Question

I have two overloads of operator(), one that takes a function reference that takes any type as its parameters and returns any type. And another one which takes a function reference that takes any type as its parameter but returns void. Upon instantiation of my class I get the following errors:

In instantiation of 'A<void, int>':
error: 'void A<T, F>::operator()(void (&)(F)) [with T = void, F = int]' cannot be overloaded
error: with 'void A<T, F>::operator()(T (&)(F)) [with T = void, F = int]'

template <typename T, typename F> struct A {
    void operator()(T (&)(F)) {}
    void operator()(void (&)(F)) {}
};

void f(int) {}

int main() {

    A<void, int> a;
    a(f);
}

These errors only occur when the first template argument T is void. I would like to know what I’m doing wrong and why I can’t overload operator() this way?

Answer 1

Well, if T is void then you have two function definitions with the exact same prototype – breaking ODR.

Try specializing your struct to prevent this:

template <typename T, typename F> struct A {
    void operator()(T (&)(F)) {}
    void operator()(void (&)(F)) {}
};

template <typename F> struct A<void, F> {
    void operator()(void (&)(F)) {}
};