为什么c的stdbool.h包含#define false false [英] Why does clang's stdbool.h contain #define false false

问题描述

在被编译器错误指出后，我注意到clang的 stdbool.h 文件包括(其中包括)以下几行:

After being pointed there by a compiler error, I noticed clang's stdbool.h file includes (among other things) the following lines:

#define bool  bool
#define false false
#define true  true

它们包含在#ifdef块中，该块间接地强制执行__cplusplus，因此即使stdbool.h是C标头，也可以使用c ++标记.

They're contained in an #ifdef block that enforces __cplusplus indirectly, hence the c++ tag even though stdbool.h is a C header.

这些定义有什么需要?我想出于某些与预处理器相关的原因而需要它们，但我想知道标准的哪一部分或哪个技术原因使之成为标准，因此clang必须包括这些内容.

What's the need for those defines? I imagine they're required for some preprocessor-related reason but I'd be interested to know what part of the standard or which technical reason makes it so clang has to include those.

推荐答案

stdbool.h是C头文件，而不是C ++头文件.通常在C ++程序中找不到它，因为true和false在C ++中已经是关键字.

stdbool.h is a C header, not a C++ header. It is not usually found in C++ programs because true and false are already keywords in C++.

因此，如果C ++程序包含stdbool.h，则很明显地表明它是移植的C程序(例如，被编译为C ++的C程序).在这种情况下，根据中的注释，G ++在C ++模式下支持stdbool.h作为GNU扩展. GCC stdbool.h :

Consequently, if a C++ program includes stdbool.h it is a fairly clear indication that it is a ported-over C program (e.g. a C program that is being compiled as C++). In this case, G++ supports stdbool.h in C++ mode as a GNU extension, per the comments from the GCC stdbool.h:

/* Supporting <stdbool.h> in C++ is a GCC extension.  */
#define _Bool        bool
#define bool        bool
#define false        false
#define true        true

...

/* Signal that all the definitions are present.  */
#define __bool_true_false_are_defined        1

同样，

Clang在C ++中支持stdbool.h以与G ++兼容. 在此故意定义这些值以匹配内置的C ++类型，而不是传统的C99定义.它们被定义为宏，以提供与 C99标准的某些兼容性，该标准要求:

Clang, likewise, supports stdbool.h in C++ for compatibility with G++. The values are intentionally defined here to match the built-in C++ type rather than the traditional C99 definitions. They are defined as macros presumably to provide some compatibility with the C99 standard, which requires:

标头应定义以下宏:bool，true，false，__bool_true_false_are_defined.

应用程序可能会取消定义，然后可能会重新定义宏bool，true和false.

An application may undefine and then possibly redefine the macros bool, true, and false.

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