问题
Is there a reason for missing transparent (template <class K> at(K&& key);) in std::map?
回答1:
My guess is that std::map::at() must be a "bounds-checked" version of std::map::operator[](). Providing a transparent version of std::map::operator[]() imposes an additional requirement on std::map::key_type and the query key type K - if the query key is not in the map, it must be inserted (with default constructed value), which means that std::map::key_type must be constructible from the the query key type.
来源：https://stackoverflow.com/questions/40767213/why-no-transparent-c1x-stdmapat