Re: [Boost-users] Newbie: What is the actual return type?

From: Doug Gregor (dgregor_at_[hidden])
Date: 2004-06-21 11:29:15

[Sorry about the top-post; switching e-mail accounts]

On Sun, 20 Jun 2004, Tim Rowe wrote:
> > You're actually getting back an (Lvalue) property map. Think of it
> > like an std::map<vertex_descriptor, std::string>, because it maps
> > from the vertices in the graph g to the vertex name (which is an
> > std::string here). The difference is that the property map is fast
> > (O(1)) whereas the std::map would be slow (O(log |V|)).
> >
> > Interior properties will get a bit easier with the next Boost
> > release.
> >
> >
> > Doug
>
> Thanks. The property_map documentation says that "the get() function
> takes a property map object [...] and a key object"; is that the get()
> that's used there?

Well, no :(. The get() being used there is actually part of the
PropertyGraph concept, documented here:

  http://www.boost.org/libs/graph/doc/PropertyGraph.html

When you have an interior property p (like vertex_name_t in
the example graph) and graph g, the expression get(p, g) gives you a
property map for that particular property and graph, that can be "indexed"
with the appropriate key (e.g., a vertex or edge descriptor).

> So vertex_name is a map of maps, with a Graph as a key?

This analogy actually works (much to my surprise!), so long as you don't
push it too far :) So yes, get(vertex_name, g) returns a map from
vertices in g to the name of the vertex.

> And where is the interface to property_map defined?
> boost_1_31_0\libs\graph\doc\using_property_maps.html says that it's
> defined in boost_1_31_0\libs\property_map\property_map.html, but that
> just
> contains another example, and I can't find which header declares it -- I
> can't see it in property_map.hpp which is where I expected it to be!

The actual struct template property_map is used (only!) to get the return
type of, e.g., get(vertex_name, g). It is defined in
boost/graph/properties.hpp, but the definition won't help much because I
don't think that's quite what you're asking for.

When you use get(vertex_name, g) you get back a property map. There's no
single type that defines a property map, because it's a concept. To find
out what you can do with a property map, check out:

  http://www.boost.org/libs/property_map/property_map.html

Specifically, you want the links to the ReadablePropertyMap,
WriteblePropertyMap, and LvaluePropertyMap concepts.

        Doug