T& operator[](const key_type& x);
Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.
Requires: key_type shall be CopyInsertable and mapped_type shall be DefaultInsertable into *this.
Returns: A reference to the mapped_type corresponding to x in *this.
Complexity: Logarithmic.
Effects: If there is no key equivalent to x in the map, inserts value_type(std::move(x), T()) into the map.
Requires: mapped_type shall be DefaultInsertable into *this.
Returns: A reference to the mapped_type corresponding to x in *this.
Complexity: Logarithmic.
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.