25 Iterators library [iterators]

25.4 Iterator primitives [iterator.primitives]

25.4.4 Range iterator operations [range.iter.ops]

25.4.4.1 General [range.iter.ops.general]

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The library includes the function templates ranges​::​advance, ranges​::​distance, ranges​::​next, and ranges​::​prev to manipulate iterators.
These operations adapt to the set of operators provided by each iterator category to provide the most efficient implementation possible for a concrete iterator type.
[Example 1: 
ranges​::​advance uses the + operator to move a random_access_iterator forward n steps in constant time.
For an iterator type that does not model random_access_iterator, ranges​::​advance instead performs n individual increments with the ++ operator.
— end example]
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The function templates defined in [range.iter.ops] are not found by argument-dependent name lookup ([basic.lookup.argdep]).
When found by unqualified ([basic.lookup.unqual]) name lookup for the postfix-expression in a function call ([expr.call]), they inhibit argument-dependent name lookup.
[Example 2: void foo() { using namespace std::ranges; std::vector<int> vec{1,2,3}; distance(begin(vec), end(vec)); // #1 }
The function call expression at #1 invokes std​::​ranges​::​distance, not std​::​distance, despite that (a) the iterator type returned from begin(vec) and end(vec) may be associated with namespace std and (b) std​::​distance is more specialized ([temp.func.order]) than std​::​ranges​::​distance since the former requires its first two parameters to have the same type.
— end example]
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The number and order of deducible template parameters for the function templates defined in [range.iter.ops] is unspecified, except where explicitly stated otherwise.

25.4.4.2 ranges​::​advance [range.iter.op.advance]

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template<input_or_output_iterator I> constexpr void ranges::advance(I& i, iter_difference_t<I> n);
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Preconditions: If I does not model bidirectional_iterator, n is not negative.
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Effects:
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template<input_or_output_iterator I, sentinel_for<I> S> constexpr void ranges::advance(I& i, S bound);
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Preconditions: Either assignable_from<I&, S> || sized_sentinel_for<S, I> is modeled, or [i, bound) denotes a range.
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Effects:
  • (4.1)
    If I and S model assignable_from<I&, S>, equivalent to i = std​::​move(bound).
  • (4.2)
    Otherwise, if S and I model sized_sentinel_for<S, I>, equivalent to ranges​::​advance(i, bound - i).
  • (4.3)
    Otherwise, while bool(i != bound) is true, increments i.
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template<input_or_output_iterator I, sentinel_for<I> S> constexpr iter_difference_t<I> ranges::advance(I& i, iter_difference_t<I> n, S bound);
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Preconditions: If n > 0, [i, bound) denotes a range.
If n == 0, [i, bound) or [bound, i) denotes a range.
If n < 0, [bound, i) denotes a range, I models bidirectional_iterator, and I and S model same_as<I, S>.
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Effects:
  • (6.1)
    If S and I model sized_sentinel_for<S, I>:
    • (6.1.1)
      If ​|n|  ≥ |bound - i|, equivalent to ranges​::​advance(i, bound).
    • (6.1.2)
      Otherwise, equivalent to ranges​::​advance(i, n).
  • (6.2)
    Otherwise,
    • (6.2.1)
      if n is non-negative, while bool(i != bound) is true, increments i but at most n times.
    • (6.2.2)
      Otherwise, while bool(i != bound) is true, decrements i but at most -n times.
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Returns: n - M, where M is the difference between the ending and starting positions of i.

25.4.4.3 ranges​::​distance [range.iter.op.distance]

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template<class I, sentinel_for<I> S> requires (!sized_sentinel_for<S, I>) constexpr iter_difference_t<I> ranges::distance(I first, S last);
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Preconditions: [first, last) denotes a range.
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Effects: Increments first until last is reached and returns the number of increments.
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template<class I, sized_sentinel_for<decay_t<I>> S> constexpr iter_difference_t<decay_t<I>> ranges::distance(I&& first, S last);
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Effects: Equivalent to: return last - static_cast<const decay_t<I>&>(first);
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template<range R> constexpr range_difference_t<R> ranges::distance(R&& r);
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Effects: If R models sized_range, equivalent to: return static_cast<range_difference_t<R>>(ranges::size(r)); // [range.prim.size]
Otherwise, equivalent to: return ranges::distance(ranges::begin(r), ranges::end(r)); // [range.access]

25.4.4.4 ranges​::​next [range.iter.op.next]

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template<input_or_output_iterator I> constexpr I ranges::next(I x);
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Effects: Equivalent to: ++x; return x;
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template<input_or_output_iterator I> constexpr I ranges::next(I x, iter_difference_t<I> n);
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Effects: Equivalent to: ranges​::​advance(x, n); return x;
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template<input_or_output_iterator I, sentinel_for<I> S> constexpr I ranges::next(I x, S bound);
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Effects: Equivalent to: ranges​::​advance(x, bound); return x;
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template<input_or_output_iterator I, sentinel_for<I> S> constexpr I ranges::next(I x, iter_difference_t<I> n, S bound);
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Effects: Equivalent to: ranges​::​advance(x, n, bound); return x;

25.4.4.5 ranges​::​prev [range.iter.op.prev]

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template<bidirectional_iterator I> constexpr I ranges::prev(I x);
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Effects: Equivalent to: --x; return x;
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template<bidirectional_iterator I> constexpr I ranges::prev(I x, iter_difference_t<I> n);
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Effects: Equivalent to: ranges​::​advance(x, -n); return x;
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template<bidirectional_iterator I> constexpr I ranges::prev(I x, iter_difference_t<I> n, I bound);
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Effects: Equivalent to: ranges​::​advance(x, -n, bound); return x;