Question

I have a question about inheritance. From this source:

gSpan.h

struct Edge {
    int from;
    int to;
    int elabel;
    unsigned int id;
    Edge(): from(0), to(0), elabel(0), id(0) {};
};
class Vertex
{
public:
    typedef std::vector<Edge>::iterator edge_iterator;

    int label;
    std::vector<Edge> edge;

    void push (int from, int to, int elabel)    //elabel代表edge label
    {
        edge.resize (edge.size()+1);
        edge[edge.size()-1].from = from;
        edge[edge.size()-1].to = to;
        edge[edge.size()-1].elabel = elabel;
        return;
    }
};

class Graph: public std::vector<Vertex> {
public:
    typedef std::vector<Vertex>::iterator vertex_iterator;

    Graph (bool _directed)
    {
        directed = _directed;
    };
    bool directed;

    Graph(): edge_size_(0), directed(false) {};
};

gSpan.cpp

std::istream &gSpan::read (std::istream &is)
{
    Graph g(directed);
    while (true) {
        g.read (is);
        if (g.empty()) break;
        TRANS.push_back (g);
    }
    return is;
}

graph.cpp

std::istream &Graph::read (std::istream &is)    
{
    std::vector <std::string> result;
    char line[1024];

    while (true) {

        if (result.empty()) {
            // do nothing
        } else if (result[0] == "t") {
                  ...
        } else if (result[0] == "v" && result.size() >= 3) {
            unsigned int id    = atoi (result[1].c_str());
            this->resize (id + 1);
            (*this)[id].label = atoi (result[2].c_str());
               ...

Why we can use (*this)[id].label in graph.cpp? (*this) is a Graph object…. if we want to use (*this)[id].label shouldn’t we have to declare std::vector<Vertex>?

Answer 1

class Graph: public std::vector<Vertex>

This means that the Graph class inherited std::vector<Vertex>, so you can do anything with it that you’d be able to do if it were a vector. And so by creating an instance of Graph, you’re effectively declaring a new vector.