Graph concepts do not work with reference types

[BenBrock]

The NWGraph graph concepts do not work with reference types. For example, the following code fails to compile:

#include <nwgraph/edge_list.hpp>

// Fails: G = nw::graph::edge_list<...>& does not fulfill `nw::graph::edge_list_graph`
template <nw::graph::edge_list_graph G>
void foo(G&&) {}

int main(int argc, char** argv) {
  nw::graph::edge_list<nw::graph::directedness::undirected, float> graph(0);

  foo(graph);

  return 0;
}

Since graphs are specializations of ranges and will often represent data structures you don't want to copy, I think it will often be useful to use forwarding references (as above) with them. Note that std::ranges concepts like range, forward_range, and random_access_range do work with references.

(For counterpoint, the iterator concepts do not work with references, which has always annoyed me. However, iterators are supposed to be cheap to copy, so I suppose the standard library is implicitly encouraging us to pass iterators by value. However, I suppose you generally won't want to pass graphs by value.)

The two issues preventing references from working are:

• vertex_id_t This could be fixed by changing the definition on line 37 of graph_traits.hpp to

template <typename G>
using vertex_id_t = typename graph_traits<std::remove_cvref_t<G>>::vertex_id_type;

• std::copyable You could also add a std::remove_cvref_t<G> on line 57 of graph_concepts.hpp. However, I'm not sure that I'm convinced graphs always need be copyable in the first place? I suppose both a container, which owns the data, as well as a copy-constructible view, would be copyable, although copying means different things in these two cases. I guess you're really disabling move-only views from being graphs here?