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).