23 Containers library [containers]

23.4 Associative containers [associative]

23.4.3 Class template map [map]

23.4.3.3 Element access [map.access]

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

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

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

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

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

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

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

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

Returns: A reference to the mapped_type corresponding to x in *this.

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

Complexity: Logarithmic.

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

Constraints: The qualified-id Compare::is_transparent is valid and denotes a type.

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

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

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

Complexity: Logarithmic.

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

Constraints: For the third and fourth overloads, the qualified-id Compare::is_transparent is valid and denotes a type.

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

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

Complexity: Logarithmic.