iterator class of concat_view does not expose reference and pointer types

[niekbouman]

hi Hui,

When using some boost lib with concat_view, I got a compiler errors about missing iterator::reference and iterator::pointer typedefs. After manually adding them (see below) things seem to work for my use case. Any particular reason for not exposing reference and pointer in the concat_view's iterator type?

// ...
  template <bool Const>
  class iterator : public xo::iter_cat_base_t<Const, Views...> {
   public:
    using value_type = xo::concat_value_t<__maybe_const<Const, Views>...>;
    using difference_type =
        common_type_t<range_difference_t<__maybe_const<Const, Views>>...>;
    using iterator_concept =
        decltype(xo::iterator_concept_test<Const, Views...>());

    // I added the two lines below
    using reference = xo::concat_reference_t<__maybe_const<Const, Views>...>;
    using pointer = xo::concat_pointer_t<__maybe_const<Const, Views>...>;

// ...

[huixie90]

These two in class typedef are not required by the standard. And in fact, by providing them will stop the iterator_traits auto generating them. You will find most of the c++20 iterators not providing these typedef. Instead, you should always query typename iterator_traits::reference

This typedef is generated from operator*

So I would argue it is a bug in boost lib( but it is likely boost lib was created pre-c++20 so it is still assuming the old iterator model)

Regarding pointer, we did it on propose that we don’t provide operator-> at all so iterator_trait::pointer will be void most likely. Note that operator-> is not required at all to be an iterator. And it does not make too much sense for concat iterator to provide one.

Cheers Hui

On Tue, 28 Nov 2023 at 22:04, niekbouman @.***> wrote:

hi Hui,

When using some boost lib with concat_view, I got a compiler errors about missing iterator::reference and iterator::pointer typedefs. After manually adding them (see below) things seem to work for my use case. Any particular reason for not exposing reference and pointer in the concat_view's iterator type?

// ... template class iterator : public xo::iter_cat_base_t<Const, Views...> { public: using value_type = xo::concat_value_t<maybe_const<Const, Views>...>; using difference_type = common_type_t<range_difference_t<maybe_const<Const, Views>>...>; using iterator_concept = decltype(xo::iterator_concept_test<Const, Views...>());

// I added the two lines below
using reference = xo::concat_reference_t<__maybe_const<Const, Views>...>;
using pointer = xo::concat_pointer_t<__maybe_const<Const, Views>...>;

// ...

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[niekbouman]

ah I see, thanks a lot for your explanation..!