"You can't forward declare classes that overload operator&"?

Question

In the Google C++ Style Guide, there's a section on Operator Overloading that has a curious statement:

Overloading also has surprising ramifications. For instance, you can't forward declare classes that overload operator&.

This seems incorrect, and I haven't been able to find any code that causes GCC to have a problem with it. Does anyone know what that statement is referring to?

Answer 1

5.3.1 of the Standard has "The address of an object of incomplete type can be taken, but if the complete type of that object is a class type that declares operator&() as a member function, then the behavior is undefined (and no diagnostic is required)."

I didn't know this either, but as another poster has pointed out, it's easy to see how it could cause a compiler to generate incorrect code.

Answer 2

I hadn't heard of it either, but this gives potentially confusing results for the same code before and after the overload:

#include <iostream>

class Foo;

void bar (Foo& foo) {
    std::cout << &foo << std::endl;
}

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

void baz (Foo& foo) {
    std::cout << &foo << std::endl;
}

int main () {
    Foo foo;

    bar(foo);
    baz(foo);

    return 0;
}

output:

0x7fff092c55df
1

Though there are other reasons why you wouldn't do that anyway - overloading address-of doesn't play nicely with stl or much generic code.