[unord.map.elem]

constexpr mapped_type& operator[](const key_type& k);

Effects: Equivalent to: return try_emplace(k).first->second;

constexpr mapped_type& operator[](key_type&& k);

Effects: Equivalent to: return try_emplace(std::move(k)).first->second;

template<class K> constexpr mapped_type& operator[](K&& k);

Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

Effects: Equivalent to: return try_emplace(std::forward<K>(k)).first->second;

constexpr mapped_type& at(const key_type& k);
constexpr const mapped_type& at(const key_type& k) const;

Throws: An exception object of type out_of_range if no such element is present.

template<class K> constexpr mapped_type&       at(const K& k);
template<class K> constexpr const mapped_type& at(const K& k) const;

Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

Throws: An exception object of type out_of_range if find(k) == end() is true.

23 Containers library [containers]