# std::unordered_set<Key,Hash,KeyEqual,Allocator>::contains

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< cpp‎ | container‎ | unordered set

bool contains( const Key& key ) const; | (1) | (since C++20)
template< class K > bool contains( const K& x ) const; | (2) | (since C++20)

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 element to search for
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

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

### Example

Run this code

    #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

### See also

find (C++11) | finds element with specific key (public member function)
count (C++11) | returns the number of elements matching specific key (public member function)
equal_range (C++11) | returns range of elements matching a specific key (public member function)

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