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Here it is written that std::ranges::size should return an unsigned integer. However, when I use it on an Eigen vector (with Eigen 3.4) the following compiles:

Eigen::VectorXd x;
static_assert(std::same_as<Eigen::VectorXd::Index,
                           decltype(std::ranges::size(x))>);

where Eigen::VectorXd::Index is notoriously a signed integer. By looking at the implementation of std::ranges::size, I noticed that the return type is inferred from the return type of x.size(), which is precisely Eigen::VectorXd::Index. Is this a bug of std::ranges::size? Or is this expected?

Update 27/12/2021

The C++ reference page linked above has eventually changed the description of the std::ranges::size function: it only returns an integer, not necessarily an unsigned one!

fdev
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    A container's `size()` method is supposed to return an unsigned integer. The problem is at the Eigen's side. – Ruslan Dec 22 '21 at 18:33
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    `Otherwise, t.size() converted to its decayed type, if ranges::disable_sized_range> is false, and the converted expression is valid and has an integer-like type.` Looks like underspecified for containers which return signed types for sizes. – SergeyA Dec 22 '21 at 18:34
  • When the call is valid, if no user-defined `size()` functions is involved, it returns an unsigned integer (or integer-class) value. Otherwise, it returns the return value of that `size()` function. – F.v.S. Oct 20 '22 at 10:16

2 Answers2

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It's very much intended that the sized_range concept and the corresponding ranges::size customization point take no position on the signed/unsigned holy wars. ranges::size does convert to unsigned when computing from the distance between begin and end, for consistency with the existing standard library, but user types are not required to agree with that choice.

T.C.
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  • Too bad, I feel it would be better if `std::ranges::size` always returned an unsigned integer and `std::ranges::ssize` always an unsigned one! Why not? What is the point? – fdev Dec 22 '21 at 20:13
  • @fdev not part of standardization so IDK, but I guess maybe they did ranges before they agreed on ssize – NoSenseEtAl Feb 01 '22 at 22:14
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Is this a bug of std::ranges::size?

No. The cppreference documentation is misleading. There is no requirement for std::ranges::size to return an unsigned integer. In this case, it returns exactly what Eigen::VectorXd::size returns.

For ranges that model ranges::sized_range, that would be an unsigned integer, but Eigen::VectorXd evidently does not model such range.

But then what is the purpose of std::ranges::ssize compared to std::ranges::size?

The purpose of std::ranges::ssize is to be a generic way to get a signed value regardless of whether std::ranges::size returns signed or unsigned. There is no difference between them in cases where std::ranges::size returns a signed type.

Is there a reference to back up what you state?

Yes. See the C++ standard:

[range.prim.size]

Otherwise, if disable_­sized_­range<remove_­cv_­t<T>> ([range.sized]) is false and auto(t.size()) is a valid expression of integer-like type ([iterator.concept.winc]), ranges​::​size(E) is expression-equivalent to auto(​t.size()).

eerorika
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    But then what is the purpose of `std::ranges::ssize` compared to `std::ranges::size`? Is there a reference to back up what you state? – fdev Dec 22 '21 at 18:57
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    Also, `Eigen::VectorXd` *does* satisfy the `std::ranges::sized_range` concept! – fdev Dec 22 '21 at 18:59
  • @fdev See edited answer for your first comment. `Eigen::VectorXd does satisfy the std::ranges::sized_range concept` I disagree; It doesn't satisfy the requirements. – eerorika Dec 22 '21 at 19:10
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    Actually it does: the only requirements to satisfy the `std::ranges::sized_range` concept are that `Eigen::VectorXd` is a range (which it is since Eigen 3.4) and that `std::ranges::size` can be called on `Eigen::VectorXd` objects. – fdev Dec 22 '21 at 19:21
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    To be fair, the cppreference documentation just mention that [`std::ranges::size()`](https://en.cppreference.com/w/cpp/ranges/size#Notes) *"Whenever ranges::size(e) is valid for an expression e, **the return type is integer-like**."*, while [`std::ranges::ssize()`](https://en.cppreference.com/w/cpp/ranges/ssize#Notes) *"If ranges::ssize(e) is valid for an expression e, **the return type is a signed-integer-like type**, i.e. an integer type for which std::is_signed_v is true, or a signed-integer-class type. "* – Bob__ Dec 22 '21 at 19:21
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    @Bob__ Yeah, those are accurate. The misleading part is on the ranges library page linked in the question. – eerorika Dec 22 '21 at 19:24