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I have a representation of a graph as a std::vector<std::unordered_set<unsigned>> neighbors , that is, vertices are integers, and for each vertex we keep a set of its neighbors. Thus, to walk all edges, I would do something like for (unsigned u = 0; u < neighbors.size(); ++u) for (unsigned v : neighbors[u]) if (u <= v) std::cout << u << ' ' << v << std::endl; Now, I would like to be able to get the same effect from for (auto e: g.edges()) std::cout << e.first << ' ' << e.second << std::endl; where g is from a class encapsulating the neighbors vector. However, everything I tried seems extremely complicated, the best version I can come up with has 50 lines, and it's hard to see that it is correct. Is there a simple way to do this? Here's my ugly version: #include <iostream> #include <unordered_set> #include <vector> typedef unsigned Vertex; class Graph { public: typedef std::unordered_set<Vertex> Neighbors; std::size_t numVertices() const { return neighbors_.size(); } Graph(std::size_t n = 0) : neighbors_(n) { } void addEdge(Vertex u, Vertex v) { neighbors_[u].insert(v); neighbors_[v].insert(u); } class EdgeIter { friend Graph; public: bool operator!=(const EdgeIter& other) { return u_ != other.u_; } void operator++() { do { ++it_; while (it_ == it_end_) { u_++; if (u_ >= neighbors_.size()) break; it_ = neighbors_[u_].cbegin(); it_end_ = neighbors_[u_].cend(); } } while (u_ < neighbors_.size() && *it_ < u_); } std::pair<Vertex, Ver

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