nested stl containers

I have a vector that contains lists, how do I add a new element list to the vector..without creating it beforehand.. i.e. , why can't I use the 'new' command like in java?

here is my code, which 'works'

Code:

#include<list>
#include<vector>

using namespace std;

class Graph {

private:
    vector<list<int> > adjacencyList;
public:
    Graph(int vertices)
    {
        for(int i=0;i<vertices;++i)
        {
            list<int> a;
            adjacencyList.push_back(a);
        }
    }
};

why doesnt the following work?

Code:

#include<list>
#include<vector>

using namespace std;

class Graph {

private:
    vector<list<int> > adjacencyList;
public:
    Graph(int vertices)
    {
        for(int i=0;i<vertices;++i)
        {
            adjacencyList.push_back(new list<int>);
        }
    }
};

thanks!!

Originally Posted by rodrigorules

why can't I use the 'new' command like in java?

C++ is not Java.

Originally Posted by rodrigorules

here is my code, which 'works'

In this case, the best solution is to use an appropriate constructor, e.g.,

Code:

Graph(int vertices) : adjacencyList(vertices) {}

Originally Posted by rodrigorules

why doesnt the following work?

You are trying to insert a pointer into a container of objects. You can accomplish what you want by writing:

Code:

for(int i=0;i<vertices;++i)
{
    adjacencyList.push_back(list<int>());
}