Why does clang's stdbool.h contain #define false false

Question

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 

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

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.

Answer 1

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++.

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 

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:

The header shall define the following macros: bool, true, false, __bool_true_false_are_defined.

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