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

What are the requirements on a class that is to be used as the key?

The map needs to be able to tell whether one key's value is less than another key's value: by default this means that (key1 < key2) must be a valid boolean expression, i.e. that the key type should implement the 'less than' operator.

The map template also implements an overloaded constructor which lets you pass-in a reference to a function object of type key_compare, which can implement the comparison operator: so that alternatively the comparison can be implemented as a method of this external function object, instead of needing to be baked in to whatever type your key is of.