template< std::sized_sentinel_for<I> I2, std::sized_sentinel_for<I> S2 >
    requires std::sized_sentinel_for<S, I2>
friend constexpr std::iter_difference_t<I2>
    operator-( const common_iterator& x, const std::common_iterator<I2, S2>& y );
(since C++20)

Computes the distance between two iterator adaptors. Two sentinels are considered equal.

Let var denote the underlying std::variant member object in std::common_iterator, the behavior is undefined if either x or y is invalid, i.e. x.var.valueless_by_exception() || y.var.valueless_by_exception() is true.

This function template is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::common_iterator<I> is an associated class of the arguments.


x, y - iterator adaptors to compute the difference of

Return value

  • ​0​ if x.var holds an S object and y.var holds an S2 object, i.e. both of them hold a sentinel.
  • Otherwise, alt_x - alt_y, where alt_x and alt_y are the alternatives held by x.var and y.var, respectively (either two iterators or one iterator and one sentinel).


#include <algorithm>
#include <iostream>
#include <iterator>
int main()
    int a[] {0, 1, 2, 3, 4, 5};
    using CI = std::common_iterator<
    CI i1{ std::counted_iterator{ a + 1, 2 } };
    CI i2{ std::counted_iterator{ a, 3 } };
    CI s1{ std::default_sentinel };
    CI s2{ std::default_sentinel };
    std::cout << (s2 - s1) << ' '
              << (i2 - i1) << ' '
              << (i1 - s1) << '\n';


0 -1 -2

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3574 C++20 variant was fully constexpr (P2231R1) but common_iterator was not also made constexpr

See also

advances the iterator adaptor
(public member function)

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