[libcxx-commits] [PATCH] D120180: [libc++][ranges] Implement changes to reverse_iterator from One Ranges Proposal.
Arthur O'Dwyer via Phabricator via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Feb 25 08:22:24 PST 2022
Quuxplusone requested changes to this revision.
Quuxplusone added inline comments.
This revision now requires changes to proceed.
================
Comment at: libcxx/include/__iterator/iterator_traits.h:145
+// The `cpp17-*-iterator` exposition-only concepts are easily confused with the Cpp17*Iterator tables, so the
+// exposition-only concepts have been banished to a namespace that makes it obvious they have a niche use-case.
namespace __iterator_traits_detail {
----------------
var-const wrote:
> Quuxplusone wrote:
> > var-const wrote:
> > > This is a drive-by change: I just found the original comment unclear.
> > Sure. FWIW, I'd just remove this comment. It kinda seems like the only point of this comment is to explain something that the comment itself says is "obvious," which means it's like //self-referentially// bad. Plus, I don't even know what it's talking about. If "the Cpp17*Iterator tables" make an appearance in libc++'s codebase, I don't know where.
> >
> > Perhaps all this comment means is: "`__iterator_traits_detail::__cpp17_iterator` is not the same as `__is_cpp17_input_iterator`." But that really //is// obvious, and only serves to raise more questions, like "Wait, why isn't it?" So again, I think the solution is to delete the confusing comment.
> I agree that the comment is still vague and confusing; it presumes the reader has a lot of context and leaves that context out. I think the most confusing part is "Cpp17*Iterator tables" (which is also the part you pointed out). Most likely, it refers to [`iterator.cpp17`](https://eel.is/c++draft/iterator.cpp17), which indeed contains some tables (though mentioning them here is at best unhelpful). I tweaked the wording a bit -- do you think it reads better now?
It's //better// now, but I would still prefer if it were just gone.
================
Comment at: libcxx/include/__iterator/reverse_iterator.h:49
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
+ static_assert(__is_cpp17_bidirectional_iterator<_Iter>::value || bidirectional_iterator<_Iter>, "The Iterator "
+ "template argument must be a bidirectional iterator.");
----------------
var-const wrote:
> Quuxplusone wrote:
> > var-const wrote:
> > > http://eel.is/c++draft/reverse.iterators#reverse.iter.requirements-1:
> > >
> > > > "The template parameter `Iterator` shall either meet the requirements of a `Cpp17BidirectionalIterator` (`[bidirectional.iterators]`) or model `bidirectional_iterator` (`[iterator.concept.bidir]`)."
> > >
> > > My understanding is that this is IFNDR, but adding a check seems like it could prevent harder-to-read errors during instantiation.
> > >
> > > There's a similar requirement for random access iterators:
> > >
> > > > "Additionally, `Iterator` shall either meet the requirements of a `Cpp17RandomAccessIterator` (`[random.access.iterators]`) or model `random_access_iterator` (`[iterator.concept.random.access]`) if the definitions of any of the members `operator+`, `operator-`, `operator+=`, `operator-=` (`[reverse.iter.nav]`), or `operator[]` (`[reverse.iter.elem]`), or the non-member operators (`[reverse.iter.cmp]`) `operator<`, `operator>`, `operator<=`, `operator>=`, `operator-`, or `operator+` (`[reverse.iter.nonmember]`) are instantiated (`[temp.inst]`)."
> > >
> > > I'm not sure that can be done with a `static_assert` -- I tried placing it into the bodies of the mentioned operators, and the compiler produces errors trying to instantiate non-existing functions before it evaluates the `static_assert`.
> > clang-format strikes again, in the formatting here; please fix it up somehow.
> > I'm ambivalent on adding this `static_assert`. It will probably //worsen// the error messages, actually, since "such-and-such operator doesn't exist when I tried to use it" is generally much easier to deal with than "this random static_assert was false instead of true." Have you actually looked at the error messages for e.g. `reverse_iterator<std::forward_list<int>::iterator>`, and //do// you think this is an improvement?
> > Here's the state of the art, for comparison: https://godbolt.org/z/nne1Mvzan
> > ```
> > __iterator/reverse_iterator.h:120:37: error: cannot decrement value of type 'std::__forward_list_iterator<std::__forward_list_node<int, void *> *>'
> > reverse_iterator& operator++() {--current; return *this;}
> > ^ ~~~~~~~
> > ```
> > I predict you'll find that this static_assert is not an improvement, and decide not to keep it.
> I'm actually formatting most of these manually (don't want to get used to the convenience of clang-format when there's a chance it's taken away :) ). Reformatted this bit with clang-format now, I think it's an improvement.
>
> Here's the full error I get with a static assert upon trying to create a reverse iterator from a forward iterator:
>
> ```
> In file included from /Users/varconst/work/llvm-project/libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/ctor.default.pass.cpp:15:
> In file included from /Users/varconst/work/llvm-project/build/include/c++/v1/iterator:669:
> In file included from /Users/varconst/work/llvm-project/build/include/c++/v1/__iterator/reverse_access.h:14:
> /Users/varconst/work/llvm-project/build/include/c++/v1/__iterator/reverse_iterator.h:49:5: error: static_assert failed due to requirement '__is_cpp17_bidirectional_iterator<forward_iterator<int *>>::value || bidirectional_iterator<forward_iterator<int *>>' "The Iterator template argument must be a bidirectional iterator."
> static_assert(__is_cpp17_bidirectional_iterator<_Iter>::value || bidirectional_iterator<_Iter>,
> ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /Users/varconst/work/llvm-project/libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/ctor.default.pass.cpp:40:13: note: in instantiation of template class 'std::reverse_iterator<forward_iterator<int *>>' requested here
> BadIter i;
> ^
> 1 error generated.
> ```
>
> Compared to the baseline (where construction succeeds but e.g. trying to increment the iterator fails):
> ```
> In file included from /Users/varconst/work/llvm-project/libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/ctor.default.pass.cpp:15:
> In file included from /Users/varconst/work/llvm-project/build/include/c++/v1/iterator:669:
> In file included from /Users/varconst/work/llvm-project/build/include/c++/v1/__iterator/reverse_access.h:14:
> /Users/varconst/work/llvm-project/build/include/c++/v1/__iterator/reverse_iterator.h:148:37: error: cannot decrement value of type 'forward_iterator<int *>'
> reverse_iterator& operator++() {--current; return *this;}
> ^ ~~~~~~~
> /Users/varconst/work/llvm-project/libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cons/ctor.default.pass.cpp:42:5: note: in instantiation of member function 'std::reverse_iterator<forward_iterator<int *>>::operator++' requested here
> ++i;
> ^
> 1 error generated.
> ```
>
> For easier comparison, the same messages with all the cruft removed:
> ```
> __iterator/reverse_iterator.h:49:5: error: static_assert failed due to requirement '__is_cpp17_bidirectional_iterator<forward_iterator<int *>>::value || bidirectional_iterator<forward_iterator<int *>>' "The Iterator template argument must be a bidirectional iterator."
> static_assert(__is_cpp17_bidirectional_iterator<_Iter>::value || bidirectional_iterator<_Iter>,
> ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ```
>
> ```
> __iterator/reverse_iterator.h:148:37: error: cannot decrement value of type 'forward_iterator<int *>'
> reverse_iterator& operator++() {--current; return *this;}
> ^ ~~~~~~~
> ```
>
> I do think that the static assertion improves the error message slightly. The error is caught early and the message explains the fundamental problem rather than a symptom of the problem. Error messages like the current one are often cited as one of the problems with C++ templates -- showing a technical detail in the instantiation. While it's not hard to figure out why a reverse iterator needs the decrement operator, it's still an additional mental step, and I think saying that the iterator has to be bidirectional is a better way to express the requirement than dealing with individual requirements of a bidirectional iterator.
Hm! I was hoping you'd quickly agree that
```
--current;
^ ~~~~~~~
```
was obviously the better error message.
```
static_assert(__is_cpp17_bidirectional_iterator<_Iter>::value || bidirectional_iterator<_Iter>,
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
sadly gives the programmer no information about how to fix the problem with their code. So that makes it a //worse// error message.
I'll pull rank on this one. :) Please remove the `static_assert`.
================
Comment at: libcxx/include/__iterator/reverse_iterator.h:139-146
+ pointer operator->() const
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
+ requires is_pointer_v<_Iter>
+ || requires(const _Iter i) { i.operator->(); }
+#endif
+ {
+ return _VSTD::addressof(operator*());
----------------
var-const wrote:
> Quuxplusone wrote:
> > I think what I'd do here is
> > ```
> > #if _LIBCPP_STD_VER > 17
> > pointer operator->() const
> > #if !defined(_LIBCPP_HAS_NO_CONCEPTS)
> > requires is_pointer_v<_Iter> || requires(const _Iter i) { i.operator->(); }
> > #endif // !defined(_LIBCPP_HAS_NO_CONCEPTS)
> > {
> > _Iter __tmp = current;
> > --__tmp;
> > if constexpr (is_pointer_v<_Iter>) {
> > return __tmp;
> > } else {
> > return _VSTD::move(__tmp).operator->();
> > }
> > }
> > #else
> > pointer operator->() const {
> > return _VSTD::addressof(operator*());
> > }
> > #endif // _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)
> > ```
> > Drive-by removed the cute spacing in `pointer operator->`.
> Done (with very slight modification).
>
> Do you think it's worth thinking whether it's possible to test for the change in the effects of this function in C++20?
Yes, you certainly could write a test for it, so sure, let's do it!
```
struct It {
[...all the usual members...]
int& operator*() const; // not defined
int *operator->() const&; // not defined
int *operator->() && { return nullptr; }
};
std::reverse_iterator<It> r;
assert(r.operator->() == nullptr);
```
================
Comment at: libcxx/include/__iterator/reverse_iterator.h:172-178
+ _LIBCPP_HIDE_FROM_ABI friend constexpr
+ iter_rvalue_reference_t<_Iter> iter_move(const reverse_iterator& __i)
+ noexcept(is_nothrow_copy_constructible_v<_Iter>
+ && noexcept(ranges::iter_move(--declval<_Iter&>()))) {
+ auto __tmp = __i.base();
+ return ranges::iter_move(--__tmp);
+ }
----------------
var-const wrote:
> Quuxplusone wrote:
> > clang-format strikes again. Ditto `iter_swap`.
> Nope, I simply prefer this formatting. Changed.
FWIW, I have no strong preference myself on whether the arguments should be vertically aligned or (2|4)-space-indented. But I do have a strong preference that when breaking a binary operation, the operator/separator should always be attached to the line //above//, not the line //below//:
```
int a = x +
y; // good
int b = x
+ y; // bad
```
(with libc++'s style for `X() : a_()` being a notable exception, and `a ? b : c` ambivalent)
================
Comment at: libcxx/test/std/iterators/predef.iterators/reverse.iterators/iterator_concept_conformance.compile.pass.cpp:64-65
+static_assert(!std::sized_sentinel_for<unsized_iterator, unsized_iterator>);
+static_assert(!std::sized_sentinel_for<std::reverse_iterator<unsized_iterator>,
+ std::reverse_iterator<unsized_iterator>>);
+
----------------
If this line is too long, consider (1) renaming `unsized_iterator` to `It`, (2) moving the tests into a (never-called) `void test() {` purely so that you can use nice short `{ }` scopes for these typedef names.
================
Comment at: libcxx/test/std/iterators/predef.iterators/reverse.iterators/iterator_concept_conformance.compile.pass.cpp:58-60
+using sized_iterator = random_access_iterator<int*>;
+static_assert( std::sized_sentinel_for<sized_iterator, sized_iterator>);
+static_assert( std::sized_sentinel_for<std::reverse_iterator<sized_iterator>, std::reverse_iterator<sized_iterator>>);
----------------
var-const wrote:
> Quuxplusone wrote:
> > These three lines don't seem to add anything, but it would be nice to have a test for some type `B` such that `bidirectional_iterator<B> && !random_access_iterator<B> && sized_sentinel_for<B, B>`. Unfortunately I think we don't have such a type in test_iterators.h, but it might take only ~15 lines to write one here (especially since you don't have to //implement// any of its methods, just declare them).
> >
> > I also think you should actually split these lines out into `reverse.iterators/sized_sentinel.compile.pass.cpp`, and add another test for a type `R` such that `essentially-random_access_iterator<R> && disable_sized_sentinel_for<R, R>`, and make sure that `requires { reverse_iterator<R>() - reverse_iterator<R>(); } == false`.
> > These three lines don't seem to add anything
>
> The main intention for these lines is to provide more context for the actual test rather than test anything on their own. Basically, "if the underlying iterators are sized, sized sentinel for `reverse_iterator` works; otherwise, it doesn't". They also verify that the reason `sized_sentinel_for` is false is because of the underlying iterators, not because it's written in such a way that it's always false.
I'd still like you to create a `reverse.iterators/sized_sentinel.compile.pass.cpp` with the two tests I suggest in the parent comment. We need to test the interaction with `disable_sized_sentinel_for` and the effect on the `reverse_iterator`'s own `operator-`.
================
Comment at: libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/sfinae.compile.pass.cpp:44
+#include "test_macros.h"
+#include "test_iterators.h"
+
----------------
I don't think you use `test_iterators.h` in this file.
================
Comment at: libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.cmp/sfinae.compile.pass.cpp:62
+template<class T> concept HasGreater = requires (T t) { t > t; };
+template<class T> concept HasGreaterOrEqual = requires (T t) { t >= t; };
+
----------------
What about `operator<=>`?
================
Comment at: libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.elem/arrow.sfinae.compile.pass.cpp:23-24
+
+template <class Iter>
+concept HasArrow = requires(Iter i) { i.operator->(); };
+
----------------
Nit.
================
Comment at: libcxx/test/std/iterators/predef.iterators/reverse.iterators/reverse.iter.nonmember/iter_move.pass.cpp:25-32
+template <class T>
+struct BidirectionalIteratorTraits {
+ using value_type = T;
+ using difference_type = ptrdiff_t;
+ using pointer = T*;
+ using reference = const T&;
+ using iterator_category = std::bidirectional_iterator_tag;
----------------
var-const wrote:
> Quuxplusone wrote:
> > Since this test is C++20-only, you don't actually need the old-school Big Five. You might not even need any of them, except probably `difference_type` because you have no `operator-`. But you definitely don't need `pointer` or `reference`, and I think not even `iterator_category`.
> `reverse_iterator` uses the typedefs from `iterator_traits` for its base class (from which it still inherits even in C++20 and above if `_LIBCPP_ABI_VERSION` is less than `2` -- when compiling the tests locally, it is `1`. `iterator_traits` require either all 5 to be defined (with the exception of `pointer` in C++20) or for the iterator to satisfy `C++17Iterator` (which seems like it won't be much of a win). I got rid of `pointer`, though it doesn't change things all that much.
Works fine for me with only `value_type` and `difference_type`.
https://godbolt.org/z/3Mv3bnn9W
I am 90% confident that you should eliminate this base class and just copy those two member typedefs into the 4 places you need them.
Ditto in `iter_swap.pass.cpp`.
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