Re: [boost] [Variant] [BGL] Variant<edge_descriptor, edge_descriptor> is not EqualityComparable

Subject: Re: [boost] [Variant] [BGL] Variant<edge_descriptor, edge_descriptor> is not EqualityComparable
From: Stefan Strasser (strasser_at_[hidden])
Date: 2009-11-12 10:19:57

Am Thursday 12 November 2009 15:00:09 schrieb Cosimo Calabrese:

> namespace std
> {
>   template <typename Graph1, typename Graph2>
>   std::ostream& operator<<(std::ostream& os,
>   const typename boost::graph_traits<boost::union_graph<Graph1, Graph2>
>
>                  >::vertex_descriptor& v)
>
>   {
>    if ( boost::get<WR1<graph_traits<Graph1>::vertex_descriptor> >( &v ) )
>     os <<
>       boost::get<WR1<graph_traits<Graph1>::vertex_descriptor>
>
>  >(v)._data;
>
>    else
>     os <<
>       boost::get<WR2<graph_traits<Graph2>::vertex_descriptor> >(v)._data;
>    return os;
>   }
> }

the template parameters (Graph1/2) must be deducable from the function 
parameter (v).
this is not possible in this example.

consider the following, simpler, case:
template<class T>
struct A{
  typedef T type;
};

template<class A_T>
void func(typename A_T::type);

int main(){
  int a;
  func<A<int> >(); //ok
  func(a); //error
}

the compiler has no way of knowing that foo is supposed to be instantiated 
with A_T == A, unless you provide an explicit template argument (which you 
don't when you call an operator).