Unable to output the costmap_2d occupancy grid array

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Here's how my code looks -

costmap_2d::Costmap2DROS *global_costmap = new costmap_2d::Costmap2DROS("global_costmap", buffer);

I have specified the following params in my configuration file -

global_frame: odom
rolling_window: true
track_unknown_space: true

plugins:
  - {name: obstacles_laser,           type: "costmap_2d::ObstacleLayer"}
  - {name: inflation,                 type: "costmap_2d::InflationLayer"}

I am able to publish the occupancy grid using the terminal by running rostopic echo -n1 /global_costmap/costmap.

However, when I add the following line to my code -

unsigned char* global_og = global_costmap->getCostmap()->getCharMap() ; 

cout << global_og[10] << endl;

I see some strange characters being published.