious issue of great debate solved by compromise. Either form is acceptable, many people, however, find the first form more pleasant. Why is the topic of many psychological studies.
There are more reasons than psychological for preferring the first style. If you use an editor (such as vi) that supports brace matching, the first is a much better style. Why? Let's say you have a large block of code and want to know where the block ends. You move to the first brace hit a key and the editor finds the matching brace. Example:
if (very_long_condition && second_very_long_condition)
To move from block to block you just need to use cursor down and your brace matching key. No need to move to the end of the line to match a brace then jerk back and forth.
{
...
}
else if (...)
{
..
}
When Braces are Needed
All if, while and do statements must either have braces or be on a singleline.
Always Uses Braces Form
All if, while and do statements require braces even if there is only asingle statement within the braces. For example:
if (1 == somevalue)
{
somevalue = 2;
}
Justification
It ensures that when someone adds a line of code later there arealready braces and they don't forget. It provides a more consistent look.This doesn't affect execution speed. It's easy to do.
One Line Form
if (1 == somevalue) somevalue = 2;
Justification
It provides safety when adding new lines while maintainng a compactreadable form.
Add Comments to Closing Braces
Adding a comment to closing braces can help when you arereading code because you don't have to find the begin braceto know what is going on.
while(1)
{
if (valid)
{
} // if valid
else
{
} // not valid
} // end forever
Consider Screen Size Limits
Some people like blocks to fit within a common screen size so scrolling is not necessary when reading code.
Indentation/Tabs/Space Policy
- Indent using 3, 4, or 8 spaces for each level.
- Do not use tabs, use spaces. Most editors can substitute spaces for tabs.
- Tabs should be fixed at 8 spaces. Don't set tabs to a different spacing, uses spaces instead.
- Indent as much as needed, but no more. There are no arbitrary rules as to the maximum indenting level. If the indenting level is more than 4 or 5 levels you may think about factoring out code.
Justification
- Tabs aren't used because 8 space indentation severely limits the number of indentation levels one can have. The argument that if this is a problem you have too many indentation levels has some force, but real code can often be three or more levels deep. Changing a tab to be less than 8 spaces is a problem because that setting is usually local. When someone prints the source tabs will be 8 characters and the code will look horrible. Same for people using other editors. Which is why we use spaces...
- When people using different tab settings the code is impossible to read or print, which is why spaces are preferable to tabs.
- Nobody can ever agree on the correct number of spaces, just be consistent. In general people have found 3 or 4 spaces per indentation level workable.
- As much as people would like to limit the maximum indentation levels it never seems to work in general. We'll trust that programmers will choose wisely how deep to nest code.
Example
void
func()
{
if (something bad)
{
if (another thing bad)
{
while (more input)
{
}
}
}
}
Parens () with Key Words and Functions Policy
- Do not put parens next to keywords. Put a space between.
- Do put parens next to function names.
- Do not use parens in return statements when it's not necessary.
Justification
- Keywords are not functions. By putting parens next to keywords keywords and function names are made to look alike.
Example
if (condition)
{
}
while (condition)
{
}
strcpy(s, s1);
return 1;
A Line Should Not Exceed 78 Characters
- Lines should not exceed 78 characters.
Justification
- Even though with big monitors we stretch windows wide our printers can only print so wide. And we still need to print code.
- The wider the window the fewer windows we can have on a screen. More windows is better than wider windows.
- We even view and print diff output correctly on all terminals and printers.
If Then Else Formatting
Layout
It's up to the programmer. Different bracing styles will yield slightly different looks. One common approach is:
if (condition) // Comment
{
}
else if (condition) // Comment
{
}
else // Comment
{
}
If you have else if statements then it is usually a good ideato always have an else block for finding unhandled cases. Maybe put a logmessage in the else even if there is no corrective action taken.
Condition Format
Always put the constant on the left hand side of an equality/inequality comparison. For example:
if ( 6 == errorNum ) ...
One reason is that if you leave out one of the = signs, the compiler will find the error for you. A second reason is that it puts the value you are looking for right up front where you can find it instead of buried at the end of your expression. It takes a little time to get used to this format, but then it really gets useful.
switch Formatting
- Falling through a case statement into the next case statement shall be permitted as long as a comment is included.
- The default case should always be present and trigger an error if it should not be reached, yet is reached.
- If you need to create variables put all the code in a block.
Example
switch (...)
{
case 1:
...
// FALL THROUGH
case 2:
{
int v;
...
}
break;
default:
}
Use of goto,continue,break and ?:
Goto
Goto statements should be used sparingly, as in any well-structured code. The goto debates are boring so we won't go into them here. The mainplace where they can be usefully employed is to break out of severallevels of switch, for, and while nesting, although the need to do such a thingmay indicate that the inner constructs should be broken out into a separatefunction, with a success/failure return code.
for (...)
{
while (...)
{
...
if (disaster)
goto error;
}
}
...
error:
clean up the mess
When a goto is necessary the accompanying label should be alone on a line andto the left of the code that follows. The goto should becommented (possibly in the block header) as to its utility and purpose.
Continue and Break
Continue and break are really disguised gotos so they are coveredhere.
Continue and break like goto should be used sparingly as they are magic incode. With a simple spell the reader is beamed to god knows where forsome usually undocumented reason.
The two main problems with continue are:
- It may bypass the test condition
- It may bypass the increment/decrement expression
Consider the following example where both problems occur:
while (TRUE)
{
...
// A lot of code
...
if (/* some condition */) {
continue;
}
...
// A lot of code
...
if ( i++ > STOP_VALUE) break;
}
Note: "A lot of code" is necessary in order that the problem cannot becaught easily by the programmer.
From the above example, a further rule may be given:Mixing continue with break in the same loop is a sure way to disaster.
?:
The trouble is people usually try and stuff too much codein between the ? and :. Here are a couple ofclarity rules to follow:
- Put the condition in parens so as to set it off from other code
- If possible, the actions for the test should be simple functions.
- Put the action for the then and else statement on a separate line unless it can be clearly put on one line.
Example
(condition) ? funct1() : func2();
or
(condition)
? long statement
: another long statement;
One Statement Per Line
There should be only one statement per line unless the statements are very closely related.
The reasons are:
- The code is easier to read. Use some white space too. Nothing better than to read code that is one line after another with no white space or comments.
One Variable Per Line
Related to this is always define one variable per line:
Not:
char **a, *x;
Do:
char** a= 0; // add doc
char* x= 0; // add doc
The reasons are:
- Documentation can be added for the variable on the line.
- It's clear that the variables are initialized.
- Declarations are clear which reduces the probablity of declaring a pointer when you meant to declare just a char.
Alignment of Declaration Blocks
- Block of declarations should be aligned.
Justification
- Clarity.
- Similarly blocks of initialization of variables should be tabulated.
- The ??and ??tokens should be adjacent to the type, not the name.
Example
DWORD mDword
DWORD* mpDword
char* mpChar
char mChar
mDword = 0;
mpDword = NULL;
mpChar = NULL;
mChar = 0;
Include static and virtual Key Words in Source File
If a method is made static or virtual in the header file thenalso include this information as a short comment in the source file. For example:
In .h file:
class PopcornPopper
{
public:
virtual void Pop(void);
static PopcornPopper* Singleton();
};
In .cpp file:
/*virtual*/ void
PopcornPopper::Pop(void)
{
}// Pop
/*static*/ PopcornPopper*
PopcornPopper::Singleton()
{
}// Singleton
A source file doesn't contain as much information as the header file because the virtualand static keywords are dropped from method implementations. This is very useful information to have when reading code, so we want to include it in the source file.
Document Null Statements
Always document a null body for a for or while statement so that it is clearthat the null body is intentional and not missing code.
while (*dest++ = *src++)
; // VOID
Exceptions
Create One Exception for Each Library
Creating very elaborate exception hierarchies is a waste of time. Nobody ends of caringand all the effort goes to waste. Instead, create one exception per library or namespaceand have an exception reason within that exception to indicate the type of the exception.
For example, for your OS encapsulation libary, make an exception called OsencapException.
By using just one exception you make it easy for people using your code to catch yourexception. If you have thousand exceptions it is impossible for anyone to handle those.And as exceptions are added you will break existing code that thinks they are handlingall your exceptions.
If you think you need to create derived exceptions the derive them from your libraries'base exception class. That allows your library users to still catch the base classexception if they wish.
Most exceptions are things the code can't do anything about anyway, so creatinglots of exceptions to express every little thing that can go wrong with a separate class and is time confusing and confusing for the user. It's not necessary.
Make sure to document that an exception is being thrown in you method comment header.
Create a macro for throwing the exception so that you can transparently include __FILE__and __LINE__ in the exception. This information is usefull when debugging. You can alsoautomatically take a time stamp and get the thread ID.
The exception should take a one or two string arguments so developers can include allthe information they need in the exception without having to create a derived exceptionto add a few more pieces of data.
Include a stack trace of where the exception happened so you can log the stacktrace.
Throwing an exception should take only one line of code. Don't uglify your codewith tons of exception handling. Make a macro that looks like:
THROW_NAMESPACE_EX_IF(cond, msg1, msg2, reason);
The condition causes the exception to be thrown when true. Msg1 and msg2 are localcontext. Reason is the specific error code for the exception, if you think youneed it.
Don't worry about running out of memory errors. Go ahead and allocate memory in yourexception code. If you are running out of memory that should be taken care of in thebad_alloc exception handler.
Selecting Between Exceptions And Asserts
If there is reason to believe that an important test could fail in the released product, it should not be tested with an assert, use an exception instead.
Two useful questions to ask yourself are:
- Should this error ever happen?
- Is the error so unexpected or potentially damaging that the system should failover if it is found?
When at the boundry of user input we should expect invalidparameters and thus should use an exception instead of DBC.
Exception Definition
There are as many definitions of exception as thereare programmers:
The Clean Code Definition
Write a function as if everything worked.
Any error handling that prevents the code from looking
like this is an exception.
The Design by Contract Definition
When an operation is invoked with it's pre-condition
satisfied, yet it cannot return with its post-condition
satisfied.
There are those who say that exceptions should NOT be used to catch things like range errors. Here's why:
Range errors fall into a class of problem that are called Programmer Errors. Programmer errors are things that should NOT occur in a Bug Free Program.Ideally, range type problems are one of those things that should be caught during development. It is for these problems that we have Design By Contract.
You program defensively by putting lots of asserts in to make sure that your program is functioning as expected. Then when you've committed a bug, like not checking user input properly, allowing an invalid index into an array to be derived, the assert macro catches it, and the program does a gracful crash.
Exceptions, on the other hand, should be used to catch problems that would arise even in a Bug Free Program, i.e. Exceptional Circumstances. The most perfect program can still be afflicted by things like shortage of memory and other resources, communications errors, and file problems. When one of these things occurs, an exception should be thrown, and caught at a point where the program can either deal with the problem, or close gracefully.
The real theory behind exceptions is to force the programmer toanticipate things the programmer has no control over. Exceptions support the following kind of scenarios:
- Logic error in the middle of a database transaction. An exception would allow the program a chance to leave the database in a consistent state. A trashed database can be very expensive.
- Logic error in the middle of a program that is using a resource ike a modem and the program runs unattended. Exception handling would give the program a chance to hang up the modem connection, rather than sit there with an connection until it's discovered the next day, running up the phone bill.
- Logic error in the middle of something like a word processor. Consider the user that has been working for a couple of hours with unsaved edits, and your assert message pops up, and the last two hours of work are pretty much lost - causing the user some misery. If you used exception handling, you could at least give the user a chance to salvage the unsaved document in a different file as part of the cleanup.
Design-by-Contract (DBC)
A design technique developed by Bertrand Meyer for producing "bug free" systems.
Design by Contract (DBC) views the relationship between a classand its clients as formal agreement, expressing each party's rights andobligation. Rights and obligations are determined by a class'specification. Correctness can only be determined in reference toan object's specification. Specifications, in DBC, are expressed throughassertions.
Design by Contract is a development approach where a specification is associatedwith every software element. These specifications (or contracts) governthe interaction of the element with the rest of the world. A contract takesform as a set of preconditions, postconditions, and invariants that arerun as the system executes. The contract is published in the comment blockof each method.
Assertion
A boolean statement that should never be false and,therefore, will only be false because of a bug.Typically assertions are checked only during debug and are not checked during production execution,although they can be left in when performance andmemory size are not issues.
Design by Contract uses three kinds of assertions:post-conditions, pre-conditions, and invariants.Pre-conditions and post-conditions apply to operations. Invariants apply to the class.
Pre-condition
A statement of how we expect the world to be beforewe execute an operation. We might design a pre-conditionof the "square" operation as this >= 0. Such a pre-condition says that it is an error to invoke "square" on a negative number.
Post-condition
A statement of what the world should look likeafter execution of an operation. For example, ifwe define the operation "square" on a number, thepost-condition would take the form result = this *this.The post-condition is a useful way of saying what we do withoutsaying how we do it- in other words, of separatinginterface from implementation.
Invariant
An invariant is an assertion about a class. For instance,an Account class may have an invariant that says thatbalance == sum(entries.amount()). The invariantis always true for all instances of the class."Always" means whenever the object is available for an operation to be invoked. During an operationinvariants may not be satisfied, but invariants musthold when an operation has finished.
Is pop'ing an empty stack an exception?
When using a stack data structure is calling pop on an emptystack an exception? This is the standard use case for theendless discussions on what is and what isn't anexception. The upshot is there is not a "right" answer.In practice, as long as the library designer completelydocuments their choice the library user shouldn'tmake a mistake.
Let's apply our definition of exception.
The Clean Code View
If you write your application assuming everything worksthen is pop returning null something you wouldn't expect?It seems reasonable that if you ask for something itmay not be there.
Before calling pop the client can call isEmpty to checkif there's anything in the list. It's still possiblein a multi-threaded environment for the stack to returnnull even if isEmpty returns true.
So don't use an exception. Pop returns data so itshould return null when empty.
The Design by Contract View
Should the library designer have as a pre-conditionthat pop shouldn't be called when the stack isempty? From an implementor point of view this couldseem reasonable. After all, why should an operation be calledwhen there's nothing there? So, pop could assert.
But, library designers should alway see their clientspoint-of-view. It's a bit extreme tocause a system crash because pop has an empty stack.There's nothing fundamentally or systemically wrong inthis scenario that would justify an assert.
It appears the pre-conditions are satisfied forpop. Would any post-conditions not be satisfied?(this defines an exception in DBC).The stack should still be empty after the operationso no post-conditions would be violated. Thus, bydefinition an exception should not be thrown.
Be Careful Throwing Exceptions in Destructors
An object is presumably created to do something. Some of thechanges made by an object should persist after an object dies(is destructed) and some changes should not. Take an objectimplementing a SQL query. If a database field is updated viathe SQL object then that change should persist after the SQLobjects dies. To do its work the SQL object probably createda database connection and allocated a bunch of memory.When the SQL object dies we want to close the database connectionand deallocate the memory, otherwise if a lot of SQL objectsare created we will run out of database connections and/or memory.
The logic might look like:
Sql::~Sql()
{
delete connection;
delete buffer;
}
Let's say an exception is thrown while deleting the database connection.Will the buffer be deleted? No. Exceptions are basically non-localgotos with stack cleanup. The code for deleting the buffer will neverbe executed creating a gaping resource leak.
Special care must be taken to catch exceptions which may occur duringobject destruction. Special care must also be taken to fully destruct an object when it throws an exception.
Usign RAII can help prevent many of not mostof these type of errors.
Working With Libaries Using All Sorts of Different Policies
On many projects we have to work with C libraries that use error returncodes, old C++ libraries that use error return codes, and newer C++ librariesthat use exceptions. How should all these different approaches work together?
Ideally exceptions should be used where possible because that's the whereall new code is going. But I also see new libraries being forced to useerror return codes because the old libraries use them and application maintainersdon't want you to use exceptions.
In C++ any method can throw an exception and you can't tell. In Java you knowwhen you are not catching an exception. Not in C++.So what happens is that if a new library comes in and throws an exception, a crticial program may start failing because an exception is not caught. Thismay make everyone use error codes instead of exceptions.
Don't give into this old timer bias. Error return codes are virtually uselessbecause they are competely ingorable. Instead, an application should just wraptheir code in a big try catch block that catches all exceptions. That wayan application won't die with prejudice and C++ libarary writers can makeuse of exceptions in their design.
Templates
Templates have never come up as an issue on project I've worked on,so my advice on templates isn't very useful.
Keeping templates simple has worked best. Just use templates for whatthey are good at, making a class work over different types. Not everythingneeds to be a template or needs to be perfectly generic. Because templatesare so complex and so difficult to debug, it's best to use them only whennecessary.
Template meta programming is too elite for me so I'll leave that to other people.
Namespaces
Create Unique Name Space Names
A namespace isn't of much use if it's not globally unique.
On way to make name spaces unique is to root tham at some naming
There are two basic strategies for naming: root that name at some namingauthority, like the company name and division name. The Java conventionis to use the inverse of the companys domain name for package names.
Personally that always seemed overkill to me, but does work in thelarge.
For internal packages just pick a short simple descriptive namespacethat will likely prevent conflict within a project.
For externally visible code you'll probably need to the fulldomain style namespace just to be safe.
Don't Globally Define using
Don't place using namespace directive at global scope in a header file.This can cause lots of magic invisible conflicts that are hard to track.Keep using statements to implementation files.
For example, let's say I want to use boost's fabulous gregorian datelibrary. In my class I want to return dates and use dates in methods.So my code ends up looking like:
#include "boost/date_time/gregorian/gregorian.hpp" //include all types plus i/o
class Calendar
{
public:
boost::gregorian::date GetEventDate(void) const;
void SetEventDate(boost::gregorian::date dateOfTheEvent);
};
Clearly these are long names and are noisy and are pain to use. So you are temptedto put at the top of you class file:
using namespace boost::gregorian; // make namespace global for this file
This would mean you could just use "date" instead of "boost::gregorian::date".That's nice. But you can't do that. If you do you are making the decision foreveryone who uses your class as well. They may have a conflict, "date" is a verycommon name afterall.
So, don't use using in you header file, but you can use it in your source file.Because it's your source file you can make the decision to use using to shortenup the names. This strategy preserves most of the convenience while being a goodcitizen.
Create Shortcut Names
You can use using to create aliases or shortcuts, that is names that are more convenient to access than long namespace names.
For example:
namespace alias = a::very::long::namespace;
class YourClass: public alias::TheirClass
{ ... }
Clearly this makes code both harder to read and easier to read. The codeis harder to read because you as a programmer have to compile the aliasdirective in your head and know what it means whenever it is used in the code.
The code is easier to read because it is less complex. Complex namespacesare distracting and confusing. Even with the additional cognitive load, usingnamespace alias make code a lot easier to read.
Miscellaneous
This section contains some miscellaneous do's and don'ts.
- Don't use floating-point variables where discrete values are needed. Using a float for a loop counter is a great way to shoot yourself in the foot. Always test floating-point numbers as <= or >=, never use an exact comparison (== or !=).
- Compilers have bugs. Common trouble spots include structure assignment and bit fields. You cannot generally predict which bugs a compiler has. You could write a program that avoids all constructs that are known broken on all compilers. You won't be able to write anything useful, you might still encounter bugs, and the compiler might get fixed in the meanwhile. Thus, you should write ``around'' compiler bugs only when you are forced to use a particular buggy compiler.
- Do not rely on automatic beautifiers. The main person who benefits from good program style is the programmer him/herself, and especially in the early design of handwritten algorithms or pseudo-code. Automatic beautifiers can only be applied to complete, syntactically correct programs and hence are not available when the need for attention to white space and indentation is greatest. Programmers can do a better job of making clear the complete visual layout of a function or file, with the normal attention to detail of a careful programmer (in other words, some of the visual layout is dictated by intent rather than syntax and beautifiers cannot read minds). Sloppy programmers should learn to be careful programmers instead of relying on a beautifier to make their code readable. Finally, since beautifiers are non-trivial programs that must parse the source, a sophisticated beautifier is not worth the benefits gained by such a program. Beautifiers are best for gross formatting of machine-generated code.
- Accidental omission of the second ``='' of the logical compare is a problem. The following is confusing and prone to error.
if (abool= bbool) { ... }Does the programmer really mean assignment here? Often yes, but usually no. The solution is to just not do it, an inverse Nike philosophy. Instead use explicit tests and avoid assignment with an implicit test. The recommended form is to do the assignment before doing the test:
abool= bbool;
if (abool) { ... }
- Modern compilers will put variables in registers automatically. Use the register sparingly to indicate the variables that you think are most critical. In extreme cases, mark the 2-4 most critical values as register and mark the rest as REGISTER. The latter can be #defined to register on those machines with many registers.
Be Const Correct
C++ provides the const key word to allow passing as parameters objects that cannot change to indicate when a method doesn't modify its object. Using const in all the right places is called"const correctness."
It's hard at first, but using const really tightens up your coding style. Const correctness grows on you.
If you don't use const correctness from the start it can be nightmareto add it in later because it causes a chain reaction of needingconst everywhere. It's better to start being const correct fromthe start or you probably won't be.
You can always cast aways constness when necessary, but it's betternot to.
For more information see Const Correctness in the C++ FAQ.
Use Streams
Programmers transitioning from C to C++ find stream IO strangepreferring the familiarity of good old stdio. Printf and gangseem to be more convenient and are well understood.
Type Safety
Stdio is not type safe, which is one of the reasons youare using C++, right? Stream IO is type safe. That's one goodreason to use streams.
Standard Interface
When you want to dump an object to a stream there isa standard way of doing it: with the << operator.This is not true of objects and stdio.
Interchangeablity of Streams
One of the more advanced reasons for using streams is thatonce an object can dump itself to a stream it can dump itselfto any stream. One stream may go to the screen, but another streammay be a serial port or network connection. Good stuff.
Streams Got Better
Stream IO is not perfect. It is however a lot better thanit used to be. Streams are now standardized, acceptably efficient,more reliable, and now there's lots of documentation on how to use streams.
Check Thread Safety
Some stream implementations are not yet thread safe. Make surethat yours is.
But Not Perfect
For an embedded target tight on memory streams do not make sense.Streams inline a lot of code so you might find the imagelarger than you wish. Experiment a little. Streams mightwork on your target.
No Magic Numbers
A magic number is a bare naked number used in source code. It's magicbecause no-one has a clue what it means including the author inside3 months. For example:
if (22 == foo) { start_thermo_nuclear_war(); }
else if (19 == foo) { refund_lotso_money(); }
else if (16 == foo) { infinite_loop(); }
else { cry_cause_im_lost(); }
In the above example what do 22 and 19 mean? If there was a number change or thenumbers were just plain wrong how would you know?
Instead of magic numbers use a real name that means something. You can use #define or constants or enums as names. Which one is a design choice. For example:
#define PRESIDENT_WENT_CRAZY (22)Now isn't that better? The const and enum options are preferablebecause when debugging the debugger has enough information to displayboth the value and the label. The #define option just shows up asa number in the debugger which is very inconvenient. The constoption has the downside of allocating memory. Only you know if thismatters for your application.
const int WE_GOOFED= 19;
enum
{
THEY_DIDNT_PAY= 16
};
if (PRESIDENT_WENT_CRAZY == foo) { start_thermo_nuclear_war(); }
else if (WE_GOOFED == foo) { refund_lotso_money(); }
else if (THEY_DIDNT_PAY == foo) { infinite_loop(); }
else { happy_days_i_know_why_im_here(); }
Error Return Check Policy
- Check every system call for an error return, unless you know you wish to ignore errors. For example, printf returns an error code but rarely would you check for its return code. In which case you can cast the return to (void) if you really care.
- Include the system error text for every system error message.
- Check every call to malloc or realloc unless you know your versions of these calls do the right thing. You might want to have your own wrapper for these calls, including new, so you can do the right thing always and developers don't have to make memory checks everywhere.
To Use Enums or Not to Use Enums
C++ allows constant variables, which should deprecate theuse of enums as constants. Unfortunately, in most compilers constants take space. Some compilers will remove constants,but not all. Constants taking space precludes them from beingused in tight memory environments like embedded systems.Workstation users should use constants and ignore the restof this discussion.In general enums are preferred to #define asenums are understood by the debugger.
Be aware enums are not of a guaranteed size. So if you have a type that can take a known range of values and it is transported in a message you can't use an enum as the type. Use the correct integer size and use constants or #define. Castingbetween integers and enums is very error prone as you couldcast a value not in the enum.
A C++ Workaround
C++ allows static class variables. These variables are available anywhere and only the expected amount of space is taken.Example
class Variables
{
public:
static const int A_VARIABLE;
static const int B_VARIABLE;
static const int C_VARIABLE;
}
Macros
Don't Turn C++ into Pascal
Don't change syntax via macro substitution. It makes the programunintelligible to all but the perpetrator.Replace Macros with Inline Functions
In C++ macros are not needed for code efficiency. Use inlines.Example
#define MAX(x,y)(((x) > (y) ? (x) : (y))// Get the maximum
The macro above can be replaced for integers with the following inline functionwith no loss of efficiency:
inline int
max(int x, int y)
{
return (x > y ? x : y);
}
Be Careful of Side Effects
Macros should be used with caution because of the potential for error wheninvoked with an expression that has side effects.Example
MAX(f(x),z++);
Always Wrap the Expression in Parenthesis
When putting expressions in macros always wrap the expression in parenthesis to avoid potential communitive operation ambiguity.Example
#define ADD(x,y) x + y
must be written as
#define ADD(x,y) ((x) + (y))
Make Macro Names Unique
Like global variables macros can conflict with macros fromother packages.- Prepend macro names with package names.
- Avoid simple and common names like MAX and MIN.
Do Not Default If Test to Non-Zero
Do not default the test for non-zero, i.e.is better than
if (FAIL != f())
even though FAIL may have the value 0 which C considers to be false. Anexplicit test will help you out later when somebody decides that a failurereturn should be -1 instead of 0. Explicit comparison should be used even ifthe comparison value will never change; e.g., if (!(bufsize % sizeof(int)))should be written instead as if ((bufsize % sizeof(int)) == 0) to reflectthe numeric (not boolean) nature of the test. A frequent trouble spot is usingstrcmp to test for string equality, where the result should never ever be defaulted. The preferred approach is to define a macro STREQ.
if (f())
#define STREQ(a, b) (strcmp((a), (b)) == 0)
Or better yet use an inline method:
inline bool
StringEqual(char* a, char* b)
{
return strcmp(a, b) == 0;
}
Note, this is just an example, you should really use the standardlibrary string type for doing the comparison.
The non-zero test is often defaulted for predicates and other functions orexpressions which meet the following restrictions:
- Returns 0 for false, nothing else.
- Is named so that the meaning of (say) a true return is absolutely obvious. Call a predicate IsValid(), not CheckValid().
The Bull of Boolean Types
Any project using source code from many sources knows the painof multiple conflicting boolean types. The new C++ standarddefines a native boolean type. Until all compilers supportbool, and existing code is changed to use it, we must still deal with the cruel world.The form of boolean most accurately matching the new standard is:
Note, the standard defines the names true and falsenot TRUE and FALSE. The all caps versions are used to not clashif the standard versions are available.
typedef int bool;
#define TRUE 1
#define FALSE 0
or
const int TRUE = 1;
const int FALSE = 0;
Even with these declarations, do not check a boolean value for equality with 1(TRUE, YES, etc.); instead test for inequality with 0 (FALSE, NO, etc.). Mostfunctions are guaranteed to return 0 if false, but only non-zero if true. Thus,
must be written
if (TRUE == func()) { ...
if (FALSE != func()) { ...
Usually Avoid Embedded Assignments
There is a time and a place for embedded assignment statements. In someconstructs there is no better way to accomplish the results without making thecode bulkier and less readable.
while (EOF != (c = getchar()))
{
process the character
}
The ++ and -- operators count as assignment statements. So, for many purposes,do functions with side effects. Using embedded assignment statements toimprove run-time performance is also possible. However, one should considerthe tradeoff between increased speed and decreased maintainability that resultswhen embedded assignments are used in artificial places. For example,
should not be replaced by
a = b + c;
d = a + r;
even though the latter may save one cycle. In the long run the time differencebetween the two will decrease as the optimizer gains maturity, while thedifference in ease of maintenance will increase as the human memory of what'sgoing on in the latter piece of code begins to fade.
d = (a = b + c) + r;
Reusing Your Hard Work and the Hard Work of Others
Reuse across projects is almost impossible withouta common framework in place. Objects conform to theservices available to them. Different projectshave different service environments making object reusedifficult.Developing a common framework takes a lot of up frontdesign effort. When this effort is not made, forwhatever reasons, there are several techniquesone can use to encourage reuse:
Ask! Email a Broadcast Request to the Group
This simple technique is rarely done. For some reasonprogrammers feel it makes them seem less capableif they ask others for help. This is silly! Do newinteresting work. Don't reinvent the same stuff overand over again.If you need a piece of code email to the group asking if someone has already done it. The results can be surprising.
In most large groups individuals have no idea what other people are doing. You may even find someone is looking for somethingto do and will volunteer to do the code for you. There's always a gold mine out there if people work together.
Tell! When You do Something Tell Everyone
Let other people know if you have done something reusable.Don't be shy. And don't hide your work to protect your pride.Once people get in the habit of sharing work everyone getsbetter.Don't be Afraid of Small Libraries
One common enemy of reuse is people not makinglibraries out of their code. A reusable class may behiding in a program directory and will never havethe thrill of being shared because the programmerwon't factor the class or classes into a library.One reason for this is because people don't like making smalllibraries. There's something about small libraries thatdoesn't feel right. Get over it. The computer doesn't carehow many libraries you have.
If you have code that can be reused and can't be placed in an existing library then make a new library. Libraries don't stay small for long if people are really thinking about reuse.
If you are afraid of having to update makefiles when librariesare recomposed or added then don't include libraries in yourmakefiles, include the idea of services. Base level makefilesdefine services that are each composed of a set of libraries. Higher level makefiles specify the services they want. When the libraries for a service change only the lower level makefiles will have to change.
Keep a Repository
Most companies have no idea what code they have. And mostprogrammers still don't communicate what they have done orask for what currently exists. The solution is to keepa repository of what's available.In an ideal world a programmer could go to a web page, browseor search a list of packaged libraries, taking what theyneed. If you can set up such a system where programmersvoluntarily maintain such a system, great. If you have alibrarian in charge of detecting reusability, even better.
Another approach is to automatically generate a repositoryfrom the source code. This is done by using commonclass, method, library, and subsystem headers that can double as manpages and repository entries.
Commenting Out Large Code Blocks
Sometimes large blocks of code need to be commented out for testing.Using #if 0
The easiest way to do this is with an #if 0 block:void
example()
{
great looking code
#if 0
lots of code
#endif
more code
}
You can't use /**/ style comments because comments can'tcontain comments and surely a large block of your code will contain a comment, won't it?
Don't use #ifdef as someone can unknowingly trigger ifdefs from thecompiler command line.
Use Descriptive Macro Names Instead of 0
The problem with #if 0is that even day later you or anyoneelse has know idea why this code is commented out. Is it becausea feature has been dropped? Is it because it was buggy? It didn'tcompile? Can it be added back? It's a mystery.Use Descriptive Macro Names Instead of #if 0
#if NOT_YET_IMPLEMENTED
#if OBSOLETE
#if TEMP_DISABLED
Add a Comment to Document Why
Add a short comment explaining why it is not implemented, obsolete or temporarily disabled.
Use #if Not #ifdef
Use #if MACRO not #ifdef MACRO. Someone might write code like:#ifdef DEBUGSomeone else might compile the code with turned-of debug info like:
temporary_debugger_break();
#endif
cc -c lurker.cpp -DDEBUG=0Alway use #if, if you have to use the preprocessor. This works fine, and does the right thing, even if DEBUG is not defined at all (!)
#if DEBUGIf you really need to test whether a symbol is defined or not, test it with the defined() construct, which allows you to add more things later to the conditional without editing text that's already in the program:
temporary_debugger_break();
#endif
#if !defined(USER_NAME)
#define USER_NAME "john smith"
#endif
Creating a C Function in C++
extern "C" void
a_c_function_in_cplusplus(int a)
{
}
__cplusplus Preprocessor Directive
If you have code that must compile in a C and C++ environment then you must use the__cplusplus preprocessor directive. For example:
#ifdef __cplusplus
extern "C" some_function();
#else
extern some_function();
#endif
Mixing C and C++
In order to be backward compatible with dumb linkers C++'s link time type safetyis implemented by encoding type information in link symbols, a processcalled name mangling. This creates a problem when linking to C codeas C function names are not mangled. When calling a C functionfrom C++ the function name will be mangled unless you turn it off. Name mangling is turned off with the extern "C" syntax. If you wantto create a C function in C++ you must wrap it with the above syntax. Ifyou want to call a C function in a C library from C++ you must wrap in theabove syntax. Here are some examples:
Calling C Functions from C++
extern "C" int strncpy(...);
extern "C" int my_great_function();
extern "C"
{
int strncpy(...);
int my_great_function();
};
No Data Definitions in Header Files
Do not put data definitions in header files.for example:/*
* aheader.h
*/
int x = 0;
- It's bad magic to have space consuming code silentlyinserted through the innocent use of header files.
- It's not common practice to define variables in the headerfile so it will not occur to devellopers to look for this whenthere are problems.
- Consider defining the variable once in a .cpp fileand use an extern statement to reference it.
- Consider using a singleton for access to the data.
Make Functions Reentrant
Functions should not keep static variables that preventa function from being reentrant. Functions can declarevariables static. Some C library functions in the past,for example, kept a static buffer to use a temporary work area. Problems happen when the function is called one ormore times at the same time. This can happen whenmultiple tasks are used or say from a signal handler.Using the static buffer caused results to overlapand become corrupted.The moral is make your functions reentrant by not using static variables in a function.Besides, every machine has 128MB of RAM now so we don'tworry about buffer space any more :-)
Use the Resource Acquisition is Initialization (RAII) Idiom
RAII is a pattern useful in making sure allocated resources get freed.What kind of resources? Memory usually. Could be file descriptors or lockstoo. Anything that is allocated at one point and needs to be deleted atanother point in your code.C++ can help make this painless because of a feature I miss a lot in Java, thatyour destructor is always called when an object leaves scope. Thisfeature has caused the creation of the Guard idiom. A Guard can bewrapped around any resource so has to free the resource when theguard goes out of scope.
If you think you are always so careful to pair new and deletes that youdon't need to use one of those pattern thingies then I am sure I'll besitting beside you someday, like I have dozens of times before, searchingfor resource leaks in your code.
The acquisition phase happens at construction. You are supposed to do all the initialization in the constructor so you can do all the cleaning upin the destructor. And if you can guarantee through some different meansthat the destructor is always called then you can be pretty sure youwon't leak.
Some intialization sequences can't be done in the constructor, particularlythose using message driven state machines, but pretty much everything elsecan. And for those things use RAII. The justification is that it will saveyour butt from the many memory links that haunt large software productswritten in C++.
The key is to make sure the destructor is always called. The easiest wayis to use a stack variable. That way you know the destructor will be calledwhen leaving scope.
For pointers you can use one of the many smart pointer variants like auto_ptr.These will make sure your pointers are deleted when memory ownership is transferredor the scope is exited. You can even use them as member variables so you don'thave to write code in the destructor.
If you can't use exceptions then this idiom isn't for you. As you are doinga lot of work in the constructor you can have errors and you'll want throwan exception.
See http://www.hackcraft.net/raii/for a better explanation of RAII.
Portability
Use Typedefs for Types
It's a good idea to typedef int8, int16, int32, int64, float32, float64, uint8, uint16, uint32, uint64, etc., instead of assuming it'll be done with int, long,float, and short.Alignment of Class Members
There seems to be disagreement on how to align class data members. Be aware that different platforms have differentalignment rules and it can be an issue. Alignment may alsobe an issue when using shared memory and shared libraries.The real thing to remember when it comes to alignment is to put the biggest data members first, and smaller members later, and to pad with char[] so that the same structure would be used no matter whether the compiler was in "naturally aligned" or "packed" mode.
For the Mac there's no blanket "always on four byte boundaries" rule -- rather, the rule is "alignment is natural, but never bigger than 4 bytes, unless themember is a double and first in the struct in which case it is 8". And that rule was inherited from PowerOpen/AIX.
Compiler Dependent Exceptions
Using exceptions across the shared library boundarycould cause some problems if the shared library andthe client module are compiled by different compilervendors.Compiler Dependent RTTI
Different compilers are not guaranteed to name typesthe same.Popular Myths
Promise of OO
OO has been hyped to the extent you'd figure it would solve world hungerand usher in a new era of world peace. Not! OO is an approach, a philosophy,it's not a recipe which blindly followed yields quality.Robert Martin put OO in perspective:
- OO, when properly employed, does enhance the reusability of software. But it does so at the cost of complexity and design time. Reusable code is more complex and takes longer to design and implement. Furthermore, it often takes two or more tries to create something that is even marginally reusable.
- OO, when properly employed, does enhance the software's resilience to change. But it does so at the cost of complexity and design time. This trade off is almost always a win, but it is hard to swallow sometimes.
- OO does not necessarily make anything easier to understand. There is no magical mapping between the software concepts and every human's map of the real world. Every person is different. What one person percieves to be a simple and elegant design, another will perceive as convoluted and opaque.
- If a team has been able, by applying point 1 above, to create a repository of reusable items, then development times can begin to shrink significantly due to reuse.
- If a team has been able, by applying point 2 above, to create software that is resilient to change, then maintenance of that software will be much simpler and much less error prone.
You can't use OO and C++ on Embedded Systems
Oh yes you can. I've used C++ on several embedded systems ashave many others. And if you can't why not? Please don't give in to vaguefeelings and prejudice. An attitude best shown with a short exchange:Rube: Our packet driver is slow. We're only getting 100 packets per second.My initial response was prompted by a general unacceptance of C++ in the projectand blaming C++ for all problems. Of course all the parts written in C andassembly had no problems :-) Embedded systems shops tend to be hardware drivencompanies and tend not to know much about software development, thus any newfangled concepts like OO and C++ are ridiculed without verbally accessiblereasons. Counter arguments like code that is fast and small and reusable don'tmake a dent. Examples like improving the speed of a driver by inlining certainmethods and not hacking the code to death gently roll into the bit bucket.
Me : Good thing you didn't do it in C++ huh?
Rube: Oh yah, it would have been really slow then!
Me : (smiled secretly to myself)
Techniques
Of course C++ can be a disaster for an embedded system when used incorrectly, which of course is true of any tool. Here's some ideas to use C++ safelyin an embedded system:- Get Some Training!
If people don't know C++ and OO then they will likely fail and blametheir tools. A good craftsperson doesn't blame their tools. Get training.Hire at least one experienced person as guide/mentor.
- Be Careful Using Streams
The streams library is large and slow. You are better off making a "fake"streams library by overloading the << operator. If you have a lot ofmemory then use streams, they are convenient and useful.
- Be Careful Using Templates
Code using templates can suffer from extreme code bloat. This is prettymuch a function of your compiler as templates can be efficiently usedwhen done correctly. Test your compiler for it how handles templates.If it doesn't make a copy per file for each template then you are in business.Templates have good time efficiency so they would be nice to use.
You can fix the template code bloat problem by using explicit instantiation. Actually, even if the compiler generates one copy per source file. This, however, is often too much programmer work to expect on a large project, so be careful. Many linkers are smart enough to strip away all but one of the copies.
Another issue to consider is template complexity. Templates can be complex for those new to C++. Bugs in templates are very hard to find and may overwhelmthe patience of users.
- Exceptions Beware
Embedded applications are usually interrupt driven and multi-threaded.Test that exceptions are thread safe. Many compilers support exceptions,but not thread safe exceptions. And you probably don't want tocall code in an interrupt that throws exceptions.
- Use Polymorphic Interfaces to Make Frameworks
When you think through your design and come up with good abstractions you willbe shocked at how little code and how little time it takes to implementnew features.
- Make an OS Encapsulation Library
Don't use your embedded OSs features directly. Create a layer thatencapsulates OS functions and use those encapsulations. Most feature liketasks, interrupts, semaphores, message queues, messages, etc. are common to all systems. With good encapsulations it's quite possible to have the same code compile for Solaris, VxWorks, Windows, and other systems. It just takes a little thought.
- ROM Beware
A lot of systems create a ROM and download code later over the network that islinked against the ROM. Something to remember is linkers will try and includeonly code that is used. So your ROM may not contain code that loaded code expectsto be there. You need to include all functions in your ROM.
- Multiple Interface Levels
Most embedded systems have a command line interface which usually requires C linkage,then they may have an SNMP interface, and they may have some sort of otherfriendly interface. Design this up front to be common across all code. It willmake your life much easier. C functions require access to global pointers so theycan use objects. The singleton pattern makes this easier.
Come up with common naming conventions. A decent one is:
- Make up a module abbreviation that can be prefixed to all calls. For example: log for the logging module.
- Encode an action after the prefix. For example: logHelp which prints help for the logging module.
- Require a certain set of functions for each sub system: For example:
- moduleHelp - prints help for the module
- modulePrint - prints the current state of the module
- moduleStart - start a module
- moduleStop - stop a module
- moduleSetDebug - set the debug level for a module. It's very nice to set debug levels on a module by module basis.
- Debug and Error System First
Make your debug and error system first so everyone writing code will use it.It's very hard to retrofit code with debug output and intelligent use of errorcodes. If you have some way to write system assert errors to NVRAM, disk, orsome other form of persistent storage so you can recover it on the next reboot.
- Think About Memory
Think how you'll share memory buffers between ISR code and task level code.Think how fast your default memory allocator is, it is probably slow. Thinkif your processor supports purify! Think how you'll track memory corruptionand leakage.
- Think About System Integrity
You need to design up front how you are going to handle watchdog functionsand test that the system is still running and not corrupted.
- Remember to Use Volatile
When using memory mapped I/O make sure that you declare the input port variables as volatile, (some compilers do this automatically), since the value can change without notice, and the optimizer could eliminate what looks like a redundant access to that variable. Notusing volatile leads to some very obscure bugs. If you suspect problemsin this area take a look at the generated code to make sure read-onlyassumptions are being made.
Sometimes the keyword volatile is ifdef'd out for portabilityreasons. Check that what you think is volatile is really declared asvolatile.
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© Copyright 1995-2006. Todd Hoff. All rights reserved.
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