What is an istream_view and when do I use one?

Question

Apparently, C++20 has a new std::istream-related construct: std::istream_view. The cppreference page on it is a stub right now^†. So, what is a "view of an istream" and what can I use it for?

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_{† - Ok, technically it redirects to a page about std::basic_istream_view and that one's a stub.}

Answer 1

An std::istream_view<T> is a range; and more specifically, a range formed as a view. This addition to the standard library is akin to what you might find under std::ranges::views - except that it's not a view of an arbitrary range, but of an std::istream.

So what "viewing" is applied to an std::istream? Recall an istream is a stream of characters, not of arbitrary T-type elements of your choice. The lazy application of parsing those characters into consecutive T's is the "viewing" of the the istream. That is, the k'th element of std::istream_view<T>(is) is what you'd get the k'th time running is >> t for t of type T.

You would use an std::istream_view (carefully) when you want to apply your code, which works with ranges, directly to input data - rather than first parsing your input into some data structure in a more "old-school" manner, then working on that structure as a range.

Other takes on what an std::istream_view is:

• @Barry has described it as the equivalent of a coroutine which parses Ts from an istream; read this answer for details (note it's about the istream view in the ranges-v3, much of which became the standard ranges library).
• @NicolBolas considers it to be the result of "wrapping std::istream_iterator<T> in a C++20 view interface".