# std::common_iterator<I,S>::common_iterator

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< cpp‎ | iterator‎ | common iterator

constexpr common_iterator() requires std::default_initializable<I> = default; | (1) | (since C++20)
constexpr common_iterator( I i ); | (2) | (since C++20)
constexpr common_iterator( S s ); | (3) | (since C++20)
template< class I2, class S2 > requires std::convertible_to<const I2&, I> && std::convertible_to<const S2&, S> constexpr common_iterator( const common_iterator<I2, S2>& x ); | (4) | (since C++20)

Constructs a new iterator adaptor, effectively initializes the underlying std::variant<I, S> member object `var` to hold an `I` (iterator) or `S` (sentinel) object.

1) Default constructor. Default-initializes `var`. After construction, `var` holds a value-initialized `I` object. Operations on the resulting iterator adaptor have defined behavior if and only if the corresponding operations on a value-initialized `I` also have defined behavior.
2) After construction, `var` holds an `I` object move-constructed from `i`.
3) After construction, `var` holds an `S` object move-constructed from `s`.
4) After construction, `var` holds an `I` or `S` object initialized from the `I2` or `S2` held by x.var, if x.var holds that alternative, respectively. The behavior is undefined if `x` is in an invalid state, that is, x.var.valueless_by_exception() is equal to true.

### Parameters

i | - | iterator to adapt
s | - | sentinel to adapt
x | - | iterator adaptor to copy

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <numeric>
    #include <vector>

    int main()
    {
        std::vector v {3,1,4,1,5,9,2};

        using CI = std::common_iterator<
                       std::counted_iterator<std::vector<int>::iterator>,
                       std::default_sentinel_t
                       >;
        CI unused; // (1)
        CI start  { std::counted_iterator{std::next(begin(v)), ssize(v)-2} }; // (2)
        CI finish { std::default_sentinel }; // (3)
        CI first  { start };  // (4)
        CI last   { finish }; // (4)

        std::copy(first, last, std::ostream_iterator<int>{std::cout, " "});
        std::cout << '\n';

        std::common_iterator<
          std::counted_iterator<
            std::ostream_iterator<double>>,
            std::default_sentinel_t>
            beg { std::counted_iterator{std::ostream_iterator<double>{std::cout,";  "}, 5} },
            end { std::default_sentinel };
        std::iota(beg, end, 3.1);
        std::cout << '\n';
    }

Output:

    1 4 1 5 9
    3.1;  4.1;  5.1;  6.1;  7.1;

### See also

operator= (C++20) | assigns another iterator adaptor (public member function)

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