[c++] What is the printf format specifier for bool?

Since ANSI C99 there is _Bool or bool via stdbool.h. But is there also a printf format specifier for bool?

I mean something like in that pseudo code:

bool x = true;
printf("%B\n", x);

which would print:

true

There is no format specifier for bool. You can print it using some of the existing specifiers for printing integral types or do something more fancy:

 printf("%s", x?"true":"false");

ANSI C99/C11 don't include an extra printf conversion specifier for bool.

But the GNU C library provides an API for adding custom specifiers.

An example:

#include <stdio.h>
#include <printf.h>
#include <stdbool.h>

static int bool_arginfo(const struct printf_info *info, size_t n,
    int *argtypes, int *size)
{
  if (n) {
    argtypes[0] = PA_INT;
    *size = sizeof(bool);
  }
  return 1;
}
static int bool_printf(FILE *stream, const struct printf_info *info,
    const void *const *args)
{
  bool b =  *(const bool*)(args[0]);
  int r = fputs(b ? "true" : "false", stream);
  return r == EOF ? -1 : (b ? 4 : 5);
}
static int setup_bool_specifier()
{
  int r = register_printf_specifier('B', bool_printf, bool_arginfo);
  return r;
}
int main(int argc, char **argv)
{
  int r = setup_bool_specifier();
  if (r) return 1;
  bool b = argc > 1;
  r = printf("The result is: %B\n", b);
  printf("(written %d characters)\n", r);
  return 0;
}

Since it is a glibc extensions the GCC warns about that custom specifier:

$ gcc -Wall -g    main.c   -o main
main.c: In function ‘main’:
main.c:34:3: warning: unknown conversion type character ‘B’ in format [-Wformat=]
   r = printf("The result is: %B\n", b);
   ^
main.c:34:3: warning: too many arguments for format [-Wformat-extra-args]

Output:

$ ./main
The result is: false
(written 21 characters)
$ ./main 1
The result is: true
(written 20 characters)

I prefer an answer from Best way to print the result of a bool as 'false' or 'true' in c?, just like

printf("%s\n", "false\0true"+6*x);

• x == 0, "false\0true"+ 0" it means "false";
• x == 1, "false\0true"+ 6" it means "true";

You can't, but you can print 0 or 1

_Bool b = 1;
printf("%d\n", b);

source

In the tradition of itoa():

#define btoa(x) ((x)?"true":"false")

bool x = true;
printf("%s\n", btoa(x));

To just print 1 or 0 based on the boolean value I just used:

printf("%d\n", !!(42));

Especially useful with Flags:

#define MY_FLAG (1 << 4)
int flags = MY_FLAG;
printf("%d\n", !!(flags & MY_FLAG));

If you like C++ better than C, you can try this:

#include <ios>
#include <iostream>

bool b = IsSomethingTrue();
std::cout << std::boolalpha << b;