[kouteiheika]

> on the contrary, std::map is a good default container

If you care about performance then it's not. (And if you don't care about performance then why are you using C++?) The standard requires that `insert` will not invalidate iterators, which basically forces everyone to implement `std::map` as a red-black tree, and those are pretty bad performance-wise on modern hardware mostly due to cache misses.

> If you need fast O(1) look up std::unordered_map is really not fit for purpose and requires you to come up with an hash function.

Modern hash table implementations (along with a modern hash function) are exactly what you should use if you need fast average-case O(1) lookup, so I'm confused why would you say that it's not fit for purpose? Unless you specifically meant only `std::unordered_map` which, yes, is pretty atrocious performance-wise (again, due to the iterator invalidation requirements).

[gpderetta]

I meant specifically std::unordered_map, because how it is specified in the standard it is very hard to implement it efficiently. If you need performance, yes, use a good hash table implementation. But even in C++ you do not need to be shaving cycles everywhere and there std::map is better than std::unordered_map.