index matrix

edit

still i can't imagine how to use such a matrix but you can create it with findNonZero()

take a look at the code below

#include <opencv2/opencv.hpp>
using namespace cv;

int main( int argc, const char** argv )
{
    Mat src= Mat::ones(5,5,CV_8UC1);
    //std::vector<Point> locations;   // you can use vector or Mat to get same result
    Mat locations;  // output, locations of non-zero pixels
    findNonZero(src, locations);
    std::cout << locations;
    return 0;
}

output locations vector is like below you can reshape it if you want

[0, 0;
 1, 0;
 2, 0;
 3, 0;
 4, 0;
 0, 1;
 1, 1;
 2, 1;
 3, 1;
 4, 1;
 0, 2;
 1, 2;
 2, 2;
 3, 2;
 4, 2;
 0, 3;
 1, 3;
 2, 3;
 3, 3;
 4, 3;
 0, 4;
 1, 4;
 2, 4;
 3, 4;
 4, 4]