Iterating over edges of a graph using range-based for

Question

I have a representation of a graph as a std::vector> neighbors, that is, vertices are integers, and for each vertex we keep

Answer 1

I believe that your internal representation of a graph, std::vector>, is what makes the code hard to write/read. Maybe another representation (e.g. std::set>) would make your code simpler. However, it's hard to tell since we don't know exactly what are the requirements of Graph.

Anyway, as pointed out by Zeta there's a bug in EdgeIter::operator !=(). For instance, the code below:

int main() {

    Graph g(5);
    g.addEdge(0, 1);
    g.addEdge(0, 2);

    auto i1 = g.edges().begin();
    auto i2 = i1;
    ++i2;
    std::cout << std::boolalpha;
    std::cout << (i1 != i2) << std::endl;
}

outputs false. Hence, the code considers that i1 and i2 are not different when they clearly are.

Update:

It's probably obvious but here is a simpler version which uses a different representation for the graph. However, I emphasize that this may not be satisfactory depending on your requirements for Graph (which I don know):

#include 
#include 
#include 

typedef unsigned Vertex;

class Graph {

public:

    typedef std::pair Edge;
    typedef std::set Edges;

    void addEdge(Vertex u, Vertex v) {
      edges_.insert({u, v});
    }

    const Edges& edges() { return edges_; }

private:

    Edges edges_;
};

int main() {

    Graph g;

    g.addEdge(1, 2);
    g.addEdge(2, 3);
    g.addEdge(1, 3);    

    for (auto e: g.edges())
        std::cout << e.first << ' ' << e.second << std::endl;
}