[libcxx-commits] [PATCH] D104262: [libcxx][ranges] Add contiguous_range.

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone added inline comments.


================
Comment at: libcxx/test/std/ranges/range.req/range.refinements/contiguous_range.compile.pass.cpp:42-46
+struct BadDataMember {
+  int *begin();
+  int *end();
+  char *data();
+};
----------------
cjdb wrote:
> zoecarver wrote:
> > cjdb wrote:
> > > Quuxplusone wrote:
> > > > I was initially confused by the name, as this class has //no// data members... but then I saw: the trick is that `X::data()` member function has a return type that is different from `std::add_pointer_t<ranges::range_reference_t<X>>`.
> > > > I think it'd be worth replacing this test (or at augmenting it) with both of these:
> > > > ```
> > > > struct WrongConstness {
> > > >     const int *begin() const;
> > > >     const int *end() const;
> > > >     int *data() const;
> > > > };
> > > > static_assert(std::ranges::random_access_range<WrongConstness>);
> > > > static_assert(!std::ranges::contiguous_range<WrongConstness>);
> > > > 
> > > > struct NotPtr {
> > > >     contiguous_iterator<const int*> begin() const;
> > > >     contiguous_iterator<const int*> end() const;
> > > >     contiguous_iterator<const int*> data() const;
> > > > };
> > > > static_assert(std::ranges::random_access_range<NotPtr>);
> > > > static_assert(!std::ranges::contiguous_range<NotPtr>);
> > > > ```
> > > > I was initially confused by the name, as this class has //no// data members... but then I saw: the trick is that `X::data()` member function has a return type that is different from `std::add_pointer_t<ranges::range_reference_t<X>>`.
> > > 
> > > I propose renaming to `BadDataMemberFunction`.
> > > 
> > > We should also have a check for when a range can satisfy `contiguous_range<R>`, but not `contiguous_range<R const>`.
> > Thanks for the suggestions. Arthur, I added your first test only (with some modification). The `NotPtr` test doesn't work because contiguous_iterator has an element_type. 
> This still isn't complete, because now we're missing a test case for when both `contiguous_range<T>` and `contiguous_range<T const>` are true, and one for when only `contiguous_range<T const>` is true.
> Arthur, I added your first test only (with some modification). The NotPtr test doesn't work because contiguous_iterator has an element_type.

Ah. My intent with `NotPtr` was that `contiguous_range<NotPtr>` would not be satisfied because `NotPtr().data()`'s return type is not `std::same_as<std::add_pointer_t<ranges::range_reference_t<T>>>`. However, I see now that the thing-we're-taking-the-return-type-of in [contiguous_range](https://en.cppreference.com/w/cpp/ranges/contiguous_range) is not `NotPtr().data()` but instead `std::ranges::data(NotPtr())`, which [actually evaluates](https://en.cppreference.com/w/cpp/ranges/data) to `std::to_address(NotPtr().begin())`, not `NotPtr().data()`, in this case.

So then actually maybe it would be worthwhile to add a test like this:
```
struct WrongObjectness { // https://godbolt.org/z/M7fbvj7fE
    const int *begin() const;
    const int *end() const;
    void *data() const;
};
static_assert(std::ranges::contiguous_range<WrongObjectness>);
```
which verifies that if `data()` exists but is completely the wrong return type, then there's no problem at all. (Did I ever mention how overcomplicated Ranges is? :P)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104262/new/

https://reviews.llvm.org/D104262