# operator-(std::common_iterator)

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< cpp‎ | iterator‎ | common iterator

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.

### Parameters

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).

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>

    int main()
    {
        int a[] {0, 1, 2, 3, 4, 5};

        using CI = std::common_iterator<
                       std::counted_iterator<int*>,
                       std::default_sentinel_t
                       >;

        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';
    }

Output:

    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

operator++operator++(int) (C++20) | advances the iterator adaptor (public member function)

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