What is problem with my code and why an error occurred "SEGMENTATION FAULT"?

let's say we are taking row = 1 and col = 3 , then my loop terminates when it reaches value of j as 2 and with error segmentation fault...

        std::cin >> row >> col;
        std::vector<std::vector<int> > arr;
        for ( int i = 0; i < row; i++ )                    // first loop
        {
            std::vector<int> temp (col, 0);
            arr . push_back (temp);
        }

        for ( int i = 0; i < arr . size (); i++ )          //second loop
        {
            for ( int j = 0; j < arr[ i ] . size (); j++ )
            {
                std::cin >> arr[ i ][ j ];
            }
        }