23 Containers library [containers]

23.4 Associative containers [associative]

23.4.3 Class template map [map]

23.4.3.3 Element access [map.access]

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constexpr mapped_type& operator[](const key_type& x);
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Effects: Equivalent to: return try_emplace(x).first->second;
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constexpr mapped_type& operator[](key_type&& x);
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Effects: Equivalent to: return try_emplace(std​::​move(x)).first->second;
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template<class K> constexpr mapped_type& operator[](K&& x);
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Constraints: The qualified-id Compare​::​is_transparent is valid and denotes a type.
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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;
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Returns: A reference to the mapped_type corresponding to x in *this.
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Throws: An exception object of type out_of_range if no such element is present.
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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;
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Constraints: The qualified-id Compare​::​is_transparent is valid and denotes a type.
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Preconditions: The expression find(x) is well-formed and has well-defined behavior.
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Returns: A reference to find(x)->second.
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Throws: An exception object of type out_of_range if find(x) == end() is true.
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Complexity: Logarithmic.