Do C++20 `std::views` store copies or references to their input ranges?

Question

C++20 adds numerous std::views::... range adaptors.

When applied to a range, do they store a copy of the range, or a reference to it?

In other words:

std::vector<A> v;
for (const auto &elem : v | std::views::reverse) {}

Does this copy the vector, or stores a reference to it?

And if the range is an rvalue, does that affect the behavior?

I'm asking about the bulk of the views in std::views, as I assume most of them share the same behavior.

Some views appear to be special-cased on this, e.g. std::views::all is said to store references to lvalues but move rvalues. It's not clear to me if the remaining views do the same thing, or do something else?

I found some related wording in RangeAdaptorClosureObject, but that deals with combining adaptors together (which copies/moves the adaptors being combined), not applying an adaptor to a range.

I tried passing a vector of a non-copyable type to std::views::reverse and didn't get an error, so I assume at least lvalues are not copied, but I can't find where this is specified.

Does [language-lawyer] indicate you asking for a quote from the standard? If cppreference is good enough, a "view" is a lightweight object that indirectly represents iterable sequences, which must be copyable in constant time. Like std::string_view. — Drew Dormann
– Drew Dormann, Commented Aug 12 at 17:38
@DrewDormann I would prefer a quote, yes. Counterexample: std::ranges::owning_view stores a copy. — HolyBlackCat
– HolyBlackCat, Commented Aug 12 at 18:19
@DrewDormann But ignoring the language-laywering, are you saying that auto view = vec | a | b dangles whatever is returned by vec | a? — HolyBlackCat
– HolyBlackCat, Commented Aug 12 at 18:40
It does not -- it effectively creates a new view that "absorbs" vec | a. So it'll work. On that topic, std::ranges does some very clever things to catch "dangling" data that earlier stdlib functions would not. Dangling references become compiler errors. — Drew Dormann
– Drew Dormann, Commented Aug 12 at 18:50
@DrewDormann Ok, so you're saying that views get copied/moved, while anything else is stored by reference? If not a standard quote, do you know any documentation on this? — HolyBlackCat
– HolyBlackCat, Commented Aug 12 at 18:51

T.C. · Accepted Answer · 2025-08-12 18:53:22Z

12

The general rules are (which are the same as all's rules):

If the input is a view, it's copied or moved depending on the value category and never taken by reference.
If the input is not a view, it's moved (into an owning_view) if an rvalue, and a reference is taken if an lvalue.

For most unary adaptors, this is done by using views::all_t directly in the spec (example). For the rest, they are specified to use CTAD (example), and the applicable deduction guide will use views::all_t on the source range (example).

answered Aug 12 at 18:53

T.C.

139k18 gold badges306 silver badges440 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Bernard Gressing Aug 13 at 0:43

The order of your two points are confusing, the 2nd one should have been stated before the first one, otherwise it's a generally ok answer.

Collectives™ on Stack Overflow

Do C++20 `std::views` store copies or references to their input ranges?

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related