Vector from vector references

[luislupe]

I have several vectors of int elements.
Now I want to create a vector of vectors, but I don't want to copy the vectors to the new one. If possible, I'd like to pass them by reference.
Please consider:

#include <iostream>
#include <vector>

using namespace std;

int main ()
{
    vector<int> v1; 
    v1.push_back(1);
    v1.push_back(2);
    v1.push_back(3);

    vector<int> v2; 
    v2.push_back(4);
    v2.push_back(5);
    v2.push_back(6);

    vector<int> v3; 
    v3.push_back(7);
    v3.push_back(8);
    v3.push_back(9);

    vector<int>& v4 = v1.front();
    vector<int>& v5 = v2.front();
    vector<int>& v6 = v3.front();

    vector<vector<int> >vall;
    vall.push_back(v4);
    vall.push_back(v5);
    vall.push_back(v6);

    for (vector<int>::size_type i = 0; i != v1.size(); ++i) {
        cout << vall[0][i] << vall[1][i] << vall[2][i] << endl;
    }   
    return 0;
}

This attempt results in error.
Is there a way to do this?

[ravenous]

I would be careful about doing this. The std::vector container stores its contents in a contiguous portion of memory. If you resize the vector at any point, either directly by calling std::vector::resize or indirectly, by calling std::vector::push_back and the space available in the currently allocated memory for the vector isn't big enough then the whole contents of the vector might be moved to a different portion of memory. This will leave your vector of references still cointaining the old memory addresses of the elements of the vector, which is almost certainly going to result in a really nasty run-time bug! If you want to do this safely, you'll probably have to manage the memory yourself in some kind of clever way that stops this from happening. If the order of the elements isn't important to you, I think that you might be able use std::set instead.

[luislupe]

The order is of the elements is important.
Those vectors that are built are not going to be modified in any way.
If possible, I just could pass a const reference to each of them (as I would as parameters of a function).

[luislupe]

I guess that:

vector<int>& v4 = v1;
vector<int>& v5 = v2;
vector<int>& v6 = v3;

does the trick. It's simpler than I thought. Or am I wrong?

[ravenous]

That will do what (I think) you want, and is also safe, since these are references to the actual vector, not the data in the vector, the vector object reference will always be valid (as long as the original vector is in scope).