Question

I am writing a Matlab extension using the C++ ublas library, and I would like to be able to initialize my ublas vectors from the C arrays passed by the Matlab interpeter.
How can I initialize the ublas vector from a C array without (for the sake of efficiency) explicitly copying the data. I am looking for something along the following lines of code:

using namespace boost::numeric::ublas;

int pv[10] = { 5, 5, 5, 5, 5, 5, 5, 5, 5, 5 };
vector<int> v (pv);

In general, is it possible to initialize a C++ std::vector from an array? Something like this:

#include <iostream>
#include <vector>
using namespace std;

int main()
{
    int pv[4] = { 4, 4, 4, 4};
    vector<int> v (pv, pv+4);

    pv[0] = 0;
    cout << "v[0]=" << v[0] << " " << "pv[0]=" << pv[0] << endl;

    return 0;
}

but where the initialization would not copy the data. In this case the output is

v[0]=4 pv[0]=0

but I want the output to be the same, where updating the C array changes the data pointed to by the C++ vector

v[0]=0 pv[0]=0

Answer 1

Both std::vector and ublas::vector are containers. The whole point of containers is to manage the storage and lifetimes of their contained objects. This is why when you initialize them they must copy values into storage that they own.

C arrays are areas of memory fixed in size and location so by their nature you can only get their values into a container by copying.

You can use C arrays as the input to many algorithm functions so perhaps you can do that to avoid the initial copy?