Disambiguating trouble-free function calls in variational class hierarchies

Question

I am trying to provide users with a class (MyGizmo below) that derives from the variational hierarchy (ObjGetter below) with a simple, uncluttered way to uniquely call a member function that takes no arguments (check () below). I can do this work with functions that take arguments (e.g. tune () below), but I have not found a way to make it work for functions that take no arguments.

struct Base { };
struct ObjA : public Base { };
struct ObjB : public Base { };
struct ObjC : public Base { };

template <class ... Obj> struct ObjGetter;

template <class Obj, class ... Tail>
struct ObjGetter<Obj, Tail ...> : public ObjGetter<Tail ...>
{
  using ObjGetter<Tail ...>::tune;  // resolve ambiguous lookups for tune()

  void tune(Obj * obj) { } // no problem with this one, disambiguated by obj type

  Obj * check() const { return 0; } // problem with this one, no arg to disambiguate
};

template <> struct ObjGetter<> { // to terminate the recursion
  void tune(void);  // needed by the using statement above but should not be used, hence different syntax
};

struct MyGizmo : public ObjGetter<ObjA, ObjC> // variadic
{
  void testit() {
    ObjA * a = 0; ObjB *b = 0; ObjC *c = 0;

    a = ObjGetter<ObjA, ObjC>::check(); // too ugly!
    c = ObjGetter<ObjC>::check(); // too ugly!

    tune(a); // no problem
    //tune(b); // correct compile-time error: no matching function for call to ‘MyGizmo::tune(ObjB*&)’
    tune(c); // no problem

    // I would like a simple syntax like this:
    //a = check<ObjA>(); // should call ObjGetter<ObjA, ObjC>::check()
    //b = check<ObjB>(); // should give a compile-time error
    //c = check<ObjC>(); // should call ObjGetter<ObjC>::check()
  }
};

I tried the following, but not completely guessed:

At first, I can use a secondary, just a template class that wraps around the hierarchy to reduce the ugly call, to have only one arg template; gives something like:

a = ObjGetterHelper<ObjA>::check(); // still ugly! MyGizmo user should not have to know about ObjGetterCore
c = ObjGetterHelper<ObjC>::check(); // too ugly!

Type2Type check() , , :

a = check(Type2Type<ObjA>()); // pretty ugly too
c = check(Type2Type<ObjC>()); // pretty ugly too

, ...

#define CHECK(X) check(Type2Type<X>())

, , g++, . - ? !

Answer 1

- check<Type> - , , .

SFINAE.

  template< class Obj2 >
  typename std::enable_if< std::is_same< Obj, Obj2 >::value, Obj * >::type
  check() const { return 0; } // perform work

  template< class Obj2 >
  typename std::enable_if< ! std::is_same< Obj, Obj2 >::value, Obj2 * >::type
  check() const { return base::template check< Obj2 >(); } // delegate

, . .