c++ - What is difference between const and non const key?

Question

What is the difference between the following two lines?

map<int, float> map_data;
map<const int, float> map_data;

Answer 1

• int and const int are two distinct types.

• std::map<int, float> and std::map<const int, float> are, similarly, different types.

The difference between std::map<const int, float> and std::map<int, float> is, to a degree, analogous to the difference between, say, std::map<int, float> and std::map<std::string, float>; you get a fresh map type for each.

In the non-const case, the internal key type is still non-const int:

std::map<const int, float>::key_type       => const int
std::map<int, float>::key_type             => int

However, map keys are semantically immutable, and all map operations that allow direct access to keys (for example, dereferencing iterators, which yields value_type) does constify the key_type:

std::map<const int, float>::value_type => std::pair<const int, float>
std::map<int, float>::value_type       => std::pair<const int, float>

So the difference may be largely invisible to you in every way that matters, if your implementation allows it.

That's not always the case, though: the standard officially requires your key type to be copyable and moveable, and some implementations re-use map nodes; under those implementations, attempting to use a const key simply won't work.