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:

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#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);
        }
    }
};

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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!!

Quote:

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Originally Posted by rodrigorules

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

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C++ is not Java.

Quote:

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Originally Posted by rodrigorules

here is my code, which 'works'

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

Code:

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

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Quote:

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Originally Posted by rodrigorules

why doesnt the following work?

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

Code:

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

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