std::map::at
mapped_type& at (const key_type& k); const mapped_type& at (const key_type& k) const; Returns a reference to the mapped value of the element identified with key k. If k does not match the key of any element in the container, the function throws an out_of_range exception.
Parameters
k
Key value of the element whose mapped value is accessed.
Member type key_type is the type of the keys for the elements in the container, defined in map as an alias of its first template parameter (Key).