calling sizeof on a function call skips actually calling the function!}

Question

I happened to stumble across this piece of code.

int x(int a){
    std::cout<<a<<std::endl;
    return a + 1;
}

int main()
{
    std::cout<<sizeof(x(20))<<std::endl;
    return 0;
}

I expected it to print 20 followed 4. But it just prints 4. Why does it happen so? Isn't it incorrect to optimize out a function, that has a side effect (printing to IO/file etc)?

Answer 1

sizeof is a compile-time operator, and the operand is never evaluated.

Answer 2

sizeof is actually an operator and it is evaluated in compile-time.

The compiler can evaluate it because the size of the return type of x is fixed; it cannot change during program execution.