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

Question

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

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!

Answer 1

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.

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^† 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.