[libcxx-commits] [PATCH] D120180: [libc++][ranges] Implement changes to reverse_iterator from One Ranges Proposal.
Konstantin Varlamov via Phabricator via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Mar 10 18:10:31 PST 2022
var-const added inline comments.
================
Comment at: libcxx/include/__iterator/reverse_iterator.h:54
+ using iterator_category = _If<__is_cpp17_random_access_iterator<_Iter>::value,
+ random_access_iterator_tag,
----------------
ldionne wrote:
> The intent of this change is so that a `reverse_iterator<contiguous-iterator>` advertises itself as a `random_access_iterator`, not a `contiguous_iterator`. Right?
I think so (it definitely has this effect), but I would have to do some archaeology to find out if that was the main intention (the One Ranges Proposal omits any rationale). Please let me know if you'd like me to confirm.
================
Comment at: libcxx/include/__iterator/reverse_iterator.h:130
+ pointer operator->() const
+#if !defined(_LIBCPP_HAS_NO_CONCEPTS)
+ requires is_pointer_v<_Iter> || requires(const _Iter i) { i.operator->(); }
----------------
ldionne wrote:
> You should be able to replace all instances of `_LIBCPP_HAS_NO_CONCEPTS` by `_LIBCPP_STD_VER > 17` now that we've bumped the compilers.
>
> For this specific instance, it means it can be eliminated altogether.
That's really cool, done.
================
Comment at: libcxx/include/__iterator/reverse_iterator.h:134-137
+ _Iter __tmp = current;
+ --__tmp;
+ if constexpr (is_pointer_v<_Iter>) {
+ return __tmp;
----------------
ldionne wrote:
> Any reason for not using verbatim what's in the standard, i.e. `prev(current)`?
I don't think there was a particularly strong reason. Changed to the Standard version.
================
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.");
----------------
ldionne wrote:
> var-const wrote:
> > Quuxplusone wrote:
> > > 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`.
> > I think both are workable, so I'm ok with removing the `static_assert`, but I still feel that saying //why// it doesn't compile is more useful than saying //what// doesn't compile.
> >
> > > sadly gives the programmer no information about how to fix the problem with their code
> > It leads the programmer in the right direction -- they can look up the `bidirectional_iterator` requirements or just do `static_assert(std::bidirectional_iterator<MySupposedlyBidirectionalIterator>)` to see what exactly is missing from their type (which might be more than just the `operator--`).
> I have to say -- I think the `static_assert` approach is superior for a couple of reasons (in no order):
>
> - Instantiating `reverse_iterator` with a non-bidi iterator is a precondition type of error, per http://eel.is/c++draft/reverse.iterators#reverse.iter.requirements-1. What we normally do for these errors is assert, or `static_assert` when detectable at compile-time. So there's a consistency argument.
> - A `static_assert` firing makes it clear that the user messed up. When a user sees a `static_assert` with a message we bothered writing for them, they immediately go "oh, I must have messed up", which provides a nicer experience than letting them scratch their head over "why is this code not compiling".
> - We have a message we can use to make it arbitrarily clear what's going wrong. We shouldn't be shy to explain the issue properly so as to make the `static_assert` undoubtedly easier to understand than the compilation error.
> - Without a `static_assert`, it's possible that some users would try to fix their code by just adding the missing operator to their iterator type, without necessarily making it a proper bidirectional iterator. So they'd still be in IFNDR-land, when we could instead have helped them understand the problem and fix their code.
> - It is generally better to fail earlier rather than later. For example, we don't want users to use `reverse_iterator<non-bidi>` successfully until they finally try to use `operator++` on it, which would start failing. For instance, without the assertion, I think users could use `reverse_iterator<non-bidi>` successfully if they only used `operator--`, right? If so, that's not a good thing.
>
>
> By the way, the above applies to anywhere in the library where we have ill-formed code that we're able to diagnose. If we agree on this, we can take it as a blanket policy to prefer `static_assert` over "lazy instantiation errors" when we have those cases.
>
> So, all that being said, my vote would go for adding a `static_assert` with a nice error message, and adding a libc++ specific test to check that we do catch it.
Restored the `static_assertion` and its test (`bad_template_argument.verify.cpp` in `libcxx` tests). Please let me know if you'd like to expand the error message.
One thing to mention, though -- the Standard's constraints are:
> The template parameter Iterator shall either meet the requirements of a `Cpp17BidirectionalIterator` (`[bidirectional.iterators]`) or model `bidirectional_iterator` (`[iterator.concept.bidir]`).
I presumed that `__is_cpp17_bidirectional_iterator` implements `Cpp17BidirectionalIterator`. However, it seems significantly more lax -- if I'm reading the implementation correctly, it only checks the `iterator_category`, not the provided operations. So the current check is significantly weaker than it should be -- any type providing the expected `iterator_category` and the other 3-4 typedefs would do. Do you think I should fix this in this patch or a follow-up? Or perhaps create a local version of `__is_cpp17_bidirectional_iterator` in this file?
> For instance, without the assertion, I think users could use `reverse_iterator<non-bidi>` successfully if they only used `operator--`, right? If so, that's not a good thing.
I tried it out to verify, and yes, functions like `operator--` and `base` do work if there is no `static_assertion`.
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https://reviews.llvm.org/D120180/new/
https://reviews.llvm.org/D120180
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