std::unordered_map::at

From cppreference.com

T& at( const Key& key );
(1) (since C++11)
const T& at( const Key& key ) const;
(2) (since C++11)

Returns a reference to the mapped value of the element with key equivalent to key. If no such element exists, an exception of type std::out_of_range is thrown.

Parameters

key - the key of the element to find

Return value

Reference to the mapped value of the requested element

Exceptions

std::out_of_range if the container does not have an element with the specified key

Complexity

Average case: constant, worst case: linear in size.

See also

access or insert specified element
(public member function)