Template function on struct members

Question

Is there a way to code a single template function able to run on different members of a given struct ?

A wrong example would look like :

struct Foo
{
  int a, b;
}

template <MEMBER x> //which does not exist 
cout_member(Foo foo)
{
  cout << foo.x << endl;
}

int main()
{
  Foo foo; 
  cout_member<a>(foo);
  cout_member<b>(foo);
  return 0;
}

I imagined an answer based on a switch, but I then wondered if this switch would be tested on run-time (what I would like to avoid) or on compile-time ?

Answer 1

As long as you want to pick up a data member from a set of data members having the same type, you can use a pointer to data member:

template <int Foo::*M>
void cout_member(Foo foo)
{
    std::cout << (foo.*M) << std::endl;
}

And use it as:

cout_member<&Foo::a>(foo);

If you want to indicate also the type, you can do this:

template <typename T, T Foo::*M>
void cout_member(Foo foo)
{
    std::cout << (foo.*M) << std::endl;
}

And use it as:

cout_member<int, &Foo::a>(foo);

---

Just out of curiosity, the second snippet would be even simpler in C++17:

template <auto M>
void cout_member(Foo foo)
{
    std::cout << (foo.*M) << std::endl;
}

See it up and running on wandbox;