Question

Is this just an implementation side effect (red-black tree) or the order is guaranteed by the c++ standard?

Answer 1

Ordered iteration is not an implementation detail; it is guaranteed by the C++ standard. It is a fundamental property of all associative containers (C++03 §23.1.2/9):

The fundamental property of iterators of associative containers is that they iterate through the containers in the non-descending order of keys where non-descending is defined by the comparison that was used to construct them. For any two dereferenceable iterators i and j such that distance from i to j is positive,

    value_comp(*j, *i) == false

value_comp is the comparator with which the map was constructed (by default, it is std::less<T>).