What's the difference between a "range" and a "view" in the rangesv3 ts?

Question

Don't find any similar answers on G search. Guess I'm struggling with a basic overview of what each one is supposed to do:

is it the case (in c++ speak) that range 'is-a' view, or vice-versa?

Is it simply that a view is read-only range? Or perhaps that the "elements" in the range (what you get when you dereference the iterator) are const in one and not in the other?

Thanks!

Don't you mean `view` and `action`? A `view` is a range, it's just reads only. — NathanOliver, Mar 02 '18 at 21:24
Range V3 has [documentation that explains this pretty well](https://ericniebler.github.io/range-v3/index.html#range-views) — Justin, Mar 02 '18 at 21:26
No, I do mean Range: reference: https://github.com/ericniebler/range-v3 — r webby, Mar 02 '18 at 21:26

score 13 · Accepted Answer · edited Oct 14 '22 at 07:29

From the docs, a range is:

A range can be loosely thought of a pair of iterators, although they need not be implemented that way.

and:

A view is a lightweight wrapper that presents a view of an underlying sequence of elements in some custom way without mutating or copying it. Views are cheap to create and copy, and have non-owning reference semantics.

A view is a range, just with more restrictions.

More formalized definitions in the TS are in the Range and View concepts. Basically, a range is something that's iterable over, and a view is a range that is semiregular^† and has constant time copy/move/assignment/begin/end/...

For instance, std::vector<char>, std::string, and std::string_view are all ranges, but only the last one is also a view.

^† While in the Ranges TS, a View was always semiregular, this restriction was relaxed in P1456. In C++20, a View is instead required to be only ~~default constructible and~~^‡ movable. The additional semantic constraints (all^‡ the operations being constant time) still hold. Notably: while a View need not be copyable, if it is copyable, then those copying operations still need to take constant time.

^‡The default construction restriction was lifted in P2325 and other requirements were further relaxed in P2415.

"a view is a range that is semiregular and has constant time copy/move/assignment." and `begin`/`end`. *All* of the required operations for `View` are required to be O(1). — Casey, Mar 03 '18 at 00:46
Thanks for the ref to the eric niebler's work. This is just like howard hinnant's chrono, where the best exposition of the material now in standard C++ is the original author's documentation. — TooTone, Aug 09 '23 at 11:55

What's the difference between a "range" and a "view" in the rangesv3 ts?

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