Also things often just don’t compose well. For example if you have a nested class that you want to use in an unordered_set in its parent class then you just can’t do it because you can’t put the std::hash specialization anywhere legal. It’s just two parts of the language which are totally valid on their own but don’t work together. Stuff like this is such a common problem in c++ that it drives me nuts
This is not true. From within your parent class you use an explicit hashing callable, and then from outside of the parent class you can go back to using the default std::hash.
The result looks like this:
struct Foo {
struct Bar {};
struct BarHasher {
std::size_t operator ()(const Bar&) const noexcept;
};
std::unordered_set<Bar, BarHasher> bar_set;
};
namespace std {
template<>
struct hash<Foo::Bar> {
std::size_t operator()(const Foo::Bar& b) const noexcept {
return Foo::BarHasher()(b);
}
};
}