Boost Graph Library cannot store references to other vertices?

Question

I'm using BGL to build a graph storing bundled vertices where one type of vertex stores a reference to the other vertex type. Both types are handled using std::variant:

struct simple_node_t {
    size_t enabled;
};

struct complex_node_t {
    bool foo1;
    size_t foo2;
    simple_node_t& control;
};

using vertex_t = std::variant<simple_node_t, complex_node_t>;

using netListGraph_t = boost::adjacency_list<
    boost::vecS,
    boost::vecS,
    boost::undirectedS,
    vertex_t>;

Vertices of type complex_node_t are created and stored like this:

// Create node
complex_node_t cpx = {
    true,
    0xDEADBEEF,
    std::get<simple_node_t>(mGraph[desc_to_simple_vertex])
};

// Add complex_node_t to graph
vertex_t vtx(cpx);
auto vertex = boost::add_vertex(vtx, mGraph);

Now the problem:

auto pVal = std::get_if<complex_node_t>(&mGraph[vertex]);

assert(pVal->foo1 == true); //OK
assert(pVal->foo2 == 0xDEADBEEF); //OK

But accessing the reference fails (invalid object)!

**pVal->control.enabled -> GARBAGE**

Storing the data by value works - but storing by reference does not.

What am I doing wrong?

PS: My example is very reduced of course... that means the vertices I want to store via reference are much bigger.

EDIT

I now changed my code:

struct complex_node_t {
    bool foo1;
    size_t foo2;
    std::reference_wrapper<simple_node_t> control;
};

and try to access elements:

if (pVal->control.get().enabled) -> **STILL GARBAGE**

Answer 1

If you store a reference inside a class, it is no longer assignable nor default-constructible.

BGL Has the concept of a descriptor here, it's an abstraction of something like an array index, but independent of the graph representation. So you could use those.

Beware of invalidation rules though: depending on the graph model[1]. See

• Should I use iterators or descriptors to keep a reference on an edge or vertex?
• BGL: storing vertex descriptors in a way that they won't invalidate
• In the Boost Graph Library, why does adding an edge invalidate Edge iterators (and other questions)?

PS. if you know your graph has reference stability for vertices you could do what you want replacing the reference with raw pointers or std::reference_Wrapper<>

---

[1] in the case of adjacency_list<> it depends on the vertex/edge container selector template arguments

Demo Code

This code demonstrates

• the mechanics of defining the graph (with self-referential descriptor types)
• how NOT to populate (see // BUG)
• how to safely populate/use the attributes instead.

Live On Coliru

#include <boost/graph/adjacency_list.hpp>
#include <variant>

using base_traits = boost::graph_traits<boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS> >;

struct simple_node_t {
    size_t enabled = 0;
};

struct complex_node_t {
    bool foo1 = false;
    size_t foo2 = 0;
    base_traits::vertex_descriptor control {};
};

using vertex_t = std::variant<simple_node_t, complex_node_t>;

using netListGraph_t = boost::adjacency_list<
    boost::vecS,
    boost::vecS,
    boost::undirectedS,
    vertex_t>;

int main() {
    {
        netListGraph_t g;
        auto control = add_vertex(simple_node_t{12}, g);
        // BUG HERE:
        add_vertex(complex_node_t{ true, 42, control }, g); // whoops, might invalidate `control`
    }

    {
        netListGraph_t g(10);
        auto control = vertex(6, g);
        g[control] = simple_node_t{12};

        auto other   = vertex(4, g);
        g[other] = complex_node_t{ true, 42, control }; // fine

        // also fine:
        std::get<complex_node_t>(g[other]).control = control;
    }
}