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3618. Unnecessary iter_move for transform_view::iterator

Section: 24.7.7.3 [range.transform.iterator] Status: WP Submitter: Barry Revzin Opened: 2021-10-12 Last modified: 2022-02-10

Priority: Not Prioritized

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Discussion:

transform_view's iterator currently specifies a customization point for iter_move. This customization point does the same thing that the default implementation would do — std::move(*E) if *E is an lvalue, else *E — except that it is only there to provide the correct conditional noexcept specification. But for iota_view, we instead provide a noexcept-specifier on the iterator's operator*. We should do the same thing for both, and the simplest thing would be:

Change 24.7.7.3 [range.transform.iterator] to put the whole body into noexcept:

constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) {
  return invoke(*parent_->fun_, *current_);
} 

And to remove this:

friend constexpr decltype(auto) iter_move(const iterator& i)
  noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) {
  if constexpr (is_lvalue_reference_v<decltype(*i)>)
    return std::move(*i);
  else
    return *i;
}

[2022-01-29; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

1. Modify 24.7.7.3 [range.transform.iterator], class template transform_view::iterator, as indicated:

   […]
   constexpr decltype(auto) operator*() const noexcept(noexcept(invoke(*parent_->fun_, *current_))) {
     return invoke(*parent_->fun_, *current_);
   }
   […]
   friend constexpr decltype(auto) iter_move(const iterator& i)
     noexcept(noexcept(invoke(*i.parent_->fun_, *i.current_))) {
     if constexpr (is_lvalue_reference_v<decltype(*i)>)
       return std::move(*i);
     else
       return *i;
   }
   […]