26 Containers library [containers]

26.4 Associative containers [associative]

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.