template function as template parameter, what is the standard?

Question

Suppose you have the following code

emplate<typename T,void (T::*m)(int)>
    struct B{

    void f(T* a,int x){
        (a->*m)(x);
    }
};


struct A{


   template<typename X> void f(int){
  }

   void wrap(int i){
     f<char>(i);
   }

   B<A,&A::f<char> > y;
};


int main(){
   A a;
}

This definition

B<A,&A::f<char> > y;
works with gcc,but not with Visual studio 2010:

error C2440: 'specialization' : cannot convert from 'overloaded-function' to 'void (__thiscall A::* )(int)'
On the contrary,

 B<A,&f<char> > y;
works with visual studio but not with gcc.

Note that B<A,&A::f<char> > y; placed in the main works for VS as well.

IS B<A,&f<char> > y; not standard? is there a way (apart wrapping the template function) to make the thing compile with both compilers?

Answer 1

On 7/18/2012 12:44 PM, Fabio DallaLibera wrote:

This definition

B<A,&A::f<char> > y;
works with gcc,but not with Visual studio 2010:

error C2440: 'specialization' : cannot convert from 'overloaded-function' to 'void (__thiscall A::* )(int)'
On the contrary,

  B<A,&f<char> > y;
works with visual studio but not with gcc.

GCC is right, VC is wrong.

IS B<A,&f<char> > y; not standard?

It is not. The only way to form a pointer-to-member is &ClassName::MemberName. The standard is very clear about it:

5.3.1p4 A pointer to member is only formed when an explicit & is used and its operand is a qualified-id not enclosed in parentheses. [ Note: that is, the expression &(qualified-id), where the qualified-id is enclosed in parentheses, does not form an expression of type “pointer to member.” Neither does qualified-id, because there is no implicit conversion from a qualified-id for a non-static member function to the type “pointer to member function” as there is from an lvalue of function type to the type “pointer to function” (4.3). Nor is &unqualified-id a pointer to member, even within the scope of the unqualified-id’s class. —end note ]

is there a way (apart wrapping the template function) to make the thing compile with both compilers?

Macros and #ifdef's, I suppose.

---

Igor Tandetnik

Answer 2

Thank you, I resorted to 

#ifdef _WIN32
#define vsFix(a,b) b
#else
#define vsFix(a,b) a::b
#endif

 B<A,&vsFix(A,f)<char> > y;

Answer 3

Fabio DallaLibera wrote:

Thank you, I resorted to

#ifdef _WIN32

_MSC_VER might be a better choice:

http://msdn.microsoft.com/en-us/library/b0084kay.aspx

---

Igor Tandetnik