Different outputs of sizeof operator in C and C++

Question

Different outputs of sizeof() operator in C and C++.

In C:

int main() 
{
    printf("%zu\n", sizeof(1 == 1));
    return 0;
}

output:

4

In C++:

int main() 
{
    std::cout << sizeof(1 == 1) << std::endl;
    return 0;
}

output:

1

Questions:

• Why are the outputs different?
• Is sizeof independent of the OS or compiler?
• Is it dependent on language?

Answer 1

According to N1570 draft (c11):

6.5.9 Equality operators

The == (equal to) and != (not equal to) operators are analogous to the relational operators except for their lower precedence. Each of the operators yields 1 if the specified relation is true and 0 if it is false. The result has type int.

Therefore, the sizeof(1 == 1) will return equal value to sizeof(int) which is implementation defined and in your case it is 4.

---

According to N4296 draft (c++14):

5.10 Equality operators

The == (equal to) and the != (not equal to) operators group left-to-right. The operands shall have arithmetic, enumeration, pointer, or pointer to member type, or type std::nullptr_t. The operators == and != both yield true or false, i.e., a result of type bool.

Therefore, the sizeof(1 == 1) will return equal value to sizeof(bool) which is implementation defined and in your case it is 1.