Question

When I compile and run this with Visual C++ 2010:

#include <iostream>

int main() {
    int subtrahend = 5;

    struct Subtractor {
        int &subtrahend;
        int operator()(int minuend) { return minuend - subtrahend; }
    } subtractor5 = { subtrahend };

    std::cout << subtractor5(47);
}

I get the correct answer, 42.

Nevertheless, the compiler complains that this is impossible:

Temp.cpp(9) : warning C4510: main::Subtractor : default constructor could not be generated
Temp.cpp(6) : see declaration of main::Subtractor

Temp.cpp(9) : warning C4512: main::Subtractor : assignment operator could not be generated
Temp.cpp(6) : see declaration of main::Subtractor

Temp.cpp(9) : warning C4610: struct main::Subtractor can never be instantiated – user defined constructor required

What’s going on?

Answer 1

The first two warnings are just letting you know that the implicitly declared member functions cannot be generated due to the presence of a reference data member.

The third warning is a Visual C++ compiler bug.

All three warnings can be ignored with no ill effects, though you can easily make all three go away by making the reference data member a pointer instead (reference data members are almost never worth the trouble).