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[range.chunk.fwd.iter, range.stride.iterator] What precondition is implied here? #5730

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hewillk opened this issue Aug 16, 2022 · 4 comments
Open

[range.chunk.fwd.iter, range.stride.iterator] What precondition is implied here? #5730

hewillk opened this issue Aug 16, 2022 · 4 comments

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@hewillk
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hewillk commented Aug 16, 2022

From [range.chunk.fwd.iter-12] / [range.stride.iterator-13]:

constexpr iterator& operator+=(difference_type n) requires random_­access_­range<Base>;

Preconditions: If n is positive, ranges​::​distance(current_­, end_­) > stride_­ * (n - 1) is true.
[Note 1: If n is negative, the Effects paragraph implies a precondition. — end note]

Effects: Equivalent to:

if (x > 0) {
  missing_ = ranges::advance(current_, n_ * x, end_);
} else if (x < 0) {
  ranges::advance(current_, n_ * x + missing_);
  missing_ = 0;
}
return *this;

Maybe it's my English comprehension issue, "If x is negative, the Effects paragraph implies a precondition." What does this mean? What precondition is implied here?
@JohelEGP
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According to https://eel.is/c++draft/description#structure.specifications-4, that'd be the Preconditions of the call(s) to ranges::advance.

@hewillk
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hewillk commented Aug 16, 2022 •
edited

Thanks for the clarification.
However, according to the description of ranges::advance in [range.iter.op.advance]:

template<input_­or_­output_­iterator I>
  constexpr void ranges::advance(I& i, iter_difference_t<I> n);

Preconditions: If I does not model bidirectional_­iterator, n is not negative.
Effects:

  • If I models random_­access_­iterator, equivalent to i += n.
  • Otherwise, if n is non-negative, increments i by n.
  • Otherwise, decrements i by -n.

There are no Preconditions for random_access_iterator, that is to say, the Precondition of calling ranges::advance here has already been satisfied, isn't it? so I don't see why need to take a Note here again.

@JohelEGP
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So, according to that, that'd be the Preconditions on i += n.
The original call ranges::advance(current_, n_ * x + missing_); is equivalent to current_ += n_ * x + missing_. The note must be referring to a precondition on the evaluation of this latter +=.

@tkoeppe
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tkoeppe commented Sep 16, 2022

Is there an editorial issue here? The wording is as requested in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1899r3.html. Do you have an improvement in mind? @cjdb, @timsong-cpp, any thoughts?

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@tkoeppe @JohelEGP @hewillk