将void作为参数替换为模板化方法

Question

In my code I have a class which registers method's of other classes:

#include <iostream>
using namespace std;

template< typename C>
  class Reg {
  public:
    template< typename R, typename A>
      void register_f( string name, R ( C:: *method_p ) ( A)) { /*registration process*/ }

//    template< typename R>
//      void register_void_f( string name, R ( C:: *method_p ) ( void)) { /*registration process*/ }
  };

  class A {
  public:
      int f( void) { return 1; }
      void g( int x) { /* some code*/ }
  };


  int main() {
      Reg< A> r;

      r.register_f( "g", &A::g);
/*1*///  r.register_f( "f", &A::f);
/*2*///  r.register_f< int, void>( "f", &A::f);
/*3*///  r.register_void_f< int>( "f", &A::f);

      return 0;
  }

http://ideone.com/X8PNLC

Uncommenting line /* 2 */ gives me an error:

template argument deduction/substitution failed:

In substitution of 'template void register_f(std::string, R (C::*)(A)) [with R = R; A = A; C = A] [with R = int; A = void]':

error: invalid parameter type 'void'

Line /* 1 / is the same as / 2 */, but with not so informative error message.

I understand that to fix the problem I can use method register_void_f , but I don't want to do it because register_f is a part of the my final API.

Question> How to fix the compilation errors without introducing register_void_f ?

I have an idea to solve it with partial specialized register_f , but I don't know how to do it since in C++ you can't partially specialize templated method.

PS> I can't use C++11.

Answer 1

Don't use void for no arguments, use nothing - something like this:

template< typename R>
     void register_void_f( string name, R ( C:: *method_p ) ()) { /*registration process*/ }

Answer 2

Overload your function:

void foo( int ) {}
double bar() { return 3.14; }

template< class R, class A >
void test(  R ( *method_p ) (A)) {  }
template< class R >
void test(  R ( *method_p ) ()) {  }

int main(){
  test(foo);
  test(bar);
}

live example

Converting this to it being methods should be easy.

Answer 3

You may use the following:

template< typename C> class Reg;

template <typename C, typename F> struct helper;

template <typename C, typename R, typename A>
struct helper<C, R (C::*)(A)>
{
    void operator() (Reg<C>& reg, const std::string& name, R (C::*method)(A)) const { /* Your implementation */}
};

template <typename C, typename R>
struct helper<C, R (C::*)()>
{
    void operator() (Reg<C>& reg, const std::string& name, R (C::*method)()) const { /* Your implementation */}
};


template< typename C>
  class Reg {
  public:
    template< typename F>
      void register_f(const std::string& name, F method) { helper<C, F>()(*this, name, method);  /*registration process*/ }

  };

And use it that way:

Reg< A> r;

r.register_f( "g", &A::g);
r.register_f( "f", &A::f);
r.register_f<int (A::*)(void)>( "f", &A::f);