Explicit instantiation of function with auto->decltype approach

Question

I want to create template function that would be able to extract any property of struct A.

Here is Source.h

struct B
{
    int bbb;
};

struct C
{
    double ccc;
};

struct A
{
    B b;
    C c;
};

template <class R>
auto foo(A* str, R getter) -> decltype(str->*getter);

Now I want use explicit instantiation approach for foo

Here comes Source.cpp:

#include "Source.h"

template <class R>
auto foo(A* str, R getter) -> decltype(str->*getter)
{
    return str->*getter;
}

If we look at Main.cpp, we can see, that without explicit instantiation in the code block above we get link error:

//MAIN.cpp
#include "Source.h"

void main()
{
    A a;
    a.b.bbb = 7;
    auto z = foo(&a, &A::b);
}

Now my question is how to explicitly instantiate foo for &A::b and &A::c types. I have tried tons of variants but nothing works. I am in visual studio 2015.

P.S. Oh, and one more. Can we make foo with default argument for R = decltype(&A::b) ?

Answer 1

There you go:

template B &foo(A*, B A::*);
template C &foo(A*, C A::*);

As for the default argument, you need defaults for both the type and the value:

template <class R = B A::*>
auto foo(A* str, R getter = &A::b) -> decltype(str->*getter);