Support more map containers

[rdong8]

I'd like to use response<vector<pair<T, U>>> or response<absl::flat_hash_map<T, U>> as my response type for HGETALL, but the adapter is hardcoded to only support the standard library associative containers:

redis/include/boost/redis/adapter/detail/adapters.hpp

Lines 487 to 530 in 6f17663

	template <class T>
	struct impl_map {
	using type = simple_impl<T>;
	};
	template <class Key, class Compare, class Allocator>
	struct impl_map<std::set<Key, Compare, Allocator>> {
	using type = set_impl<std::set<Key, Compare, Allocator>>;
	};
	template <class Key, class Compare, class Allocator>
	struct impl_map<std::multiset<Key, Compare, Allocator>> {
	using type = set_impl<std::multiset<Key, Compare, Allocator>>;
	};
	template <class Key, class Hash, class KeyEqual, class Allocator>
	struct impl_map<std::unordered_set<Key, Hash, KeyEqual, Allocator>> {
	using type = set_impl<std::unordered_set<Key, Hash, KeyEqual, Allocator>>;
	};
	template <class Key, class Hash, class KeyEqual, class Allocator>
	struct impl_map<std::unordered_multiset<Key, Hash, KeyEqual, Allocator>> {
	using type = set_impl<std::unordered_multiset<Key, Hash, KeyEqual, Allocator>>;
	};
	template <class Key, class T, class Compare, class Allocator>
	struct impl_map<std::map<Key, T, Compare, Allocator>> {
	using type = map_impl<std::map<Key, T, Compare, Allocator>>;
	};
	template <class Key, class T, class Compare, class Allocator>
	struct impl_map<std::multimap<Key, T, Compare, Allocator>> {
	using type = map_impl<std::multimap<Key, T, Compare, Allocator>>;
	};
	template <class Key, class Hash, class KeyEqual, class Allocator>
	struct impl_map<std::unordered_map<Key, Hash, KeyEqual, Allocator>> {
	using type = map_impl<std::unordered_map<Key, Hash, KeyEqual, Allocator>>;
	};
	template <class Key, class Hash, class KeyEqual, class Allocator>
	struct impl_map<std::unordered_multimap<Key, Hash, KeyEqual, Allocator>> {
	using type = map_impl<std::unordered_multimap<Key, Hash, KeyEqual, Allocator>>;
	};

It's also missing the new ones from C++23 (ie. flat_map). It would ideally support anything with a "map-like" interface.

[anarthal]

Thanks for reporting. I think we have two (potentially compatible) courses of action:

1. We support "map-like" as you propose. We currently use insert() as part of the map implementation, so this would allow you to use the absl containers, the C++23 containers and the ones in Boost.Unordered, but not vector<pair<T, U>> because vector has no insert.
2. We make the adapter interface public.

I think 2 should be the ultimate goal, but starting with 1 seems wiser. The adapter interface looks pretty convoluted at the moment, so we'd probably have to revisit it before documenting it.

@mzimbres what do you think?

[mzimbres]

Hi, I will investigate.

[mzimbres]

Hi, support for response<vector<pair<T, U>>> is now available in the develop branch https://github.com/boostorg/redis/blob/develop/example/cpp20_containers.cpp#L94-L96