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 08:30:31

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?

Cheers,
Cosimo Calabrese.