P2636R2
References to ranges should always be viewable

1. Revision History

1.1. Revision 2 - 2023-01-14

• Rewrote and expanded the section Implication for the "meaning" of views.

• Expanded the section on Lifetime to include many more examples.

• Added std::views::maybe_ref to the Workarounds section.

• Renamed the types in the examples for easier understanding.

• Fixed various issues that Barry Revzin reported.

• Removed "7.3. P2387" again which was available in a draft of this paper.

• There was no consensus on pursuing the changes proposed here, so this revision merely documents the last state of the proposal.

1.2. Revision 1 - 2022-10-14

• Some language corrections and smaller additions to text.

• Added "The solution (TL;DR)".

• Added "Implication for the "meaning" of views".

• Added section on std::views::ref.

• Feature test macro added to proposed wording.

1.3. Revision 0 - 2022-09-14

• Initial version.

2. Introduction

When dealing with views, two concepts are important: view ("ranges that are views themselves") and viewable_range ("ranges that can be turned into views"). Initially, the requirements for the first were very strict, encompassing only non-owning, semi-regular types. The second concept encompassed "views and any lvalue-references to ranges".

Over time, both concepts were relaxed considerably. Views can now be move-only (P1456) and owning (P2415), rvalues of non-views are now viewable––in fact most ranges people encounter are now viewable. However, in one particular case, the concepts are now stricter than before: lvalue-references to certain ranges, move-only views, are not viewable. This means that while almost all non-view ranges can be easily composed with view adaptors, certain views now do not qualify for this.

This issue was briefly discussed on the LWG reflector in 2020 where another member highlighted the wide-ranging impact of this change:

I’m concerned about having components that work with ranges but not views, given that views are supposed to be just ranges with additional properties.

A possible "fix" was mentioned in that discussion:

I can envision an alternate design in which views:all(E) does admit move-only lvalue views [...] I think that design change needs LEWG approval [...]

But apparently there was no support for this change or no will to pursue it further. This paper picks up that discussion and argues why such a change would indeed make a lot of sense.

2.1. The problem (TL;DR)

void foobar(ranges::forward_range auto && r)
{
    auto v = r | views::take(3);
}

In the "one ranges proposal" and in multiple shipped compiler versions, the above code was well-formed for all forward_ranges. Since P1456, the function template is now underconstrained and will become ill-formed for certain inputs.

2.2. The solution (TL;DR)

Change the definition of views::all (and viewable_range) so that:

• rvalues are moved

• lvalues are copied if that is possible and cheap

• other lvalues are wrapped in ref_view except lvalues to views

This means that all lvalue ranges become viewable. The only non-viewable ranges are non-movable rvalues.

3. Detailed examples

To illustrate the different behaviours, I have chosen to provide examples that start out with (1) a copyable view, (2) a copyable non-view range, (3) a move-only non-view range, and (4) a move-only view.

As example types for non-copyable ranges, I use MoveOnlyRange and MoveOnlyView. MoveOnlyRange could refer to a user-provided type that generates objects upon iteration (e.g. records of data from a stream); it is a move-only single-pass input-range.

The type MoveOnlyView is the same as MoveOnlyRange except that it also inherits from the empty base class ranges::view_base. This has no influence on the semantics of the type other than it begin treated as a view by current standard libraries. While we might suggest to users that such ranges should always be implemented as views, we cannot assume that this will always be the case (there are ranges that are neither containers nor views!).

Note, that these examples are also discussed in the presentation I gave in Kona.

3.1. Views from lvalue reference

string_view s{"foobar"};
auto v = s | views::take(3);

string s{"foobar"};
auto v = s | views::take(3);

MoveOnlyRange g{};
auto v = g | views::take(3);

MoveOnlyView g{};
auto v = g | views::take(3);

Before P1456 and P2415, all of these snippets would have been well-formed. Currently, (4) is not, because lvalue references to move-only views are not viewable.

This also means that code is rejected by current compilers that was accepted by previous versions of the same compiler (GCC, MSVC). While this is true for other C++20 features, the particular changes to the view concepts were discussed mainly as relaxing constraints and not as introducing new ones.

3.2. View composition (starting from lvalue reference)

string_view s{"foobar"};
auto v1 = s  | views::take(3);
auto v2 = v1 | views::transform(/**/);

string s{"foobar"};
auto v1 = s  | views::take(3);
auto v2 = v1 | views::transform(/**/);

MoveOnlyRange g{};
auto v1 = g  | views::take(3);
auto v2 = v1 | views::transform(/**/);

MoveOnlyView g{};
auto v1 = g  | views::take(3);
auto v2 = v1 | views::transform(/**/);

(4) is still ill-formed; the rest are well-formed.

3.3. Views from rvalues

auto v = string_view{"foobar"} | views::take(3);

auto v = string{"foobar"} | views::take(3);

auto v = MoveOnlyRange{} | views::take(3);

auto v = MoveOnlyView{} | views::take(3);

All of these snippets are currently well-formed code. In the original ranges proposal, only (1) would have been well-formed, because the others are rvalues of non-views; (4) would not have been regarded as a view, because it is move-only.

3.4. View composition (starting from rvalue)

auto v1 = string_view{"foobar"} | views::take(3);
auto v2 = v1 | views::transform(/**/);

auto v1 = string{"foobar"} | views::take(3);
auto v2 = v1 | views::transform(/**/);

auto v1 = MoveOnlyRange{} | views::take(3);
auto v2 = v1 | views::transform(/**/);

auto v1 = MoveOnlyView{} | views::take(3);
auto v2 = v1 | views::transform(/**/);

(2)-(4) are not well-formed, because v1 in the respective snippet is move-only. Note, how the well-formed snippets in this example are different from those created from lvalues. If the | views::transform(/**/) had been appended to the first line and no v2 created, it would have worked. This means that––contrary to what many previous ranges guides suggest––it makes an important difference now whether you compose the view incrementally or in one line. And it appears that "creating in one line" needs to be recommended.

3.5. Generic code

Returning to the problem of generic code, assume the following:

(definitions for to_upper and to_lower omitted)

void print_up_low(ranges::forward_range auto && rng)
{
    for (char c : rng | views::transform(to_upper))
        cout << c;
    cout << '\n';
    for (char c : rng | views::transform(to_lower))
        cout << c;
}

print_up_low(string_view{"foobar"});
print_up_low(string_view{"foobar"} | views::take(3));

print_up_low(string{"foobar"});
print_up_low(string{"foobar"} | views::take(3));

string_view s{"foobar"};
print_up_low(s);
print_up_low(s | views::take(3));

string s{"foobar"};
print_up_low(s);
print_up_low(s | views::take(3));

These examples show that it is easy to run into problems with "move-only views", even when just using simple adaptors on types likes strings and string_views. Is the behaviour shown above intended? Do we expect developers to understand why 1 of 8 calls to print_up_low() is ill-formed and why?

Curiously, it is now the example of "creating a view in one line" that is ill-formed, while previous examples suggested that exactly this usage pattern increases the likelihood of the code being well-formed.

4. Proposal

This papers suggests making all previously shown ill-formed snippets well-formed by changing the viewable_range concept and the definition of the views::all adaptor.

4.1. Wording

26.4.5 Other range refinements [range.refinements]

Change the definition of viewable_range to:

template<class T>
  concept viewable_­range =
    range<T> &&
    ((view<remove_cvref_t<T>> && constructible_­from<remove_cvref_t<T>, T>) ||

(!view<remove_cvref_t<T>> && (is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>))));

is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>));

[26.7.6.1 General [range.all.general]]

Change the definition of views:all to

• (2.1) decay-copy(E) if that expression is well-formed and the decayed type of E models view.
• (2.2) Otherwise, ref_­view{E} if that expression is well-formed.
• (2.3) Otherwise, owning_­view{E}.

[17.3.2 version.syn]

Bump the __cpp_lib_ranges version.

4.2. Impact

These changes are breaking insofar as all concept changes are always potentially breaking. However, this proposal very clearly relaxes the constraints on viewable_range so the scope is broadened, and in practice more code will work than before and not less. In fact, this will unbreak code that was well-formed in earlier published versions of the IS (and is well-formed in released versions of GCC and MSVC) but that has been broken in the meantime.

Should LEWG accept this proposal for C++23, it could ask LWG to denote it as a C++20 defect fix. There is precedent for this, because the view and viewable_range concepts have already been changed multiple times with wide-ranging modifications being back-ported to C++20 and even older compiler versions (GCC10, GCC11); although technically these changes were feature changes more than fixes. The current changes are much smaller in scope and re-establish initial ranges behaviour.

5. "Workarounds"

For the previously shown ill-formed snippets, it is possible to work around the problem by any or either of the following methods:

1. move-ing the initial range into the pipe (resulting in an decay_copy or owning_view); or

2. manually wrapping the initial range in ref_view

Since most users never directly interact with views::all, ref_view and owning_view, I assume that it will be very hard to explain where (and which of) these workarounds are necessary. Furthermore, this creates a burden on generic code that will either need to if constexpr on ranges passed to it by reference or blindly wrap them in ref_view to make them viewable.

Note also that "just adding ranges::ref_view" in a generic context may have undesired side-effects. Even for ranges that are already viewable, it is not a no-op; e.g. it prevents copyable views from being captured by value which changes their behaviour with regard to const (they are now shallow const but would have been deep const before). It also changes type-specific behaviour in view adaptor objects like the "same-type optimisations" in views::take and views::drop.

5.1. std::views::ref

It was suggested that having views::ref as a shortcut for ranges::ref_view would alleviate the problems presented here. While I do not object to having such a view factory, I do not see how it solves the fundamental problem of a bad default.

It is of no consequence to almost all algorithms whether they are operating on a move-only view or a copyable range, so we shouldn’t require a workaround.

5.2. std::views::maybe_ref

During the Kona meeting, Barry Revzin proposed a range adaptor closure object that acts like views::all for all viewable ranges and like (a potential) views::ref for non-viewable ranges, in particular move-only views.

This avoids the indirection for copyable views, but it does not solve the fundamental problem of a bad default. Such an adaptor would need to be inserted into every generic algorithm that uses range composition.

If that is going to be the official recommendation, why can’t we just make views::all behave in that way by default?

6. Lifetime

Previous mailing list comments suggested that the changes proposed here would create issues with view lifetime. The following three examples explore some of those implications.

6.1. Returning views on local variables

string_view s{"foobar"};
return s  | views::take(3);

string s{"foobar"};
return s | views::take(3);

MoveOnlyRange g{};
return g | views::take(3);

MoveOnlyView g{};
return g | views::take(3);

Returning a view created from a local variable can create a dangling reference. This has always been the case and is inherent to ranges::ref_view (and independent of the underlying range). While it could be argued that preventing it for some ranges is beneficial, it is unclear why this should only be the case for move-only views and not other ranges; especially since example (2) is likely to appear more often than example (4) and the range in (3) is semantically identical to the range in (4).

On a sidenote: Now that we have move-only owning views, one could make an argument for making all uses of ranges::ref_view explicit (removing the option from views::all and recommending that users move into a view composition by default or explicitly opt into having a reference if desired). However, such a change would be quite disruptive, so pursuing this argument at this point seems futile.

6.2. Returning views on in-parameters

6.2.1. Capturing in-parameter as lvalue

auto foo(auto && rng)
{
  return rng | views::take(3) | views::transform(/**/);
}

The rvalue cases are essentially the same as in the previous example, but for the lvalue cases one would become well-formed (and not dangling) with the changes proposed here.

6.2.2. Splitting range composition into multiple lines

auto foo(auto && rng)
{
  auto v = rng | views::take(3);
  return v | views::transform(/**/);
}

The concern was raised that splitting a single statement into multiple ones would create dangling cases from previously ill-formed cases. When capturing the range as lvalue from an in-parameter (which may be an lvalue or rvalue itself), this is not the case; the same cases are ill-formed/dangling as before.

6.2.3. Forwarding the in-parameter

auto foo(auto && rng)
{
  return FWD(rng) | views::take(3) | views::transform(/**/);
}

Allthough this is not the usually recommended form of invoking range compositions, forwarding into the range composition fixes all the dangling cases. This proposal does not affect that.

6.2.4. Splitting range composition into multiple lines + FWDing

auto foo(auto && rng)
{
  auto v = FWD(rng) | views::take(3);
  return v | views::transform(/**/);
}

The concern was raised that splitting a single statement into multiple ones would create dangling cases from previously ill-formed cases. When we forward or move values into the range composition, this is true. However, we need to be aware of three things here:

1. The recommended way of dealing with ranges in generic code, is to capture by && and pass on as lvalue, not to forward/move (see also below). Since that is the easier/shorter syntax, it is reasonable to assume that developers who make a choice to forward/move, do so intentionally and know what they are doing.

2. If you explicitly forward/move something into the initialising expression of a local variable, it is obvious that that variable can take ownership of the respective dynamic memory; that’s the whole point.

3. There is a very easy fix: move(v) into the returning expression, as well; this will make all examples well-formed and non-dangling. It is just a matter of forwarding/moving consistently. Either you don’t do it all, or you do it for all "lines" of the range composition.

7. Implication for the "meaning" of views

Objections to this proposal include that it would distort the meaning of views and that »[t]he entire point of a view is that this is something that should be taken by value and owned«.

Note, that this proposal does not change the definition of view at all!

So what is implied here, is that the meaning of view is actually hidden in the definition of viewable_range (which itself uses the definition of view…). We will encounter this logical problem again below.

In any case, I disagree with the objection raised and want to argue instead that (a) views are not generally taken by value; and (b) the view concept is not useful (in most generic code) anyway.

In fact, part of this argument is already made in P2415:

The initial thought […] might be that […] we write algorithms that take views by value: […] and we definitely do encourage people to take specific views by value - such as span and string_view […] Either way, we very much want range-based algorithms to be able to operate on, well, ranges, so these are always written instead to take ranges by forwarding reference:

template <input_range R>
void some_algo(R&& r);

→ we do not actually take views by value in generic code, and we do not use the view concept to constrain algorithms.

As far as "the meaning of views" is concerned, I would like to have a closer look how this changed from early proposals to the current state, and how "views" and "range composition" are related.

Evidently, viewable_range is used to constrain "what goes into" a range composition, and view "what comes out", so the implementors of range composition facilities (need to) care about these concepts. But it does not yet provide evidence of their meaning to the users of range composition.

It should also be noted that there are views that are independent of range composition (e.g. span, string_view), and that range factories (e.g. views::iota) can be used standalone, as well.

7.1. P0896: The One Ranges proposal

P0896: The One Ranges proposal

1. ranges::view: a semi-regular range with O(1) operations

2. ranges::viewable_range: any lvalue of a range (including views) or rvalue of a view

The definition of view is very straight-forward here; in particular the notion of "copying something in constant time" is easy to explain. Views were often summarised as "light-weight ranges" or "non-owning ranges". Furthermore, it is easy to illustrate these properties by using string_view and span (the simplest views) as examples. Importantly, one does not need to refer to "range composition" at all, in explaining what a view is.

Since it was/is recommended, in algorithms code, to pass ranges by lvalue into range compositions, the role of ranges::viewable_range is purely technical. Users neither constrain their algorithms with it, nor do they explicitly try to make their types model this concept. The meaning of viewable_range is not obvious, but it does not need to be; as far as users are concerned, it could have been exposition-only.

Generic ranges algorithms were not (and are not) constrained with the view concept, but its "cheap copy" properties might allow its use e.g. as constraints in contexts that rely on pass-by-value. Furthermore, views were the only ranges that you could pass to range compositions as rvalues, so there was a clear incentive/benefit to opting into the view concept for a user-defined type: it enabled usage patterns that were not possible for types that did not model view.

7.2. P1456 → LWG3481 → P2415 (current state)

P1456: Move-only views
LWG3481: viewable_range mishandles lvalue move-only views
P2415: What is a view?

1. ranges::view: a range that is copyable in O(1) or not copyable at all (more technical requirements ommitted)

2. ranges::viewable_range: any rvalue of a movable range (including views) or lvalue of range except move-only views

Views, the types of objects returned by range compositions, can now own their elements and need no longer be copyable at all. This makes it more difficult to explain what a view is, and P2415 can only give this answer: