Logical operator execution order

Question

I have though about operator overloading and come with an interesting code:

#include <iostream>

class A {
public:
    operator bool() {
        return true;
    }

    bool operator!() {
        return false;
    }
};

int main() {
    A a;

    if (!a) {
        std::cout << "HELLO";
    } else {
        std::cout << "WORLD";
    }
    std::cout << std::endl;

    return 0;
}

What will be called first and what after? And why? Is this described anywhere in the cppreference?

PS For downvoters and others who think I could not execute this code by myself. I could. I did. I have changed it many times to see it's behavior. So what? This is not an explanation. I have asked for a reference which clearly states what rule this code obeys. The information how it works on my machine does not answer the question - what if this is not even portable across different environments (OSes, maybe processors, and so on)?

Answer 1

How it works is simple. The compiler parses the source and sees if(!a) . It then checks if A defines an operator! . Just so happens that it does. So that gets called.

If it had seen if(a) it would have checked that an A is convertible to something which can be used in the condition of an if . Just so happens that it is indeed convertible.

If there had been no operator! , the compiler would have instead checked that an A can be converted to something which may be logically negated. Then the conversion to bool would have been performed.

---

BTW, the conversion will happen even in surprising places. For instance a + 1 will compile. Not what we want, I think. It's best to allow it only where a bool is contextually expected. You do that by marking the conversion operator explicit:

explicit operator bool() {
    return true;
}

Answer 2

!a is no more than syntactic sugar for a.operator!() , which you have defined: that's the compiler's preferred choice.

So the conversion to bool operator is never a candidate.

You could contrive the latter by writing

if (!(bool)a) {