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

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< cpp‎ | container‎ | unordered set

void clear() noexcept; |  | (since C++11)

Erases all elements from the container. After this call, size() returns zero.

Invalidates any references, pointers, or iterators referring to contained elements. May also invalidate past-the-end iterators.

### Parameters

(none)

### Return value

(none)

### Complexity

Linear in the size of the container, i.e., the number of elements.

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <unordered_set>

    int main()
    {
        std::unordered_set<int> container{1, 2, 3};

        auto print = [](const int& n) { std::cout << ' ' << n; };

        std::cout << "Before clear:";
        std::for_each(container.begin(), container.end(), print);
        std::cout << "\nSize=" << container.size() << '\n';

        std::cout << "Clear\n";
        container.clear();

        std::cout << "After clear:";
        std::for_each(container.begin(), container.end(), print);
        std::cout << "\nSize=" << container.size() << '\n';
    }

Possible output:

    Before clear: 1 2 3
    Size=3
    Clear
    After clear:
    Size=0

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2550 | C++11 | for unordered associative containers, unclear if complexity is linear in the number of elements or buckets | clarified that it's linear in the number of elements

### See also

erase (C++11) | erases elements (public member function)

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