25 Iterators library [iterators]

25.5 Iterator adaptors [predef.iterators]

25.5.5 Common iterators [iterators.common]

25.5.5.6 Comparisons [common.iter.cmp]

template<class I2, sentinel_for<I> S2>
  requires sentinel_for<S, I2>
friend constexpr bool operator==(
  const common_iterator& x, const common_iterator<I2, S2>& y);

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.

Returns: true if i == j, and otherwise get<i>(x.v_) == get<j>(y.v_), where i is x.v_.index() and j is y.v_.index().

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template<class I2, sentinel_for<I> S2>
  requires sentinel_for<S, I2> && equality_comparable_with<I, I2>
friend constexpr bool operator==(
  const common_iterator& x, const common_iterator<I2, S2>& y);

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.

Returns: true if i and j are each 1, and otherwise get<i>(x.v_) == get<j>(y.v_), where i is x.v_.index() and j is y.v_.index().

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template<sized_sentinel_for<I> I2, sized_sentinel_for<I> S2>
  requires sized_sentinel_for<S, I2>
friend constexpr iter_difference_t<I2> operator-(
  const common_iterator& x, const common_iterator<I2, S2>& y);

Preconditions: x.v_.valueless_by_exception() and y.v_.valueless_by_exception() are each false.

Returns: 0 if i and j are each 1, and otherwise get<i>(x.v_) - get<j>(y.v_), where i is x.v_.index() and j is y.v_.index().