[eigen] Using Eigen::Map to use eigen operations on my data

[Robert Lupton the Good <rhl@xxxxxxxxxxxxxxxxxxx>]

I must be missing something.  The following code prints:
[1 2 3 4]
1 0
0 1

although I thought that the point of Map was to create an object that  
shared memory with an underlying array.  What am I Missing?

				R

#include <iostream>
#include "Eigen/Core.h"

using namespace std;
using namespace Eigen;

void identity(MatrixXf *mat) {
    (*mat)(0, 0) = (*mat)(1, 1) = 1;
    (*mat)(1, 0) = (*mat)(0, 1) = 0;
}

int main() {
    float data[4] = { 1, 2, 3, 4 };
    MatrixXf mat2x2 = MatrixXf(Map<MatrixXf>(data, 2, 2));

    identity(&mat2x2);

    cout << "[" << data[0] << " " << data[1] <<  " " << data[2] <<  "  
" << data[3] << "]\n" << mat2x2 << endl;
}