#if defined(...)的含义

最新推荐文章于 2025-07-10 19:35:38 发布

文刀劉Michael

最新推荐文章于 2025-07-10 19:35:38 发布

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分类专栏： C/C++ 文章标签： behavior c

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本文深入探讨了C语言预处理器中#define操作符的使用方法，包括其作用、语法、注意事项以及与其他预处理器指令的交互。通过具体实例展示了如何利用#define进行宏定义，以简化代码并提高开发效率。

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#if defined(...)

The special operator defined is used in #if and #elif expressions to test whether a certain name is defined as a macro. defined name and defined (name) are both expressions whose value is 1 if name is defined as a macro at the current point in the program, and 0 otherwise. Thus, #if defined MACRO is precisely equivalent to #ifdef MACRO.

defined is useful when you wish to test more than one macro for existence at once. For example,

#if defined (__vax__) || defined (__ns16000__)

would succeed if either of the names __vax__ or __ns16000__ is defined as a macro.

Conditionals written like this:

#if defined BUFSIZE && BUFSIZE >= 1024

can generally be simplified to just #if BUFSIZE >= 1024, since if BUFSIZE is not defined, it will be interpreted as having the value zero.

If the defined operator appears as a result of a macro expansion, the C standard says the behavior is undefined. GNU cpp treats it as a genuine defined operator and evaluates it normally. It will warn wherever your code uses this feature if you use the command-line option -pedantic, since other compilers may handle it differently.