[boost] Simulating partial ordering (was Re: Boost.Format final s ubmission)

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2002-02-23 10:47:14

David Abrahams wrote:
> > But you still end up with the following ambiguity on MSVC:
> >
> > template<typename T>
> > struct A {
> > };
> >
> > template<typename T>
> > void foo( T const& ) {}
> >
> > template<typename T>
> > void foo( A<T> const& ) {}
> >
> > int main(int, char*[])
> > {
> >     A<int> a_int;
> >
> >     foo( a_int );
> >
> >     return 0;
> > }
> >
> > error C2667: 'foo' : none of 2 overload have a best conversion
> >
> > Anyone knows a fix?
> 
> It depends whether 'A' is supposed to represent a 
> user-defined type or a template supplied by the library. If the 
> latter, you can intrude on foo and detect that T is an A<T> using 
> the sizeof() trick, then dispatch as appropriate. Otherwise, I'm 
> afraid there's no way.

    #include <iostream>

    template< typename T > struct A {};

    namespace aux {
    template< typename T >
    struct foo_impl
    {
        template< typename U >
        static void foo(A<U> const&, long)
        {
            std::cout << "foo(A<U> const&)\n";
        }

        static void foo(T const&, int)
        {
            std::cout << "foo(T const&)\n";
        }
    };
    }

    template< typename T >
    void foo(T const& t)
    {
        aux::foo_impl<T>::foo(t, 1L);
    }

    int main()
    {
        foo(int());
        foo(A<int>());
        return 0;
    }

That's more, I belive that the technique allows you to simulate partial
ordering of any number of overloaded function templates, not just 2 of them.

Aleksey