做{}而（0） - 宏安全性 [英] do{}while(0) - macro safety

问题描述

我们知道，多行的宏都在做的是围封，而（0）为他们循环安全地在code的任何地方包括在内。

We know that multiline macros have to be inclosed in a do while(0) loop for them to be safely included anywhere in the code.

例如，这是100％安全的：

For example this is 100% safe:

#define swap( a , b )       \
        do                  \
        {                   \
            int t = a ;     \
            a = b ;         \
            b = t ;         \
        }                   \
        while(0)

我想知道这是否是也可以放心与循环外code的一部分的宏，如果它不是，在什么可能发生的情况的问题。

I would like to know if it is also safe to have a macro with a part of the code outside of the loop, and if it isn't, in what circumstances could problems occur.

#define swap( a , b )       \
        int t ;             \   //should be in the same scope where the macro was called and before the loop
        do                  \
        {                   \
            t = a ;         \
            a = b ;         \
            b = t ;         \
        }                   \
        while(0)

有没有一种安全的方式来实现这一目标，将有什么改变或什么规则应该跟着我，如果它是不是安全。

Is there a safe way to achieve this, what would have to be changed or what rules should I follow if it isn't safe.

推荐答案

这不是在许多方面的安全。首先，如果任何变量称为T已经存在，它不会编译。

It is not safe in many ways. First, if any variable called t already existed, it wouldn't compile.

除此之外，你必须要小心这样的话：

Apart from that, you have to be careful with things like:

if (...)
    swap(a,b);

如将它扩展为：

if (...)
    int t;
    do { ... } while(0);

注意，因为如果没有括号的，如果块主体将只是一个声明。

Note that the if body would be only a declaration because there are no braces for the if block.

你为什么不声明T内的做阻止？

Why don't you declare t inside the do block?

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