Disambiguating argument-less function calls in variadic class hierarchies

Question

I am trying to provide users of a class (MyGizmo below) that derives from a variadic hierarchy (ObjGetter below) with a simple, uncluttered way to unambiguously call a member function that takes no arguments (check() below). I can make this work with functions that take arguments (like 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 have tried the following but am not fully satistified:

First I can use a secondary, simply-templated class that gets carried around in the hierarchy, to reduce the ugly call to have just one template arg; yields something like:

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

I can use a Type2Type helper and give check() an argument, this works fine, looks like this:

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

I could use macros but I don't want to go there...

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

I think that template aliases will provide a solution but I am using g++ which does not support them yet. Is there anything else in the meantime? Thanks much!

Answer 1

You need a member function template check<Type> with some kind of structure to delegate up the inheritance chain if the type does not match the head of the variadic list.

This is a classic problem for 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

Works the same as my other answer. I'll leave that one as an example of baroque stupidity.