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.


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


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


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

See also

assigns another iterator adaptor
(public member function)

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