c++ problem with vector push_back

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 开发者_StackOverflow社区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,

I suspect that you mean Rand instead of MTRand:

Rand.rand()

MTRand is the name of the type. Rand is the name of the instance you created.

Just a guess, but did you try

 xy.push_back(pair<double, double>(MTRand.rand(), MTRand.rand())

according to the declation of xy?

EDIT: seems the OP has changed it's code, now my answer does not match the new question any more. Nevertheless, hope my answer was useful.

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) << " ";