do...while(0)的妙用

来源：互联网发布：叶根友特楷简体 mac版编辑：程序博客网时间：2024/06/04 17:44

在C++中，有三种类型的循环语句：for, while, 和do...while，但是在一般应用中作循环时，我们可能用for和while要多一些，do...while相对不受重视。
但是，最近在读我们项目的代码时，却发现了do...while的一些十分聪明的用法，不是用来做循环，而是用作其他来提高代码的健壮性。

1. do...while(0)消除goto语句。
通常，如果在一个函数中开始要分配一些资源，然后在中途执行过程中如果遇到错误则退出函数，当然，退出前先释放资源，我们的代码可能是这样：
version 1

bool Execute()
{
   // 分配资源
   int *p = new int;
   bool bOk(true);

   // 执行并进行错误处理
   bOk = func1();
   if(!bOk)
   {
      delete p;
      p = NULL;
      return false;
   }

   bOk = func2();
   if(!bOk)
   {
      delete p;
      p = NULL;
      return false;
   }

   bOk = func3();
   if(!bOk)
   {
      delete p;
      p = NULL;
      return false;
   }

   // ..........

   // 执行成功，释放资源并返回
    delete p;
    p = NULL;
    return true;

}

这里一个最大的问题就是代码的冗余，而且我每增加一个操作，就需要做相应的错误处理，非常不灵活。于是我们想到了goto:
version 2

bool Execute()
{
   // 分配资源
   int *p = new int;
   bool bOk(true);

   // 执行并进行错误处理
   bOk = func1();
   if(!bOk) goto errorhandle;

   bOk = func2();
   if(!bOk) goto errorhandle;

   bOk = func3();
   if(!bOk) goto errorhandle;

   // ..........

   // 执行成功，释放资源并返回
    delete p;
    p = NULL;
    return true;

errorhandle:
    delete p;
    p = NULL;
    return false;

}

代码冗余是消除了，但是我们引入了C++中身份比较微妙的goto语句，虽然正确的使用goto可以大大提高程序的灵活性与简洁性，但太灵活的东西往往是很危险的，它会让我们的程序捉摸不定，那么怎么才能避免使用goto语句，又能消除代码冗余呢，请看do...while(0)循环：
version3

bool Execute()
{
   // 分配资源
   int *p = new int;

   bool bOk(true);
   do
   {
      // 执行并进行错误处理
      bOk = func1();
      if(!bOk) break;

      bOk = func2();
      if(!bOk) break;

      bOk = func3();
      if(!bOk) break;

      // ..........

   }while(0);

    // 释放资源
    delete p;
    p = NULL;
    return bOk;

}

“漂亮！”，看代码就行了，啥都不用说了...

2 宏定义中的do...while(0)
如果你是C++程序员，我有理由相信你用过，或者接触过，至少听说过MFC, 在MFC的afx.h文件里面，你会发现很多宏定义都是用了do...while(0)或do...while(false)，比如说：
#define AFXASSUME(cond) do { bool __afx_condVal=!!(cond); ASSERT(__afx_condVal); __analysis_assume(__afx_condVal); } while(0)
粗看我们就会觉得很奇怪，既然循环里面只执行了一次，我要这个看似多余的do...while(0)有什么意义呢？
当然有！
为了看起来更清晰，这里用一个简单点的宏来演示：
#define SAFE_DELETE(p) do{ delete p; p = NULL} while(0)
假设这里去掉do...while(0),
#define SAFE_DELETE(p) delete p; p = NULL;
那么以下代码：
if(NULL != p) SAFE_DELETE(p)
else ...do sth...
就有两个问题，
1) 因为if分支后有两个语句，else分支没有对应的if，编译失败
2) 假设没有else, SAFE_DELETE中的第二个语句无论if测试是否通过，会永远执行。
你可能发现，为了避免这两个问题，我不一定要用这个令人费解的do...while, 我直接用{}括起来就可以了
#define SAFE_DELETE(p) { delete p; p = NULL;}
的确，这样的话上面的问题是不存在了，

但是我想对于C++程序员来讲，在每个语句后面加分号是一种约定俗成的习惯，这样的话，以下代码:
if(NULL != p) SAFE_DELETE(p);
else ...do sth...
其else分支就无法通过编译了（原因同上），所以采用do...while(0)是最好的选择了。

也许你会说，我们代码的习惯是在每个判断后面加上{}, 就不会有这种问题了，也就不需要do...while了，如：
if(...)
{
}
else
{
}
诚然，这是一个好的，应该提倡的编程习惯，但一般这样的宏都是作为library的一部分出现的，而对于一个library的作者，他所要做的就是让其库具有通用性，强壮性，因此他不能有任何对库的使用者的假设，如其编码规范，技术水平等。

此处，do while（0）可在在函数中模拟goto语句的效果，从而实现return 所达不到的效果。

Linux FAQ/DoWhile0

Why do a lot of #defines in the kernel use do { ... } while(0)?

There are a couple of reasons:

(from Dave Miller) Empty statements give a warning from the compiler so this is why you see #define FOO do { } while(0).
(from Dave Miller) It gives you a basic block in which to declare local variables.
(from Ben Collins) It allows you to use more complex macros in conditional code. Imagine a macro of several lines of code like:
```
#define FOO(x) \        printf("arg is %s\n", x); \        do_something_useful(x);
```
Now imagine using it like:
```
if (blah == 2)        FOO(blah);
```
This interprets to:
```
if (blah == 2)        printf("arg is %s\n", blah);        do_something_useful(blah);;
```
As you can see, the if then only encompasses the printf(), and the do_something_useful() call is unconditional (not within the scope of the if), like you wanted it. So, by using a block likedo { ... } while(0), you would get this:
```
if (blah == 2)        do {                printf("arg is %s\n", blah);                do_something_useful(blah);        } while (0);
```
Which is exactly what you want.
(from Per Persson) As both Miller and Collins point out, you want a block statement so you can have several lines of code and declare local variables. But then the natural thing would be to just use for example:
```
#define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }
```
However that wouldn't work in some cases. The following code is meant to be an if-statement with two branches:
```
if (x > y)        exch(x,y);          // Branch 1else          do_something();     // Branch 2
```
But it would be interpreted as an if-statement with only one branch:
```
if (x > y) {                // Single-branch if-statement!!!        int tmp;            // The one and only branch consists        tmp = x;            // of the block.        x = y;        y = tmp;};                           // empty statementelse                        // ERROR!!! "parse error before else"        do_something();
```
The problem is the semi-colon (;) coming directly after the block. The solution for this is to sandwich the block between do and while (0). Then we have a single statement with the capabilities of a block, but not considered as being a block statement by the compiler. Our if-statement now becomes:
```
if (x > y)        do {                int tmp;                tmp = x;                x = y;                y = tmp;        } while(0);else        do_something();
```
(from Bart Trojanowski) gcc adds Statement-Expressions which provide an alternative to the do-while-0 block. They provide the above mentioned benefits and are slightly more legible.
```
#define FOO(arg) ({         \           typeof(arg) lcl; \           lcl = bar(arg);  \           lcl;             \    }) 
```