mapped_type& operator[](const key_type& k);
Effects: Equivalent to: return try_emplace(k).first->second;
mapped_type& operator[](key_type&& k);
Effects: Equivalent to: return try_emplace(move(k)).first->second;
mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;
Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.
Throws: An exception object of type out_of_range if no such element is present.