Template help

[Anonymous via Digitalmars-d-learn]

One way I've used it in code

struct MapBy(T,string key) if (hasDeepMember!(T,key)) {
	alias key_t = DeepMemberType!(T,key);
	private const(T)[key_t] _map;

	bool has(key_t id) const nothrow {
		if ((id in _map) != null) return true;
		else return false;
	}

	void put(in ref T val) pure nothrow {
		auto id = val.Extract!key;
		_map[id] = val;
	}

	ref const(T) get(key_t id) const nothrow {
		assert(has(id));
		return _map[id];
	}
}