23 Containers library [containers]

23.5 Unordered associative containers [unord]

23.5.3 Class template unordered_map [unord.map]

23.5.3.3 Element access [unord.map.elem]

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

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

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

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

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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;

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

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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.

Preconditions: The expression find(k) is well-formed and has well-defined behavior.

Returns: A reference to find(k)->second.

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

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constexpr optional<mapped_type&>       lookup(const key_type& k);
constexpr optional<const mapped_type&> lookup(const key_type& k) const;
template<class K> constexpr optional<mapped_type&>       lookup(const K& k);
template<class K> constexpr optional<const mapped_type&> lookup(const K& k) const;

Constraints: For the third and fourth overloads, the qualified-ids Hash::is_transparent and
Pred::is_transparent are valid and denote types.

Preconditions: The expression find(k) is well-formed and has well-defined behavior.

Returns: find(k)->second if contains(k) is true, otherwise nullopt.

Complexity: Average case constant, worst case linear in size().