Re: [Boost-users] [BGL] graph constness and vertex/edge iterators

From: Douglas Gregor (gregod_at_[hidden])
Date: 2004-05-11 11:39:07

On Tuesday 11 May 2004 12:27 pm, Cyrille Damez wrote:
> So if I understood correctly, I was misunderstanding what a descriptor is
> supposed to be. In my particular case, the graph structure is actually a
> triangle mesh: the graph vertices are the mesh triangles (resp. the graph
> edges are mesh edges) and have non-const access to their edges (resp. the
> triangles that contain them).

Interesting.

> Therefore, returning non-const iterators that could in turn return
> descriptors was conflicting with the constness of the graph.

This is correct.

> So if I'm not mistaken, a descriptor should be something like a container
> for whatever additional information I am supposed to carry with my graph
> (here geometric data like 3D positions, normals, etc.), and shouldn't have
> the right to modify anything related to the graph topology. Therefore I
> should make all topological informations const in the vertex_descriptor and
> edge_descriptor classes and only enable changes to the topology through the
> mesh/graph object.

Descriptors are more like a key or index into a container that represents 
those attributes... So if all your vertices are stored into a vector, the 
descriptor could be the index into that vector. If your vertices were stored 
in an STL map, the descriptor could be the key into that map. In both cases, 
it's the constness of the container (vector or map) that matters, not the 
constness of the descriptor (key or index).

        Doug