#if defined(...)

最新推荐文章于 2025-07-10 19:35:38 发布

转载最新推荐文章于 2025-07-10 19:35:38 发布 · 517 阅读

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本文介绍了C预处理器中defined操作符的使用方法及其特点。defined操作符用于判断宏是否被定义，可以单独测试宏的存在与否，也可以与其他条件结合使用。文章还探讨了defined操作符在宏展开过程中的行为，并提供了具体的示例。

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.