24 Containers library [containers]

24.4 Associative containers [associative]

24.4.4 Class template map [map] Element access [map.access]

mapped_type& operator[](const key_type& x);
Effects: Equivalent to: return try_emplace(x).first->second;
mapped_type& operator[](key_type&& x);
Effects: Equivalent to: return try_emplace(std​::​move(x)).first->second;
mapped_type& at(const key_type& x); 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.