# std::unordered_set<Key,Hash,KeyEqual,Allocator>::begin, std::unordered_set<Key,Hash,KeyEqual,Allocator>::cbegin

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< cpp‎ | container‎ | unordered set

iterator begin() noexcept; | (1) | (since C++11)
const_iterator begin() const noexcept; | (2) | (since C++11)
const_iterator cbegin() const noexcept; | (3) | (since C++11)

Returns an iterator to the first element of the `unordered_set`.

If the `unordered_set` is empty, the returned iterator will be equal to end().

### Parameters

(none)

### Return value

Iterator to the first element.

### Complexity

Constant.

### Notes

Because both `iterator` and `const_iterator` are constant iterators (and may in fact be the same type), it is not possible to mutate the elements of the container through an iterator returned by any of these member functions.

### Example

Run this code

    #include <iostream>
    #include <unordered_set>

    struct Point { double x, y; };

    int main()
    {
        Point pts[3] = {{1, 0}, {2, 0}, {3, 0}};

        // points is a set containing the addresses of points
        std::unordered_set<Point*> points = { pts, pts + 1, pts + 2 };

        // Change each y-coordinate of (i, 0) from 0 into i^2 and print the point
        for (auto iter = points.begin(); iter != points.end(); ++iter)
        {
            (*iter)->y = ((*iter)->x) * ((*iter)->x); // iter is a pointer-to-Point*
            std::cout << "(" << (*iter)->x << ", " << (*iter)->y << ") ";
        }
        std::cout << '\n';

        // Now using the range-based for loop, we increase each y-coordinate by 10
        for (Point* i : points)
        {
            i->y += 10;
            std::cout << "(" << i->x << ", " << i->y << ") ";
        }
    }

Possible output:

    (3, 9) (1, 1) (2, 4)
    (3, 19) (1, 11) (2, 14)

### See also

end cend (C++11) | returns an iterator to the end (public member function)
begin cbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)

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