boost::graph compilation issue with dynamic_properties and write_graphviz

Question

This question is about boost::graph and how to handle properties associated to vertices (and/or edges). I am quite confused about handling this, but I suspect it could be a template-related issue.

Let's say I have this graph definition:

struct myVertex_t {
    int color;
};

typedef boost::adjacency_list<
    boost::vecS,                   // edge container
    boost::vecS,                   // vertex container
    boost::undirectedS,            // type of graph
    myVertex_t,                    // vertex properties
    boost::property<               // edge properties
        boost::edge_color_t,             // ???
        boost::default_color_type        // enum, holds 5 colors
    >
> myGraph_t;

AFAIK, this way of storing properties for vertices is called "bundle properties" and seems to be a third way of storing this information, although it is said in the manual that:

There are two kinds of graph properties: interior and exterior.

Back to my main question. Now, I can instanciate and printout a graph using the "dot" format this way:

 int main()
 {
    myGraph_t g;
    boost::add_edge(0, 1, g);

    boost::dynamic_properties dp;
    dp.property("color",   boost::get( &myVertex_t::color,  g ) );
    dp.property("node_id", boost::get( boost::vertex_index, g ) );
    boost::write_graphviz_dp( std::cout , g, dp);
 }

Online here

This is based on this answer in a similar question, and compiles fine.

Now I want to separate the printing in a separate function, so I write the same code in a templated function, just replacing concrete types with template type arguments:

template<typename graph_t, typename vertex_t>
void RenderGraph( const graph_t& g )
{
    boost::dynamic_properties dp;
    dp.property( "color",   boost::get( &vertex_t::color,    g ) );
    dp.property( "node_id", boost::get( boost::vertex_index, g ) );
    boost::write_graphviz_dp( std::cout, g, dp );
}

int main()
{
    myGraph_t g;
    boost::add_edge(0, 1, g);

    RenderGraph<myGraph_t,myVertex_t>( g );
}

But this does not compile:

property_map.hpp:361:44: error: assignment of read-only location ...

Any ideas what I did wrong ?

sehe sehe · Accepted Answer · 2015-12-08T16:41:13

property_map.hpp:361:44: error: assignment of read-only location ...

Yeah, sadly the fact that g is const there makes the default property factory function illegal. Dynamic properties are constructed in a writable way if the model allows it:

Requirements: PropertyMap must model Readable Property Map or Read/Write Property Map.

Because the property map is writable, the dynamic property compiles the writing branch too.

You'd have to take the argument as non-const or manually override the Property traits for the underlying maps (see the comments here (Cut set of a graph, Boost Graph Library) for an example).

You might consider reporting this as a usability issue, as logically, the properties should be const there.

boost::graph compilation issue with dynamic_properties and write_graphviz

1 Answers