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std::common_iterator<I,S>::common_iterator - cppreference.com

From cppreference.com

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.

If x.var.valueless_by_exception() is true, the behavior is undefined.

(until C++26)

If x.var.valueless_by_exception() is true:

(since C++26)

Parameters

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

Example

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

int main()
{
    std::vector v{3, 1, 4, 1, 5, 9, 2};
    
    template<typename Iter>
    using CommonIter = std::common_iterator<std::counted_iterator<Iter>,
                                            std::default_sentinel_t>;
    
    using CI = CommonIter<std::vector<int>::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';
    
    CommonIter<std::ostream_iterator<double>>
        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