铸造0到void [英] casting 0 to void

问题描述

可能重复：结果
  为什么（无效）0空操作C和C ++？

在我执行的C ++（Visual Studio 2008中实现）我看到下面的行＆LT;＆了cassert GT;

On my implementation of C++ (Visual Studio 2008 implementation) I see the following line in <cassert>

#ifdef  NDEBUG
#define assert(_Expression) ((void)0)

我不明白，需要投0作废。在我看来，那

I do not understand the need to cast 0 to void. It seems to me that

#ifdef  NDEBUG
#define assert(_Expression) (0)

甚至干脆

#ifdef  NDEBUG
#define assert(_Expression) 0

会做，考虑到环境中，断言（表达式）可以使用。

那么，什么是0类危险的内部而不是类型0的无效在这种情况下？任何 现实的的例子吗？

So, what's the danger of 0 of type int instead of 0 of type void in this case? Any realistic examples?

推荐答案

复杂的前pression的唯一目的（无效）0 是为了避免编译器警告。如果你只是有一个赤裸裸的，无用的前pression，编译器会发出警告说没有效果的前pression。但是，通过明确铸造的东西无效表示您的意思是要做到这一点。

The only purpose of the complicated expression (void)0 is to avoid compiler warnings. If you just had a naked, useless expression, the compiler might warn about an expression that has no effect. But by explicitly casting something to void you indicate that you mean to do this.

（想想这将是用户如何混乱，如果编译器突然说，警告：前pression 0; 没有任何效果。的时候，你所做的一切切换到发布模式。）

(Think about how confusing it would be to the user if the compiler suddenly said, "Warning: expression 0; has no effect.", when all you've done is switched to release mode.)

这也是在C，在那里你会说（空白）的printf常见的做法（你好）; 来告诉你有意选择忽略编译器该函数的返回值。

This was also common practice in C, where you'd say (void)printf("Hello"); to tell the compiler that you intentionally chose to ignore the return value of the function.

的（无效）投是不是由特定的实现仅仅是一种选择;它要求的C标准。引用了2011 ISO C标准（类似的措辞出现在1990年和1999年版）：

The (void) cast is not merely a choice by a particular implementation; it's required by the C standard. Quoting the 2011 ISO C standard (similar wording appears in the 1990 and 1999 editions):

如果 NDEBUG 是在源文件中的点定义为宏名
  其中， ＆LT; ASSERT.H＆GT; 包括，在 断言 宏被定义为简单

If NDEBUG is defined as a macro name at the point in the source file where <assert.h> is included, the assert macro is defined simply as

的#define断言（忽略）（（无效）0）

#define assert(ignore) ((void)0)

C ++标准要求的＆LT的内容;了cassert＆GT; 头是一样的标准C ＆LT; ASSERT.H＆GT; 头。

The C++ standard requires the contents of the <cassert> header to be the same as the Standard C <assert.h> header.

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