VS2010 SFINAE and function overload error - visual-c++

Question

I have code like this, that compiles fines in all compiler I've tested except VS2010.
I'm trying not to use C++11 specific features here, so it can still compile on outdated compiler like gcc 4.1.
#include <iostream>
using namespace std;
// Simplified variant class
struct Var
{
template <class T>
Var(T t) {}
Var(void) {}
};
// Simplified argument array class
struct FuncArgs
{
};
/** Make a wrapper around the given function */
template <int line, typename Ret, Ret Func()>
Var WrapFuncT(const FuncArgs & args)
{
Var ret;
ret = Func(); return ret;
}
/** Make a wrapper around the given function */
template <int line, void Func()>
Var WrapFuncT(const FuncArgs & args)
{
Var ret;
Func(); return ret;
}
// Unary
template <int line, typename Ret, typename Arg1, Ret Func(Arg1)>
Var WrapFuncT(const FuncArgs & args)
{
Var ret; Arg1 arg;
ret = Func(arg);
return ret;
}
template <int line, typename Arg1, void Func(Arg1)>
Var WrapFuncT(const FuncArgs & args)
{
Var ret; Arg1 arg;
Func(arg);
return ret;
}
// Binary
template <int line, typename Ret, typename Arg1, typename Arg2, Ret Func(Arg1, Arg2)>
Var WrapFuncT(const FuncArgs & args)
{
Var ret; Arg1 arg1; Arg2 arg2;
ret = Func(arg1, arg2);
return ret;
}
template <int line, typename Arg1, typename Arg2, void Func(Arg1, Arg2)>
Var WrapFuncT(const FuncArgs & args)
{
Var ret; Arg1 arg1; Arg2 arg2;
Func(arg1, arg2);
return ret;
}
#define WrapFunc(X, Y, ...) &WrapFuncT<__LINE__, X, Y, ## __VA_ARGS__ >
int testFunc()
{
return 42;
}
void testFunc2(int value)
{
cout<<value<<endl;
}
typedef Var (*NamedFunc)(const FuncArgs &);
int main()
{
NamedFunc a, b;
a = WrapFunc(int, testFunc);
b = WrapFunc(int, testFunc2);
}
Visual studio 2010 compiler chokes on this with error:
In line 'a = WrapFunc(int, testFunc);' : error C2440: 'specialization' : cannot convert from 'int (__cdecl *)(void)' to 'void (__cdecl *const )(int)'
This conversion requires a reinterpret_cast, a C-style cast or function-style cast
error C2973: 'Type::WrapFuncT' : invalid template argument 'int (__cdecl *)(void)'
In line 'template <int line, typename Arg1, void Func(Arg1)>' : see declaration of 'Type::WrapFuncT'
Seems like VS2010 does not find the former definition template < int line, typename Ret, Ret Func(void) > with Ret = int for int testFunc(void) function, and instead tries and errors on the template < int line, typename Arg1, void Func(Arg1) >.
If I comment the later, then it compiles fine, so it is able to find the former overload.
I've tried to solve this in numerous way, none worked, as I need to "capture" a pointer to function in a same signature function Var (*) (const FuncArgs &)

Answer 1

You may try a generic template function and using specialization with structure, something like:
namespace detail
{
// class to specialize for each function type
template <int line, typename F, F f> struct helper_wrapper;
// partial specialization
template <int line, typename Ret, Ret (&Func)()>
struct helper_wrapper<line, Ret (&)(void), Func>
{
Var operator()(const FuncArgs&) const
{
Var ret;
ret = Func();
return ret;
}
};
// partial specialization
template <int line, void (&Func)()>
struct helper_wrapper<line, void (&)(), Func>
{
Var operator()(const FuncArgs&) const
{
Var ret;
Func();
return ret;
}
};
// partial specialization
template <int line, typename Ret, typename Arg1, Ret (&Func)(Arg1)>
struct helper_wrapper<line, Ret (&)(Arg1), Func>
{
Var operator()(const FuncArgs&) const
{
Var ret;
Arg1 arg;
ret = Func(arg);
return ret;
}
};
// partial specialization
template <int line, typename Arg1, void (&Func)(Arg1)>
struct helper_wrapper<line, void (&)(Arg1), Func>
{
Var operator()(const FuncArgs&) const
{
Var ret;
Arg1 arg;
Func(arg);
return ret;
}
};
// other partial specialization omitted.
}
// The general function
template <int line, typename F, F f>
Var WrapFuncT(const FuncArgs& arg) { return detail::helper_wrapper<line, F, f>()(arg); }
// The helper macro
#define WrapFunc(X, Y) &WrapFuncT<__LINE__, X, Y>
And then call it that way:
a = WrapFunc(int(&)(), testFunc);
b = WrapFunc(void(&)(int), testFunc2);