Can we define boolean without a value? - c

Question

Can we define boolean without a value?

Answer 1

To use Boolean variables in C you need to #include <stdbool.h> in your headers.

Answer 2

You can very much define a boolean variable without a value, as evidenced by the following code (but note the second header include, see below for explanation):
#include <stdio.h>
#include <stdbool.h>
int main (void) {
bool u, t = true, f = false;
if (t) puts("t is true"); else puts("t is false");
if (f) puts("f is true"); else puts("f is false");
if (u) puts("u is true"); else puts("u is false");
return 0;
}
This outputs (for me):
t is true
f is false
u is false
However, you should be aware of the normal rules regarding initialisation of variables where you don't make it explicit. In the code given, u will hold some arbitrary value and, indeed, using a higher warning level of gcc makes that clear:
prog.c: In function ‘main’:
prog.c:8:8: warning: ‘u’ is used uninitialized in this function [-Wuninitialized]
8 | if (u) puts("u is true"); else puts("u is false");
| ^
Note that, if you want to use bool, true, and false, these are not keywords, they're instead defined in the stdbool header file so you need to include that.
Otherwise all you get is the _Bool type(1) and neither of the nice constants, so you'll have to define some yourself or use 0 and 1 (although any non-zero value converts to 1).
Since this is scarcely a step up for doing the same thing with int, you probably want to do it properly (i.e., use the header).
Note further that this is only available as of C99. The presence of an #include <conio.h> in your code may indicate that you're using a hideously outdated implementation of C (it was a common header for DOS-era compilers such as Turbo C).
If that is the case, it's likely you don't even have a stdbool header that you can use, so you'll have to stick with int, 0, and 1.
Alternatively, you may be able to create your own my_stdbool.h such as the following, assuming there's no naming conflict as previously mentioned (most code I've seen that defines the constants tends to use the upper-case FALSE and TRUE in keeping with commonly-used guidelines):
#ifndef XYZZY_STDBOOL_H_INCLUDED
#define XYZZY_STD_BOOL_H_INCLUDED
typedef bool int;
#define false 0
#define true 1
#endif
(1) Uglier, yes, but it ensures there's no clash between code that may have been written before this became part of the standard since names beginning with _X (where X is any upper-case letter or another _) are reserved for the implementation. Hence you shouldn't be using them.

Answer 3

In C, you can declare a boolean variable using the _Bool type. Following as an example. However, it is not a recommended method.
#include <stdio.h>
int main()
{
printf("Hello, World!\n");
_Bool k=0, l = 1, m = 324234;
printf("%d\n", k);
printf("%d\n", l);
printf("%d\n", m);
return 0;
}
You can further use a macro to define _Bool as bool.
#include <stdio.h>
typedef _Bool bool;
int main()
{
printf("Hello, World!\n");
bool k=0, l = 1, m = 324234;
printf("%d\n", k);
printf("%d\n", l);
printf("%d\n", m);
return 0;
}