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: Cosimo Calabrese (cosimo.calabrese_at_[hidden])
Date: 2009-11-12 08:49:56

Cosimo Calabrese wrote:
> Andrew Sutton wrote:
>>> // EqualityComparable concept, requested by IncidenceGraph concept
>>> template <typename Graph1, typename Graph2>
>>> inline bool operator!=(
>>>  const typename boost::graph_traits<
>>>                           boost::union_graph<Graph1, Graph2>
>>>                 >::edge_descriptor& left,
>>>  const typename boost::graph_traits<
>>>                           boost::union_graph<Graph1, Graph2>
>>>                 >::edge_descriptor& right )
>>> {
>>>    return !( left == right );
>>> }
>>>
>>>
>>>  UG ug( g1, g2 );
>>>
>>>  graph_traits<UG>::edge_descriptor e1;
>>>  graph_traits<UG>::edge_descriptor e2;
>>>
>>>  tie( e1, existence ) = edge( v1, v2, g );
>>>  tie( e2, existence ) = edge( v1, v2, g );
>>>
>>>  e1 != e2;
>>> }
>>>
>>
>>  I looks like the inequality operator is provided in terms of edge
>> descriptors, but you're actually comparing variants. You may be able 
>> to get
>> away with writing something like:
>>
>> template <typename G1, typename G2>
>> bool operator!=(
>>     typename graph_traits<union_graph<G1, G2>>::edge_descriptor left,
>>     typename graph_traits<union_graph<G1, G2>>::edge_descriptor right)
>> { ... }
>>
>> That should generate an inequality operator over variants, but prevent 
>> the
>> template from becoming overly general, since its explicitly "specialized"
>> for your union_graph.
>>
>> Maybe?
>>
>> Andrew Sutton
> 
> I'm in agreement with you, but that code doesn't work. I don't know why, 
> I've tried it yesterday. This is my version. I've written it in the 
> global namespace, not in boost namespace:
> 
> template <typename G1, typename G2>
> bool operator!=( typename boost::graph_traits<boost::union_graph<G1, G2> 
>  >::edge_descriptor left,
>                  typename boost::graph_traits<boost::union_graph<G1, G2> 
>  >::edge_descriptor right )
> {
>     return !( left == right );
> }
> 
> Instead it works the follow, more general:
> 
> template <typename T0_, typename T1>
> inline bool operator!=( const boost::variant<T0_, T1>& left,
>                         const boost::variant<T0_, T1>& right )
> {
>     return !( left == right );
> }
> 
> I've written it in the same position of the first version, but it 
> provides the operator!=() for all the variant specialization...
> 
> What do you think about?
> 

I've tried to make esplicit the edge_descriptor definition, without 
using graph_traits class:

template <typename G1, typename G2>
bool operator!=(
boost::variant<
   boost::WR1<typename boost::graph_traits<G1>::edge_descriptor>,
   boost::WR2<typename boost::graph_traits<G2>::edge_descriptor> >& left,
boost::variant<
   boost::WR1<typename boost::graph_traits<G1>::edge_descriptor>,
   boost::WR2<typename boost::graph_traits<G2>::edge_descriptor> >& right )
{
     return !( left == right );
}

but doesn't work, the compile doesn't resolve it.

But this works:

template <typename T1, typename T2>
bool operator!=( boost::variant<boost::WR1<T1>, boost::WR2<T2> >& left,
                  boost::variant<boost::WR1<T1>, boost::WR2<T2> >& right )
{
     return !( left == right );
}

It is less general that the template <typename T0_, typename T1> 
version, but it involves the vertex_description too.

It's make me crazy...

Thanks,
Cosimo Calabrese.