26 Containers library [containers]

26.4 Associative containers [associative]

26.4.4 Class template map [map] map element access [map.access]

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