CODEWITHSUNDEEP
clogical-operatorsreturn-type
Utkan Gezer
Utkan Gezer

Reputation: 3069

Is it safe to negate the result of an && (logical and) operation?

I vaguely know that the return type of a Boolean operator is int in C, but is it guaranteed to be the unflavored int, and not, say, unsigned int?

I am generally concerned if I am allowed to negate the result of a Boolean operation and expect it to be either 0 or -1, and not 2^32 - 1. See the following example:

int main(void) {
    int a = -(5 == 5);
    // Is "a" guaranteed to be -1 here?
}

Upvotes: 2

Views: 81

Answers (2)

Thomas Jager
Thomas Jager

Reputation: 5265

The result of a comparison and logical operators is the value 0 or 1 of type int. int is specified to hold at least the integer range [−32,767, +32,767].

-(5 == 5) is therefore always -1, and -(5 == 4) is 0,

Doing this is however confusing. The results of these operators is usually treated in a boolean-like way, and using the numerical value of the result might make it harder to understand the code.

Using the ternary operator may make the intent more clear:

(5 == num) ? -1 : 0

Using a number of compilers on the Compiler Explorer, I didn't see a difference in the generated code.

Upvotes: 3

pmg
pmg

Reputation: 108968

See C11 6.5.8 and C11 6.5.9.

Yes, the operator == yields a result of type int with value 0 or 1.

Upvotes: 1

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