c++ problem with vector push_back

Question

UPDATE: The following code gives me an error

Graph.cpp: In function 'std::ostream& operator<<(std::ostream&, const Graph&)': Graph.cpp:43: error: passing 'const std::map >, std::less, std::allocator > > > >' as 'this' argument of '_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = long int, _Tp = std::vector >, _Compare = std::less, _Alloc = std::allocator > > >]' discards qualifiers Graph.cpp:44: error: passing 'const std::map >, std::less, std::allocator > > > >' as 'this' argument of '_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = long int, _Tp = std::vector >, _Compare = std::less, _Alloc = std::allocator > > >]' discards qualifiers make[2]: * [build/Debug/GNU-MacOSX/Graph.o] Error 1

class Graph {
public:
    Graph();
    Graph(const Graph& orig);
    virtual ~Graph();

    void clear();
    void rgg(lint, double);
    bool is_directed() { return directed; }
    friend ostream& operator<< (ostream&, const Graph&); 


private:
    map< lint, vector<lint> > adjList;
    vector< pair<double, double> > xy;
    double radius;
    MTRand Rand;
    bool directed;
};


void Graph::rgg(lint n, double r) {
    radius = r; directed = false;

    clear(); 

    for(lint i = 0; i < n; i++)
        xy.push_back(pair<double, double>(Rand.rand(), Rand.rand()));
}

ostream& operator<< (ostream& os, const Graph& inGraph) {
    for(lint i = 0; i < inGraph.nodes; i++) {
        os << i << " ";
        if( inGraph.adjList.find(i) != inGraph.adjList.end() ) {
            for(lint idx = 0; idx < (inGraph.adjList[i]).size(); idx++ )
                os << inGraph.adjList[i].at(idx) << " ";

        }
        os << endl;
    }
}

Thank you in advance,

You have a destructor and copy-constructor, but I don't see a copy assignment operator. Why? — GManNickG
Why have you completely changed the question? Now none of the answers already posted make sense which will completely confuse anyone reading the question in the future. Please revert your change and post a new question if you want further help. — Mark Byers

UncleBens UncleBens · Accepted Answer · 2011-01-15T11:16:27

The cause of your problems is that map's operator[] is a mutable operation (if the key doesn't exist, it will be added to the map).

You will have to use the iterator returned from find():

map< lint, vector<lint> >::const_iterator it = inGraph.adjList.find(i);
if (it != inGraph.adjList.end();
for(lint idx = 0; idx < it->size(); idx++ )
    os << it->at(idx) << " ";

c++ problem with vector push_back

3 Answers