Is there pointer to member traits or something like this?

Question

Based on other my question.

Consider the following code

template<typename T, int N>
struct A {
  typedef T value_type; // save T to value_type
  static const int size = N; // save N to size
};

Look, I can use value_type and size as template parameter.

typedef A<int, 2> A1;
typedef A<A1::value_type, A1::size + 3> A2;  // OK,  A2 is A<int,5>

Now I want to do the same with pointer to member:

struct Foo {
    int m;
    int r;
};

template<int Foo::*Mem>
struct B {
   static int Foo::* const mp;
};

template<int Foo::*Mem>
int Foo::* const B<Mem>::mp = Mem; // Save pointer to member

But I get error.

typedef B<&Foo::m> B1;
typedef B<B1::mp>  B2;  // DOES NOT WORK

How to make last line to work? Or how to get similiary result?

Note. I know that it does not work. No links to C++ Standard is needed. I need workaround.

Answer 1

Are you trying to solve any real-world-problem or are you just stretching the c++ language to its limits?

Answer 2

Mike is right that it should compile. This is a bug in VS.

Answer 3

It shouldn't work according to C++ Standard 5.19/2:

Other expressions are considered constant-expressions only for the purpose of non-local static object initialization (3.6.2). Such constant expressions shall evaluate to one of the following:
— a null pointer value (4.10),
— a null member pointer value (4.11),
— an arithmetic constant expression,
— an address constant expression,
— a reference constant expression,
— an address constant expression for a complete object type, plus or minus an integral constant expression,
or
— a pointer to member constant expression.

It is not the answer to the original question, but it is the answer to this wrong statement.

Answer 4

Not totally an answer but do accessing Mem directly work?

ie: B1::Mem instead of B1::mp.

I'm pretty sure the standard do not allow it since we normally typedef template name when it's a type instead of accessing it directly, but technically it could maybe allow it (not sure what would be the implication). Where your solution is probably sure not to work since it require initialisation of a static member that is done at runtime (correct me if I'm wrong), so it cannot be accessed at compile time like you want to do.

Maybe you could try giving us the bigger picture of what you're trying to do with your trait/policy class to see if a suitable workaround is possible.