std::views ?

[Stefan Zager]

I don't pay close attention to the adoption of newer C++ features, but I just ran up against this one.

Sure would be nice for what I'm doing if I could use std::ranges::transform_view.

[Victor Vianna]

They are banned here and the docs link to the thread with the rationale, in case you want to revisit any of the arguments there.

[Stefan Zager]

I'm trying to replace this very succinct use of views (which compiles and passes tests):

```

  auto LayersThatShouldPushProperties() const {
    return std::views::transform(layers_that_should_push_properties_,
                                 [this](int id) { return find(id)->get(); });

```

... with a custom iterator class, but hitting a compiler error I can't quite figure, and I'd appreciate any available help. Here's the code:

```

  class DirtyLayerIterator {
   public:
    DirtyLayerIterator(const OwnedLayerImplList& layer_list,
                       SetType::const_iterator cur)
        : layer_list_(layer_list), cur_(cur) {}
    DirtyLayerIterator(const DirtyLayerIterator&) = default;
    DirtyLayerIterator(DirtyLayerIterator&&) = default;
    DirtyLayerIterator& operator=(const DirtyLayerIterator&) = default;
    DirtyLayerIterator& operator=(DirtyLayerIterator&&) = default;
    LayerImpl* operator*() { return layer_list_->find(*cur_)->get(); }
    DirtyLayerIterator& operator++() {
      ++cur_;
      return *this;
    }
    DirtyLayerIterator operator++(int) {
      DirtyLayerIterator result(*this);
      ++*this;
      return result;
    }

   private:
    raw_ref<const OwnedLayerImplList> layer_list_;
    SetType::const_iterator cur_;
  };
  class DirtyLayerRange {
   public:
    explicit DirtyLayerRange(const OwnedLayerImplList& layer_list)
        : layer_list_(layer_list) {}
    DirtyLayerRange(const DirtyLayerRange&) = default;
    DirtyLayerRange(DirtyLayerRange&&) = default;
    DirtyLayerRange& operator=(const DirtyLayerRange&) = default;
    DirtyLayerRange& operator=(DirtyLayerRange&&) = default;
    DirtyLayerIterator begin() const {
      return {*layer_list_,
              layer_list_->layers_that_should_push_properties_.begin()};
    }
    DirtyLayerIterator end() const {
      return {*layer_list_,
              layer_list_->layers_that_should_push_properties_.end()};
    }
    size_type size() const {
      return layer_list_->layers_that_should_push_properties_.size();
    }

   private:
    raw_ref<const OwnedLayerImplList> layer_list_;
  };
  static_assert(std::ranges::range<DirtyLayerRange>);

```

The `static_assert` fails with:

../../cc/layers/layer_collections.h:164:17: error: static assertion failed
  164 |   static_assert(std::ranges::range<DirtyLayerRange>);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../cc/layers/layer_collections.h:164:17: note: because 'DirtyLayerRange' does not satisfy 'range'
gen/third_party/libc++/src/include/__ranges/concepts.h:52:3: note: because 'ranges::begin(__t)' would be invalid: call to deleted function call operator in type 'const __begin::__fn'
   52 |   ranges::begin(__t); // sometimes equality-preserving
      |   ^
1 error generated.

SOS!

[Daniel Cheng]

1. Your iterator probably needs to be default constructible, though I think that's usually a problem for `std::ranges::end()`, not `std::ranges::begin()`.

2. If you write out the `std::ranges::begin()` call explicitly, clang will usually tell you why `std::ranges::begin()` would be invalid in a more helpful way. Sadly the actual error from `std::ranges::ranges<SomeRangeTYpe>` is not that helpful most of the time.

(If you have a draft CL uploaded somewhere, I can help as well)

Daniel

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[Daniel Cheng]

I've also filed crbug.com/493534962 to consider allowing more use of range views.

Daniel