# 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)
bool contains( const Key& key, std::size_t hash ) const; | (1) | (since C++20)
template< class K > bool contains( const K& x ) const; | (2) | (since C++20)
template< class K > bool contains( const K& x, std::size_t hash ) 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 only participates in overload resolution if the qualified-id Hash::transparent_key_equal is valid and denotes a type. This assumes that such Hash is callable with both K and Key type, and that its key_equal is transparent, which, together, allows calling this function without constructing an instance of `Key`.

If present, the `hash` parameter may be used to pass a pre-computed hash value as an optimization. The behavior is undefined if invoking hash_function() on the key passed do not produce the same value as `hash`.

### Parameters

key | - | key value of the element to search for
x | - | a value of any type that can be transparently compared with a key
hash | - | the hash value of the 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};

        if(example.contains(2)) {
            std::cout << "Found\n";
        } else {
            std::cout << "Not found\n";
        }
    }

Output:

    Found

### See also

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

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