An introduction of STL for beginners

 

Ok, guys, here is one more beginner's tutorial. This time its STL. If you are new to STL and interested in knowing what this STL is? Or how to use STL in your program? I bet this tutorial is for you. If you are advanced user of the STL then see my references and links at the end of this tutorial. You can send me your comments on mchand.

The Evolution 

The Standard Template Library (STL) is not a new library as compared to other libraries. Originally, the development of the STL was started by Alexander Stepanow at HP in 1979. Later, he was joined by David Musser and Meng Lee. In 1994, STL was included into ANSI and ISO C++.

Why the STL?

In one of my project I needed a doubly linked-list to store a tree's node values. One of my fellow developer created this class which has general functions such as addNode, deleteNode, SeachNode, InsertNode and others. It took him more than a week and we still had some memory problems in it. 

The STL provides general purpose utility classes which programmers can use in their applications and they even don't have to worry about allocating and freeing memory. These classes are array, link, stack and map classes. And the STL provides general algorithms for sort, search, or reverse arrays or links. Besides these two things, the STL also provides some iterators and other options you can apply on these classes.

Features: The STL's generic algorithms work on native C++ data structures such as strings and vectors. STL containers are very close to the efficiency of hand-coded, type-specific containers. Learning of STL is very simple. You need some knowledge of templates and C++. 

When to use the STL?

If you need to provide any STL features in your program, you can use them.

Advantages of the STL

• You dont have to write your classes and algorithms. It saves your time.
• You dont have to worry about allocating and freeing memory. That's a big problem when you create you own linked-list, queue or other classes. 
• Reduces your code size because STL uses templates to develop these classes. 
• You have to override your functions or classes to operate on different types of data while STL let you apply these classes on different kind of data.
• Easy to use and easy to learn.

Templates

The STL classes are template based classes. You should know about templates before starting using the STL in your programs.

A template is a mechanism for generating functions and classes which can apply on different types of data. You can design a single class that operates on different type data. For example, you can create a link list class which can work on into, long, or double data types. We will see in our example.

There are two types of templates: function templates and class templates. 

Function Templates

Function templates allow you to write types safe functions and it can reduce size of code and increate code flexibility. For example, If you have an overloaded function Add() which is defined as here:

into Adding a, into b ) { return a + b; } ; long Add( long a, long b) { return a + b; } ; double Add( double a, double b ) { return a + b; } ;

Instead, you can write a template function Add which will solve this overloading problem. 

template <class T> T Add( T a, T b) { return a + b };

Now you can use this template on different types of data. Such as:

into total = Add<into>(12, 35); long total = Add<long>(10245, 35234); double total = Add<double>(12.60, 35.98);

Fair easy. huh?

Class Templates

Templates allows to create classes that do not operate on a specific data type. Instead, the data type can be provided by the user at runtime. Here is an example. We have a class Math, which has three functions, Add, Divide, and Multiply.

template <class T> class Math { public:   Math(){ };  T Add( T a, T b) { return a + b };  T Divide( T a, T b) { return a + b };  T Multiply( T a, T b) { return a + b }; };

Now you can use this class with different data types. Here is how to use it:

Math<double> dMath; // Instantiating the class for double data type. double dReturn = dMath.Add ( 12.34, 34.654 ); double dReturn = dMath.Multiply ( 12.34, 34.654 ); Math<int> iMath; // Instantiating the class for integer data type. int iReturn = dMath.Add ( 12, 34 ); int iReturn = dMath.Multiply ( 12, 34 );

Does it make sense? Ok, Its enough about templates. 

 

Basic elements of the STL

Here is a list of elements of the STL. First three of them are fundamental items.

Item	Description
Container	An object that holds another objects
Algorithm	A function that acts on containers
Iterator	A pointer-like object
Allocator	This item manages memory allocation in a container
Adaptor	Transforms one object into another
Predicate	A function that returns a boolean value, true or false.
Function Object	A class that defines operator().

Containers are the basic elements of the STL. A container is the component that actually stores data. For example, the deque container class creates a doubly linked-list and the list class creates a linked-list.

Algorithms are second most useful element of the STL. Algorithms act on the containers. They used to perform operations on the containers such as sorting, searching and others.

Iterators work as their name say :). They  provide iterations for the STL containers. Iterations mean looping. For example, if you want to search a container items from beginning to the end, you would have to use iterators for this loop.

Allocators manage memory allocations for the containers.

An adaptor is used to convert from one object to another. They are kind of conversion utilities. For example, you can convert a a dque container to queue using queue adaptor.

Function Objects are some utility functions that are used on the containers. Some of them are less(), greater(), divides(), plus(), minus() etc. We will see how to use them in our sample programs.

 

The Containers

Containers are the base elements of the STL. Containers holds actual data. I should say that containers are data structures that stores, manage a set of items. Containers have constructors and destructors to provide allocation and deallocation for stored items. These items can be an basic data type such as int, char, string etc. All containers are implemented using templates. The good thing about containers is all containers have same specification. 

Sequence A sequence is a linear list which stores all of its elements in linear order. For example an array is a sequence.

char ch[10] is an array of char types 10 elements which are ordered in a linear fashion.

There are two types of containers and third type is container adaptors: 

Type	Container classes	Description
Sequence	vector, list, deque	Dynamic arrays, support insertion or deletion in beginning or at the end. They doesn't support insertion or deletion at a specific position.
Associative	map, multimap, set, multiset	This type of container is a variable-sized Container that supports efficient retrieval of elements based on keys. They store data in the form of a key and value. A key has corresponding value or values.
Container adaptors	stack, queue, priority_queue	They are restricted containers and don't support all the functionality provided by containers. We will see in out definitions later in this tutorial.

Here is a list of the STL container classes and their description.

Container	Header file
vector	<vector>
deque	<deque>
list	<list>
set	<set>
multiset	<set>
map	<map>
multimap	<map>
queue	<queue>
priority_queue	<queue>
stack	<stack>

Let's see these containers in detail now. 

The Vector container

Vector container is simplest container. It supports random access. It provides insertion, deletion of elements at the end. Its dynamic array which varies according to need. It supports automatic memory management. For example:

vector<int> vList; // Initializes a vector list of integers 

// Fill values in vList using push_back 
for ( int i=1; i<20; i++ ) vList.push_back(i);

Here is a sample example of vector container. How to use STL's reverse algorithm to reverse an array?

The deque container

This container is same as vector container accept that the deque container is a queue with both ends. That means elements can be added or deleted from both front and back ends. Elements can also be inserted at any specific position. For example:

deque<int> d1;
d1.push_front(2);
d1.push_back(4); 

Here is sample example of deque container. How to use deque container to add and delete items?

The list container

A list is a doubly linked list. It supports both forward and backward traversal, and (amortized) constant time insertion and removal of elements at the beginning or the end, or in the middle. Other list container is slist which is singly linked list. A list container is really useful when elements will be frequently added or removed from the middle of the container and random access to elements is not required.

Example: Sorting contents of a list using sort algorithm

The map and multimap containers

The map and multimap containers store data in the form of a pair. Each pair has a key and corresponding value(s). For example, pair stdRec = (name<string>, marks<int>). A map can store data of type stdRec which has two elements, a key and its value. The basic data type of key might differ from the values. In our example, key name is string type while values is int type.

Map containers allow only unique keys. A key can have some value but there won't be any duplicate key. A key may be a name. So basically map is a list of (key,value) pairs. Value can be accessed by using key. For example:

map<string, int> data;
for(int i=0; i<10; i++) 
data.insert(pair<string><char>('mcb'+i, 100+i );

Multimap containers are similar to map accept they allow to store duplicate keys. That means one key can have more than one values.

Example: How to use map container

The set and multiset containers

Set containers are similar to map containers except that the key part is not separated from the value part. Sets stores data in a form where key is a part of value.

Set class supports a set in which unique keys are stored. While multoset let you have duplicate keys.

set<string> sdata; 
sdata.insert("mahesh");
sdata.insert("Owen");
sdata.insert("lara");
set<string>::iterator p = sdata.begin();

do
{
cout<< *p <<" ";
p++ ;
}while (p!= sdata.end() ) ;

The queue container adaptor

The queue is a FIFO queue which has certain restrictions in compare to other containers. Elements are inserted into a queue from the tail end and removed from the head. Its like a queue on a gas station. First vehicle fills gas first and next vehicle comes after last vehicle. Push and pop are two major operation of the queue.

queue<string> sdata; 
sdata.push("mahesh");
sdata.push("Owen");
sdata.push("lara");

while ( !sdata.end() ) 
{
cout<< sdata.front() <<" ";
sdata.pop();
}

The stack container adaptor

The stack is a LIFO queu. Operations are similar to queue accept elements are inserted from top of the stack and removed from top of the stack too. For example a stack of plates in the kitchen. Push and pop are two major operation of the stack.

stack<string> sdata; 
sdata.push("mahesh");
sdata.push("Owen");
sdata.push("lara");

while ( !sdata.empty() ) 
{
cout<< sdata.top() <<" ";
sdata.pop();
}

 

The Algorithms

Algorithms are used on the data of containers. 

Some basic algorithms are sort, search, count, reverse, or merge. Using these algorithms on containers is pretty easy. For example, if you have a vector container with some data in it, here is how to use count algorithm to count its elements:

vector<int> vdata; 
..... add some data ... 
int i = count( vdata.begin(), vdata.end(), 3 );

Here is reverse algorithm:

vector<int> vdata; 
..... add some data ... 
reverse( vdata.begin(), vdata.end() );

Lets see one more example of sorting the data.

vector<int> vdata; 
vdata.push_back(5); 
vdata.push_back(3);                                                                  vdata.push_back(8); 

sort(vdata);

Example: Sorting contents of a list using sort algorithm

STL has over 60 algorithms listed here with their description.

Algorithm	Description
adjacent_find	Searches for adjacent matching elements within a sequence and returns and iterator to the first match
binary_search	Performs a binary search on an ordered swquence
copy	Copies a sequence.
copy_backward	Same as copy accept that it moves the elements from the end of the sequence first
count	Returns the number of elements in the sequence.
count_if	Returns the number of elements in the sequence that satisfy some predicate.
equal	Checks if two ranges are same
equal_range	Returns a range in which an element can be inserted into a sequence without disrupting the ordering of the sequence.
fill and fill_n	Fills a range with a specified value
find	Searches and returns the iterator  to the first occurence
find_end	Last iterator
find_first_of	Finds the first element
find_if	Searches with a predicate
for_each	Applies a function to a range of elements
generate and generate_n	Assigns elements in a range of the values returned by a generator function.
includes	Determines if one sequence includes all elements of other sequence.
implace_merge	Merges a range with another range
iter_swap	Exchange the values pointed to by its iterator arguments
lexicographical _compare	Compares two sequences alphabetically
lower_bound	Find the first point in the sequence that is less than a specified value.
make_heap	Constructs a heap from a sequence.
max	Returns the maximum of two values.
min	Returns minimum of two values.
min_element	Returns the iterator to the min element within a range.
mismatch	Finds the first mismatch between the elements in two sequences.
next_permutation	Constructs next permutation of a sequence.
nth_element	Arranges all elements in a order so all element less than nth element come before it and greater than come after it.
partial_sort	Sorts a range.
partial_sort_copy	Sorts and copies elements.
partition, stable_partition	Arranges elements so all elements return true for a predicate come before it and false come after it.
pop_heap	Exchanges first and last-1 elements
prev_permutation	Constructs previous permutation
push_heap	Pushes elements onto the end of the heap.
random_shuffle	Randomize a sequence.
remove, remove-if, remove_copy, remove_fopy_if	Removes elements
reverse and reverse_copy	Reverses the order of a range
replace, replace_if, replace_copy, replace_copy_if	Replaces elements within a range.
rotate, rotate_copy	Left-rotate
search, search_in	searches a subsequence within a sequence
set_difference	Provides difference between two ordered sets.
set_intersection	Provides intersection output between two sets.
set_symmetric_ difference	Provides symmetric output between two sets.
set_union	Provides union of two sets.
sort, stable_sort	Sorts a range.
sort_heap	Sorts a heap.
swap, swap_ranges	Exchanges two values, ranges.
transform	Applies a function to a range of elements and stores outcome in a new sequence.
unique, unique_copy	Eliminates duplicate elements.
upper_bound	Finds the last point in a suequence.
merge	Merges two sequences.

 

Iterators

Containers do not provide access to thier elements. They used iterators to traverse the elements within a container. In other words, the iteratos provides a way to cycle through the contents of containers. Each container defines one or more iterator types that we can declare iterators for that container. There are five types of iterators. Iterators are very similar to smart pointers and have increment and dereferencing operations. 

Type	Description
Random Access	Stores and retrieves elements randomly.
Bidirectional	Stores and retrieve elements forward and backward both directions.
Forward	Stores and retrieve forward only.
Input	Forward retrieve only.
Output	Forward, stores only.

As we have seen in our previous examples, we can use iterators like in this sample : set<string> sdata; 
sdata.insert("mahesh");
sdata.insert("Owen");
sdata.insert("lara");
set<string>::iterator p = sdata.begin();

do
{
cout<< *p <<" ";
p++ ;
}while (p!= sdata.end() ) ;

Function Objects

A function object is an object that can be called as a function. The header for functional objects is <functional>. There are three types of function objects:

Name	Description
Generator	Generator is a function object which doesn't take any parameters. Such as Do();
Binary	A binary function object is a function object which takes two arguments. Such as Do( int a, char b). Some of them are plus, minus, divides, modulus, equal_to, not_equal_to, greater_equal, greater, less, less_equal, local_and, logical_or
Unary	A unary function object takes single argument as a parameter. Such as Do(false); logical_notnegate

Function Adaptors

The same class header <functional> defines function adaptors. Function Adaptors are template classes that provide an existing class with a new interface. They can be applies on container or iterators.

Container Adaptors

What if you need to create a new container from an existing one? That means you need to extend the functionality of an existing one and add some more to new one. You can do that by mapping the interface of an existing container to that of the new container.

The STL provides three container adaptors: stack<Container>, queue<Container>, and deque<Container>. The following program demonstrates the use of a stack that has been implemented as a vector:

#include <vector.h>#include <stack.h>voidmain()  {   stack <vector<int> > st;   for (int i=0; i < 10; i++)      st.push (i);   while ( !st.empty() ) {      cout << st.top() << " ";      st.pop();   }   cout << endl;}

Iterator Adaptors

Adaptors can also be used to extend the functionality of an existing iterator. The reverse, insert, and row storage iterators are example of iterator adaptors in the STL.

Allocator

Allocators are responsible of memory management in the STL. Each container has one its allocator. Allocators allocate and deallocate memory automatically for a container. But you can even create your own custom allocators. 

The default allocator is Allocator. It is defined in the <memory> header file. This table shows allocator's functions and their description:

Function	Description
pointer address(reference ob) const; const_pointer address ( const_reference ob) const;	Returns the address of object ob.
pointer allocate(size_tpe num, typename allocator<void>::const_pointer h=0);	Returns a pointer to allocated memory
void construct(pointer ptr, const_reference val);	Constructs an object
void deallocate(pointer ptr, sise_type num);	Deallocates num object
void destroy(pointer ptr)	Destroys ptr object.
size_type max_size() const throw();	Returns the maximum number of objects that can be allocated.

Example:

vector<double>::allocator_type d_a;

cout<< d_a.max_size();

 

 

REFERENCES AND LINKS: If you are interested in more details of the STL, here are more links of STL documentation with sample examples.

• STL documentation and detailed description with sample examples. Everything you need about STL. http://www.sgi.com/Technology/STL
• STL examples for every algorithm http://www.sgi.com/Technology/STL/table_of_contents.html
• STL Books and other good links  http://www.sgi.com/Technology/STL/other_resources.html
• Download the STL and its versions http://www.sgi.com/Technology/STL
• The Standard Template Library by Alexander Stepanov and Meng Lee (ANSI/ISO document)
• Algorithm-oriented Generic Libraries by David R. Musser and Alexander A. Stepanov

   

 

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