C++20 ranges and sorting

I'm dealing with the last big 4 of C++ 20, attempting to learn the new main features. Trying some code from the web related to ranges, I've written:

std::vector ints{ 6, 5, 2, 8 }; auto even = [](int i) { return 0 == i % 2; }; // ranges... auto rr = ints | std::views::filter(even) | std::views::transform([](auto i) { return i * i; }) | std::views::reverse; 

Then I would sort, like range-v3 does with |action::sort, but I've understand that this implementation is not the same.

The way I've found to sort is:

ints = std::vector(std::ranges::begin(rr), std::ranges::end(rr)); std::ranges::sort(ints); 

Am I wrong? Does anybody know how to sort with pipe style the view ?

1 Answer

Then I would sort, like range-v3 does with |action::sort ...

No, you can't actually sort rr like this:

rr |= ranges::actions::sort; // error 

because rr is a view. While views can provide mutable access to the underlying range, sort additionally needs the range to support random access. A lazily generated view like rr does not allow this.

You can create a vector from rr as you have done, and then you can use actions on that range:

ints |= ranges::actions::sort; // ok 

c++20, however, doesn't have any actions (hopefully we'll get them in c++23), so until then you'll have to call the algorithm sort without the pipe syntax:

std::ranges::sort(ints); // ok