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 a massive waste of memory. My data ranges from -250000 to 250000, but I'll only have a few thousand points at the most.

Currently I'm creating a std::string with the two coordinates (i.e. "12x45") and using it as a key. This doesn't seem like the best way to do it.

My other thoughts were to use an int64 and shove the two int32s into it and use it as a key.

Or to use a class with the two coordinates. What are the requirements on a class that is to be used as the key?

What is the best way to do this? I'd rather not use a map of maps.

Answer 1

Use std::pair<int32,int32> for the key:

std::map<std::pair<int,int>, int> myMap;  myMap[std::make_pair(10,20)] = 25; std::cout << myMap[std::make_pair(10,20)] << std::endl; 

Answer 2

I usually solve this kind of problem like this:

struct Point {     int x;     int y; };  inline bool operator<(const Point& p1, const Point& p2) {     if (p1.x != p2.x) {         return p1.x < p2.x;     } else {         return p1.y < p2.y;     } }