Re: [Boost-users] Boost Graph: why can't get() take a const argument?

Subject: Re: [Boost-users] Boost Graph: why can't get() take a const argument?
From: Geoff Hilton (geoff.hilton_at_[hidden])
Date: 2010-05-04 10:38:50

On 03/05/2010 7:33 PM, Trevor Harmon wrote:
> Hi,
>
> I have a question about line 54 of the canonical_ordering example:
>
> graph g(6);
> ...
> property_map<graph, edge_index_t>::type e_index = get(edge_index, g);
>
> If I'm reading the Boost source code correctly, the get function's graph
> argument is declared const. This makes sense intuitively because, well,
> why would the graph need to be modified in order to retrieve a property?
> So the graph parameter passed to get() could be made const, correct?
> Like this:
>
> graph g(6);
> ...
> const graph g2 = g;
> property_map<graph, edge_index_t>::type e_index = get(edge_index, g2);
>
> However, the above change gives a compiler error:
>
> conversion from
> ‘boost::adj_list_edge_property_map<boost::undirected_tag, int, const
> int&, size_t, const boost::property<boost::edge_index_t, int,
> boost::no_property>, boost::edge_index_t>’ to non-scalar type
> ‘boost::adj_list_edge_property_map<boost::undirected_tag, int, int&,
> size_t, boost::property<boost::edge_index_t, int, boost::no_property>,
> boost::edge_index_t>’ requested
>
> Can anyone explain why? Thank you,
>
> Trevor

I've run into this myself (and asked about it in this list somewhere as 
well), the short answer is that it's easier to simply pass in a 
non-const graph. I can't remember the long answer, but I believe it has 
to do with the implementation of property_map<..>::type and const_type 
and the property (eg. edge_index_t) template parameter. Note though that 
I could (and may well) be wrong.

Geoff