Static member function pointer as template argument - c++

Question

I get this compile error with the latest VC++ compiler (Nov 2012 CTP) when using static member function pointer as template argument:
error C2027: use of undefined type 'wrapper<int (int,int),int A::f1(int,int)>'
But when using free function, everything works ok.
I looked up some similar bugs in g++( pointer to static member function is "invalid" as a template argument for g++ ), but there it explicitly states that argument is invalid. What is so different about static functions?
I'm casting the function to void(*)(void) because construct like <typename T_Ret, typename... T_Args, T_Ret(*)(T_Args...)> don't compile for some other urelated reasons.
struct A
{
static int f1(int a, int b)
{
return a + b;
}
};
int f2(int a, int b)
{
return a + b;
}
template <typename Sig, void(*fnc)(void)>
struct wrapper;
template <void(*fnc)(void), typename T_Ret, typename... T_Args>
struct wrapper<T_Ret (T_Args...), fnc>
{
static bool apply()
{
// get some ints here
int a = 1;
int b = 2;
typedef T_Ret (fnc_ptr*)(T_Args...);
int res = ( (fnc_ptr)fnc )(a, b);
// do smth with result
res;
return true; // or false
}
};
int main()
{
bool res;
res = wrapper<decltype(A::f1), (void(*)(void))A::f1>::apply(); // error
res = wrapper<decltype(f2), (void(*)(void))f2>::apply(); // compiles ok
return 0;
}
EDIT:
Ok, I narrowed the issue to decltype.
When I write the type explicitly, everything works:
res = wrapper<int(int, int), (void(*)(void))A::f1>::apply(); // compiles ok

Answer 1

EDIT:
Looks like it's a compiler bug: http://channel9.msdn.com/Series/C9-Lectures-Stephan-T-Lavavej-Core-C-/STLCCSeries6#c634886322325940618
Workaround:
Change decltype(A::f1) to decltype(&A::f1) which changed its output from int(int, int) to int (__cdecl *)(int,int). And change
template <void(*fnc)(void), typename T_Ret, typename... T_Args>
struct wrapper<T_Ret (T_Args...), fnc>
to
template <void(*fnc)(void), typename T_Ret, typename... T_Args>
struct wrapper<T_Ret (*)(T_Args...), fnc>
Working code:
struct A
{
static int f1(int a, int b)
{
return a + b;
}
};
template <typename Sig, void(*fnc)(void)>
struct wrapper;
template <void(*fnc)(void), typename T_Ret, typename... T_Args>
struct wrapper<T_Ret (*)(T_Args...), fnc>
{
static bool apply()
{
// get some ints here
int a = 1;
int b = 2;
typedef T_Ret (*fnc_ptr)(T_Args...);
int res = ( (fnc_ptr)fnc )(a, b);
// do smth with result
res;
return true; // or false
}
};
int main()
{
bool res;
res = wrapper<decltype(&A::f1), (void(*)(void))A::f1>::apply();
return 0;
}

Answer 2

You could try something like this:
#include <iostream>
using namespace std;
struct A
{
static int f1(int a, int b)
{
return a + b;
}
};
int f2(int a, int b)
{
return a + b;
}
template <typename T, T X>
struct wrapper
{
template <typename... Args>
static bool value(Args... blargs)
{
return X(blargs...) == 3;
}
};
int main()
{
bool res;
res = wrapper<decltype(&A::f1), &A::f1>::value(1,2);
cout << res << endl;
return 0;
}
But seriously, this is so much easier:
#include <iostream>
using namespace std;
int main()
{
bool res;
res = A::f1(a, b) == 3;
cout << res << endl;
return 0;
}