Good or bad code?

I am writing a function that takes a reference P to a
std::vector<std::vector<int> >. Before calling this function I don't know
the dimensions of P. I need to define those in the function and I need to
read form all elements in P. Here is what I do:

void testVector(std::vector<std::vector<int> > & P)
{

 // Currently hardcoded
  int rows = 3;
  int columns = 3;

  // Initializing vectors of vectors
  P.resize(rows);
  for (int i=0; i<rows; i++)
  {
    P[i].resize(columns);
    for (int j=0; j<columns; j++)
    {
      P[i][j] = 33;
    }
  }

  // Reading from vector of vectors.
  int a = 0;
  for (int i=0; i<rows; i++)
  {
    for (int j=0; j<columns; j++)
    {
      P[i][j] = a;
      a++;
    }
  }

}

int main() {

  std::vector<std::vector<int> > P;
  testVector(P);

  int rows = P.size();
  int columns = P[0].size();
  // Printing
  for (int i=0; i<rows; i++)
  {
    for (int j=0; j<columns; j++)
    {
      std::cout << P[i][j] << std::endl;
      // P[i][j] = 33;
    }
  }

return 0;

}

But is this how it should be done? Seems a bit clumsy to me.