sfinae enabler

gistfile1.cpp

/*
 * third.cpp
 * Date: 2012-11-01
 * Author: Karsten Ahnert (karsten.ahnert@gmx.de)
 */

#include <iostream>
#include <boost/utility/enable_if.hpp>

#define tab "\t"

using namespace std;

template <int N>
struct A { };

template <int N>
struct B { };

template< class T > struct is_a_or_b {};

template<int N> struct is_a_or_b< A<N> > { const static bool value = true; };
template<int N> struct is_a_or_b< B<N> > { const static bool value = true; };

template< class Class , class Enabler = void > struct dispatcher;

template< class T >
struct dispatcher< T , typename boost::enable_if_c< is_a_or_b< T >::value > ::type >
{
    const static size_t value = 2;
};

template<int N>
struct dispatcher< A<N> >
{
    const static size_t value = 1;
};



int main( int argc , char *argv[] )
{
    cout << dispatcher< A<1> >::value << endl;
    cout << dispatcher< B<1> >::value << endl;
    return 0;
}

gistfile1.cpp:43:13: error: ambiguous partial specializations of 'dispatcher<A<1>, void>'
    cout << dispatcher< A<1> >::value << endl;
            ^
gistfile1.cpp:28:8: note: partial specialization matches [with T = A<1>]
struct dispatcher< T , typename boost::enable_if_c< is_a_or_b< T >::value > ::type >
       ^
gistfile1.cpp:34:8: note: partial specialization matches [with N = 1]
struct dispatcher< A<N> >
       ^
1 error generated.