std::unordered_set contains() method

since C++20

// (1) Const version only
bool contains( const Key& key ) const;

// (2) Const version only
template< class K >
bool contains( const K& x ) const;

• (1) Checks if there is an element with key equivalent to key in the container.
• (2) Checks if there is an element with key that compares equivalent to the value x. This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

Parameters

• key - key value of the elements to count
• x - a value of any type that can be transparently compared with a key

Return value

true if there is such an element, otherwise false.

Complexity

Average case, constant - O(1).
Worst case, linear in size of the container - O(size()).

Exceptions

(none)

Example

Main.cpp

#include <iostream>
#include <unordered_set>
 
int main()
{
    std::unordered_set<int> example = {1, 2, 3, 4};
 
    for(int x: {2, 5}) {
        if(example.contains(x)) {
            std::cout << x << ": Found\n";
        } else {
            std::cout << x << ": Not found\n";
        }
    }
}

Output

2: Found
5: Not found