How do you forward declare the `bool` type in a C header file?

Question

Consider a header file, myheader.h with the following function header:

bool isValid(char c);

And whose implementation file, myimplementation.c , contains:

#include <stdbool.h>

bool isValid(char c) {
  return c == 'Y' || c == 'N';
}

If one would like to avoid include ing stdbool.h in the header file, how does one properly "forward declare" the bool datatype in the header file?

Answer 1

If you want "bool", you should include "stdbool.h":

https://en.cppreference.com/w/c/types/boolean

Boolean type support library

The C programming language, as of C99, supports Boolean arithmetic with the built-in type _Bool (see _Bool). When the header <stdbool.h> is included, the Boolean type is also accessible as bool.

Macros

Macro name Expands to ---------- ---------- bool _Bool true integer constant 1 false integer constant 0 __bool_true_false_are_defined integer constant 1

If including <stdbool.h> in every source file that uses "bool" really annoys you, Ted Lyngmo made a good suggestion: simply define your own header, and include stdbool.h there.

Answer 2

First, there is no need to declare bool ; you can simply declare the function as _Bool isValid(char c); .

Second, C 2018 7.18 2 says <stdbool.h> defines bool to be a macro that is replaced by _Bool . You can simply do this yourself with #define bool _Bool . 7.18 4 says that a program may undefine and redefine bool . To be pedantic and extra safe in conforming to that, you could use:

#undef bool
#define bool _Bool