Type of pointer to an element from the base class

Question

I have a problem with item pointers. The following code cannot compile using both Oracle Solaris Studio 12.2 CC and cygwin GCC 4.3.4, but it works with Microsoft Visual C ++ 2010:

struct A {
  int x;
};

struct B : public A {
};

template<typename T> class Bar {
public:
  template<typename M> void foo(M T::*p);
};

int main(int, char *[]) {
    Bar<B> bbar;
    bbar.foo(&B::x);
    return 0;
}

In the closest to the last line, both compilers mentioned above cannot find a match for Bar<B>::foo(int A::*). I wrote a simple test to confirm that the type of expression &B::xis actually int A::*:

// ...

static void foo(int A::*p) {
  std::cout << "A" << std::endl;
}

static void foo(int B::*p) {
  std::cout << "B" << std::endl;
}

int main(int, char *[]) {
    foo(&B::x);  // prints "A", even on MS VC++ 2010 
    return 0;
}

The following workaround works with GCC (not yet tested with Oracle CC), but with a VC ++ error due to ambiguity:

template<typename T> class Bar {
public:
  template<typename M> void foo(M T::*p);
  template<typename M, typename _T_base> inline void foo(M _T_base::*p) {
      foo(static_cast<M T::*>(p));
  }
};

My question is: Which behavior is correct? VC ++ seems to be doing an implicit raise from int A::*to int B::*to satisfy the member function template call, should the other two compilers not do the same?

Answer 1

int A::* int B::*, . , , , <int> B::foo , , foo2, , B::foo .

struct A {
  int x;
};

struct B : public A {
};

template <typename T> class Bar {
public:
  template<typename M> void foo(M T::*p);
};

template<typename M> void foo2(M B::*p);

int main(int, char*[]) {
  Bar<B> bbar;
  bbar.foo<int>(&B::x);
  foo2(&B::x); // error, but foo2<int>(&B::x) would work.
  return 0;
}

, , <int>. 14.8.2.1p3:

, A A ( A, ). , :

• P , A ( , ) cv-, A.
• A , A (conv.qual).
• P - , P template-id, A A. , P template-id, A , A.

"P" - : M B::*p, M. "A" - : int A::*. P A, , , , , ( const/volatile , X* to const X* int X::* const int X::*).

, , <int>.