CODEWITHSUNDEEP
cboolean
Willis Hershey
Willis Hershey

Reputation: 1574

Addition with _Bool type in C

In a piece of code I'm working on, I need to add 1 to an unsigned long if some _Bool is true.

I've been using

unsigned long l;
_Bool b;
... //stuff happens to both variables
if(b)
    ++l;

But I read in the C standard that the _Bool type is classified as one of the "standard unsigned integer types" and it is guaranteed to express true as 1 and false as 0.

I replaced my code with this seemingly equivalent expression

unsigned long l;
_Bool b;
... //stuff happens to both variables
l+=b;

My compiler didn't complain and it seems to be working beautifully.

Is this guaranteed to work, or am I just exploiting the implementation of _Bool in my compiler?

Upvotes: 1

Views: 615

Answers (2)

Keith Thompson
Keith Thompson

Reputation: 263357

It's almost guaranteed to work.

The standard doesn't quite say that a _Bool object can only hold the values 0 and 1. And in fact sizeof(_Bool) is at least 1, which means it has at least 8 bits, which means that it can in principle hold at least 256 distinct values.

If your code that assigned a value to b is reasonably sane, you can safely assume its value is either 0 or 1. All conversions from other types to _Bool are guaranteed to yield a value of 0 or 1.

But you can store a value other than 0 or 1 in a _Bool object, though any method of doing so is likely to have undefined behavior.

An example:

#include <stdio.h>
#include <string.h>
int main(void) {
    _Bool b;
    char c = 42;
    memcpy(&b, &c, 1);
    long n = 0;
    n += b;
    printf("n = %ld\n", n);
}

The output on my system is

n = 42

But as long as you don't do silly things like this, and treat _Bool with some minimal respect, you should be ok.

Upvotes: 3

user14107922
user14107922

Reputation:

Is this guaranteed to work

Yes, booleans in C are just integers, so adding a boolean to an integer is perfectly acceptable under any compiler I know of. C is weakly typed so there is no distinct difference between boolean and integer values. The only significance of a variable declared as _Bool is that it automatically converts any value assigned to it to either 1 or 0.

_Bool x = 3; //x will be 1
int y = 7;
int z = y + x; //z will be 8

Upvotes: 1

Related Questions