std::unordered_map::count

From cppreference.com

size_type count( const Key& key ) const;
(1) (since C++11)

Returns the number of elements with key that compares equal to the specified argument key, which is either 1 or 0 since this container does not allow duplicates.

Parameters

key - key value of the elements to count

Return value

Number of elements with key key, that is either 1 or 0.

Complexity

Constant on average, worst case linear in the size of the container.

See also

finds element with specific key
(public member function)
returns range of elements matching a specific key
(public member function)