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

[Zoe Carver via Phabricator via libcxx-commits]

zoecarver 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();
+};
----------------
Quuxplusone wrote:
> zoecarver wrote:
> > zoecarver wrote:
> > > Quuxplusone wrote:
> > > > 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)
> > > > when both contiguous_range<T> and contiguous_range<T const> are true,
> > > 
> > > That's `test_range<contiguous_iterator>`.
> > > 
> > > >  and one for when only contiguous_range<T const> is true.
> > > 
> > > Added.
> > > So then actually maybe it would be worthwhile to add a test like this:
> > 
> > I considered adding a test like this. I don't see how it's really different than the const tests (in both cases it fails because the types are not the same). I think it would be hard to write an implementation that said "if this is the same return type with different constness, that's bad, but if it's a totally unrelated types, that's OK."
> > 
> > I suppose it wouldn't hurt to have a case where they both returned const pointers. I'll add a similar test but where the return type is `const char *`. 
> Wait, on this //latest// point I think I'm right and you're confused. My `WrongObjectness` test above is supposed to //pass//, not fail, //even though// the return type of `data` is not the same as the return type of `begin` — because it is //so different// that we wrap around and become a contiguous range again! (Try the godbolt link.)
> If my `WrongObjectness` test did just fail, then it would be boring, I'd agree.
Oh, interesting. I see (because `ranges::data` won't pick the `data` member function). Sorry I didn't see this comment until I landed the patch. I'll make a follow up commit. 


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