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3325. Constrain return type of transformation function for transform_view

Section: 26.7.9.2 [range.transform.view] Status: C++20 Submitter: United States Opened: 2019-11-06 Last modified: 2022-01-15

Priority: 0

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Discussion:

Addresses US 303

The transform_view does not constrain the return type of the transformation function. It is invalid to pass a void-returning transformation function to the transform_view, which would cause its iterators' operator* member to return void.

Proposed change:

Change the constraints on transform_view to the following:

template<input_range V, copy_constructible F>
  requires view<V> && is_object_v<F> &&
           regular_invocable<F&, range_reference_t<V>> &&
           can-reference<invoke_result_t<F&, range_reference_t<V>>>
class transform_view;

Jonathan Wakely:

The NB comment says "The transform_view does not constrain the return type of the transformation function. It is invalid to pass a void-returning transformation function to the transform_view, which would cause its iterators' operator* member to return void."

[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]

Proposed resolution:

This wording is relative to N4835.

1. Modify 26.7.9.2 [range.transform.view], class template transform_view synopsis, as indicated:

   namespace std::ranges {
     template<input_range V, copy_constructible F>
       requires view<V> && is_object_v<F> &&
                regular_invocable<F&, range_reference_t<V>> &&
                can-reference<invoke_result_t<F&, range_reference_t<V>>>
     class transform_view : public view_interface<transform_view<V, F>> {
       […]
     };
   […]
   }