What is the best way to use two keys with a std::map?

Question

I have a std::map that I\'m using to store values for x and y coordinates. My data is very sparse, so I don\'t want to use arrays or vectors, which would result in

Answer 1

Hope you will find it useful:

map> troyka = { {4, {{5,6}} } };
troyka[4][5] = 7;