How to ensure that a std::map is ordered?

You don’t have to do anything. The map will be in ascending order according to the values of the key.

Internally, the map performs a comparison between keys to order its elements. By default, it uses std::less<KEY>, which is equivalent to bool operator<(int, int) for integers. For user defined types, you have to options:

1. Implement a bool operator<(const MyType&, const MyType&) implementing a strict weak ordering comparison between your user defined types. Use this if your type has a natural ordering
2. Provide a binary functor that implements strict weak ordering, which you can pass as the 3rd template parameter to the map. Use this if your type does not have a natural ordering, or if you want to build the map with an ordering different to that used by std::less<Key> via the bool operator<(...) from point 1.

What typically happens behind the scenes is that the map is implemented as a self-balancing binary tree, and the strict weak ordering is used to place new elements in the map, and to determine whether two elements are equal. As an aside, the same logic applies to std::set, where the key and value are one and the same.