C++: disambiguate this code at compile time?

Question

i tried to find a way to disambiguate this code (at compile time) (since two days :-) -> get_value is ambugiuous.

#include <iostream>

template <typename T>
struct type2type {};

template<class T, int val>
struct BASE
{
  static constexpr int get_value ( type2type< T > )
  {
    return val;
  }
};

class X {};
class Y {};

struct A :
  public BASE< X, 1 >,
  public BASE< Y, 0 >
{};

int main ( int argc, char **argv )
{
  A a {};
  std::cout << a.get_value ( type2type< X >{} ) << std::endl;
}

This is a working runtime solution.

#include <iostream>

template <typename T>
struct type2type {};

template<class T>
struct VIRTUAL
{
  int get_value () const
  {
    return get_value_from_BASE ( type2type< T > {} );
  }
private:
  virtual int get_value_from_BASE ( type2type< T > ) const = 0;
};

template<class T, int val>
class BASE :
  public VIRTUAL< T >
{
  virtual int get_value_from_BASE ( type2type< T > ) const override
  {
    return val;
  }
};

class X {};
class Y {};

struct A :
  public BASE< X, 1 >,
  public BASE< Y, 0 >
{};

int main ( int argc, char **argv )
{
  A a {};
  std::cout << a.::VIRTUAL< X >::get_value () << std::endl;
}

Is there a solution?

Note: a possible way that i found is over std::is_base_of<>, but this is very limited ( template instantiation depth )

Answer 1

This is an ambiguous name lookup, which in the case of multiple inheritance hides the names in the look-up. It doesn't even get to checking which overload to use.

You can fix this by adding the following to struct A's definition:

using BASE<X,1>::get_value;
using BASE<Y,0>::get_value;

These two statements add the name get_value from both base classes to A, and thus the compiler can then move on with its dreary life and check them as overloads.

Answer 2

Building on Atash's answer: Assuming you don't want to retype the list of base classes in the list of bases and in the using declarations, you can use an indirection like this:

#include <iostream>

template <typename T>
struct type2type {};

template<class T, int val>
struct BASE
{
  static constexpr int get_value ( type2type< T > const& )
  {
    return val;
  }
};

class X {};
class Y {};

template <typename...> struct AUX;

template <typename Base, typename... Bases>
struct AUX<Base, Bases...>: Base, AUX<Bases...> {
    using Base::get_value;
    using AUX<Bases...>::get_value;
};

template <typename Base>
struct AUX<Base>: Base {
    using Base::get_value;
};

struct A :
    public AUX<BASE< X, 1 >, BASE< Y, 0 > >
{
};

int main ()
{
  A a {};
  std::cout << a.get_value ( type2type< X >() ) << std::endl;
}