Boost.Graph and Graphviz nested subgraphs

问题

I expected the code

#include <boost/graph/graphviz.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/subgraph.hpp>
#include <iostream>

using namespace boost;

using attrs_t = std::map<std::string, std::string>;

using graph_t = adjacency_list<
    vecS, vecS, directedS,
    property<vertex_attribute_t, attrs_t>,
    property<edge_index_t, int, property<edge_attribute_t, attrs_t>>,
    property<graph_name_t, std::string,
        property<graph_graph_attribute_t, attrs_t,
            property<graph_vertex_attribute_t, attrs_t,
                property<graph_edge_attribute_t, attrs_t>>>>>;

int main()
{
    char names[] = {"AB"};
    enum {A, B, N};

    subgraph<graph_t> main(N);
    subgraph<graph_t>& sub1 = main.create_subgraph();
    subgraph<graph_t>& sub2 = sub1.create_subgraph();

    add_vertex(A, sub1);
    add_vertex(B, sub2);
    add_edge(A, B, main);

    get_property(main, graph_name) = "G0";
    get_property(sub1, graph_name) = "clusterG1";
    get_property(sub2, graph_name) = "clusterG2";

    write_graphviz(std::cout, main, make_iterator_vertex_map(names));
}

to generate the graph on the left, whereas I have got the right one:

The output is:

digraph G0 {
subgraph clusterG1 {
subgraph clusterG2 {
//B;
}
//A;
}
A -> B;
}

The commented node statements are where the hierarchy information is lost (these lines I do not have in my output). How can I avoid that?

If I add both vertices into the same subgraph:

add_vertex(A, sub1);
add_vertex(B, sub1);
add_edge(A, B, main);

the connection A -> B appears in the scope of clusterG1 and from how I understand it, that is where the mentioned vertices will also be implicitly declared.

I am using Boost 1.68.0

回答1:

Good spot. The linked answer actually has UB, as your own answer explains. The map passed in to the write_graphviz function isn't actually supposed to have the node_id for the graphviz output. Instead, that map was assumed to be a vertex_index_t propertymap.

That was an assumption I probably held from boost::print_graph (graph_utility.hpp) that does that such property-map.

To make it work safely I'd modify the example to employ write_graphviz_dp - using dynamic properties:

int main() {
    boost::dynamic_properties dp;
    dp.property("node_id", boost::make_transform_value_property_map<std::string>(&name_for_index, boost::identity_property_map{}));
    write_graphviz_dp(std::cout, create_data<subgraph<Graph> >(), dp);
}

I opted to use a transform function to get a name for any vertex descriptor, not wanting to assume anything about the number of vertices anymore either, I wrote the more general function to generate names like "A",...,"Z","AA",...,"ZZ" etc.:

static std::string name_for_index(intmax_t index) {
    std::string name;

    do {
        name += 'A' + (index%26);
        index /= 26;
    } while (index);

    return name;
}

Live On Coliru

Retaining Subgraph Information

The above overload doesn't have the subgraph support. So, instead let's fix the vertex_attribute map to have the expected vertex labels:

int main() {
    auto g = create_data<subgraph<Graph> >();

    for (auto vd : make_iterator_range(vertices(g))) {
        put(get(vertex_attribute, g), vd, 
                GraphvizAttributes{
                    {"label", name_for_index(vd)}
                });
    }

    write_graphviz(std::cout, g);
}

Now it does print:

Live On Coliru

digraph G0 {
subgraph clusterG1 {
graph [
label=G1];
node [
color=red, shape=Mrecord];
0[label="Vertex A"];
1[label="Vertex B"];
0 -> 1;
}
subgraph clusterG2 {
graph [
fillcolor=lightgray, label=G2, style=filled];
node [
shape=circle];
4[label="Vertex E"];
2[label="Vertex C"];
5[label="Vertex F"];
4 -> 5;
2 -> 5;
}
3[label="Vertex D"];
1 -> 2;
1 -> 3;
4 -> 1;
5 -> 3;
}

Which renders as

回答2:

The trouble was caused by supplying make_iterator_vertex_map(names), which returns chars from names ('A', 'B'). This is incompatible with the expected (and default) vertex_index properties (0, 1). After the fix, I get:

digraph G0 {
subgraph clusterG1 {
subgraph clusterG2 {
1;
}
0;
}
0 -> 1;
}

vertex_marker, which is std::vector<bool> of 2 elements in my case, not 67 and more was expected to be indexed by vertex indices. I was experiencing undefined behavior,
and if ( vertex_marker[pos] ) was never fulfilled.

The responsible portion of boost::detail::write_graphviz_subgraph:

// Print out vertices and edges not in the subgraphs.

typename graph_traits<Graph>::vertex_iterator i, end;
typename graph_traits<Graph>::edge_iterator ei, edge_end;

for(boost::tie(i,end) = vertices(g); i != end; ++i) {
  Vertex v = g.local_to_global(*i);
  int pos = get(vertex_id, v); // 66, 65, should be 1, 0
  if ( vertex_marker[pos] ) {  // out-of-bounds access
    vertex_marker[pos] = false;
    out << escape_dot_string(pos);
    make_vertex_attributes_writer(g.root())(out, v);
    out << ";" << std::endl;
  }
}

A static_assert/SFINAE of the vertex_id parameter type against Graph's vertex_id type (which can be done immediately in boost::write_graphviz) would be greatly welcome. On a side note, the hardcoded int pos = ... does not look too professional...

And if we want to keep the custom names, we have to provide them via label attributes instead.

来源：https://stackoverflow.com/questions/54599136/boost-graph-and-graphviz-nested-subgraphs

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