std::common_iterator::common_iterator(3)

NAME
std::common_iterator::common_iterator - std::common_iterator::common_iterator

Synopsis
   constexpr common_iterator() requires std::default_initializable<I> (1) (since C++20)
   = default;
   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> && (4) (since C++20)
            std::convertible_to<const S2&, S>
   constexpr common_iterator( const common_iterator<I2, S2>& x );
   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= assigns another iterator adaptor
   (C++20) (public member function)