P1456R1 Move-only views

Author: zygoloid

source/ranges.tex

@@ -720,7 +720,7 @@
 the requirement that the number of elements in the range can be determined
 in constant time using the \tcode{ranges::size} function.
 The \libconcept{view} concept specifies requirements on a \libconcept{range} type
-with constant-time copy and assign operations.
+with constant-time destruction and move operations.
 
 \pnum
 Several refinements of \libconcept{range} group requirements
@@ -898,19 +898,31 @@
 
 \rSec2[range.view]{Views}
 
+\pnum
+The \libconcept{view} concept specifies the requirements of a \libconcept{range} type
+that has constant time move construction, move assignment, and destruction;
+that is, the cost of these operations is
+% FIXME: Shouldn't this be "independent of the number of elements"?
+not proportional to the number of elements in the \tcode{view}.
+
 \begin{itemdecl}
 template<class T>
   concept @\deflibconcept{view}@ =
-    range<T> && semiregular<T> && enable_view<T>;
+    range<T> && movable<T> && default_constructible<T> && enable_view<T>;
 \end{itemdecl}
 
 \begin{itemdescr}
-% FIXME: This should explicitly say when view is modeled.
 \pnum
-The \libconcept{view} concept specifies the requirements of a \libconcept{range} type
-that has constant time copy, move, and assignment operators; that is, the cost of
-these operations is not proportional to the number of elements in the
-\tcode{view}.
+\tcode{T} models \libconcept{view} only if:
+\begin{itemize}
+\item
+\tcode{copy_constructible<T>} is \tcode{false}, or
+\tcode{T} has \bigoh{1} copy construction; and
+
+\item
+\tcode{copyable<T>} is \tcode{false}, or
+\tcode{T} has \bigoh{1} copy assignment.
+\end{itemize}
 
 \pnum
 \begin{example}
@@ -925,7 +937,7 @@
 \end{itemize}
 
 Most containers\iref{containers} are not views since
-copying the container copies the elements,
+destruction of the container destroys the elements,
 which cannot be done in constant time.
 \end{example}
 \end{itemdescr}
@@ -2701,6 +2713,8 @@
 \rSec3[range.filter.view]{Class template \tcode{filter_view}}
 
 \indexlibraryglobal{filter_view}%
+\indexlibrarymember{base}{filter_view}%
+\indexlibrarymember{end}{filter_view}%
 \begin{codeblock}
 namespace std::ranges {
   template<input_range V, indirect_unary_predicate<iterator_t<V>> Pred>
@@ -2722,7 +2736,8 @@
       requires viewable_range<R> && constructible_from<V, all_view<R>>
     constexpr filter_view(R&& r, Pred pred);
 
-    constexpr V base() const;
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
 
     constexpr iterator begin();
     constexpr auto end() {
@@ -2764,17 +2779,6 @@
 and initializes \tcode{pred_} with \tcode{std::\brk{}move(pred)}.
 \end{itemdescr}
 
-\indexlibrarymember{base}{filter_view}%
-\begin{itemdecl}
-constexpr V base() const;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to: \tcode{return base_;}
-\end{itemdescr}
-
 \indexlibrarymember{begin}{filter_view}%
 \begin{itemdecl}
 constexpr iterator begin();
@@ -3116,6 +3120,8 @@
 \rSec3[range.transform.view]{Class template \tcode{transform_view}}
 
 \indexlibraryglobal{transform_view}%
+\indexlibrarymember{base}{transform_view}%
+\indexlibrarymember{size}{transform_view}%
 \begin{codeblock}
 namespace std::ranges {
   template<input_range V, copy_constructible F>
@@ -3138,7 +3144,8 @@
       requires viewable_range<R> && constructible_from<V, all_view<R>>
     constexpr transform_view(R&& r, F fun);
 
-    constexpr V base() const;
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
 
     constexpr iterator<false> begin();
     constexpr iterator<true> begin() const
@@ -3190,17 +3197,6 @@
 and \tcode{fun_} with \tcode{std::move(fun)}.
 \end{itemdescr}
 
-\indexlibrarymember{base}{transform_view}%
-\begin{itemdecl}
-constexpr V base() const;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to: \tcode{return base_;}
-\end{itemdescr}
-
 \indexlibrarymember{begin}{transform_view}%
 \begin{itemdecl}
 constexpr iterator<false> begin();
@@ -3804,6 +3800,10 @@
 \rSec3[range.take.view]{Class template \tcode{take_view}}
 
 \indexlibraryglobal{take_view}%
+\indexlibrarymember{base}{take_view}%
+\indexlibrarymember{begin}{take_view}%
+\indexlibrarymember{end}{take_view}%
+\indexlibrarymember{size}{take_view}%
 \begin{codeblock}
 namespace std::ranges {
   template<view V>
@@ -3820,7 +3820,8 @@
       requires constructible_from<V, all_view<R>>
     constexpr take_view(R&& r, range_difference_t<V> count);
 
-    constexpr V base() const;
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
 
     constexpr auto begin() requires (!@\placeholder{simple-view}@<V>) {
       if constexpr (sized_range<V>) {
@@ -3905,17 +3906,6 @@
 and \tcode{count_} with \tcode{count}.
 \end{itemdescr}
 
-\indexlibrarymember{base}{take_view}%
-\begin{itemdecl}
-constexpr V base() const;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to: \tcode{return base_;}
-\end{itemdescr}
-
 \rSec3[range.take.sentinel]{Class template \tcode{take_view::sentinel}}
 
 \indexlibraryglobal{take_view::sentinel}%
@@ -4436,6 +4426,9 @@
 \rSec3[range.join.view]{Class template \tcode{join_view}}
 
 \indexlibraryglobal{join_view}%
+\indexlibrarymember{base}{join_view}%
+\indexlibrarymember{begin}{join_view}%
+\indexlibrarymember{end}{join_view}%
 \begin{codeblock}
 namespace std::ranges {
   template<input_range V>
@@ -4464,6 +4457,9 @@
       requires viewable_range<R> && constructible_from<V, all_view<R>>
     constexpr explicit join_view(R&& r);
 
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
+
     constexpr auto begin() {
       return iterator<@\placeholder{simple-view}@<V>>{*this, ranges::begin(base_)};
     }
@@ -4649,11 +4645,11 @@
 \effects
 Equivalent to:
 \begin{codeblock}
-auto update_inner = [this](range_reference_t<Base> x) -> decltype(auto) {
+auto update_inner = [this](range_reference_t<Base> x) -> auto& {
   if constexpr (ref_is_glvalue) // \tcode{x} is a reference
-    return (x);                 // \tcode{(x)} is an lvalue
+    return x;
   else
-    return (parent_->inner_ = views::all(x));
+    return (parent_->inner_ = views::all(std::move(x)));
 };
 
 for (; outer_ != ranges::end(parent_->base_); ++outer_) {
@@ -4922,6 +4918,9 @@
 \rSec3[range.split.view]{Class template \tcode{split_view}}
 
 \indexlibraryglobal{split_view}%
+\indexlibrarymember{base}{split_view}%
+\indexlibrarymember{begin}{split_view}%
+\indexlibrarymember{end}{split_view}%
 \begin{codeblock}
 namespace std::ranges {
   template<auto> struct @\placeholdernc{require-constant}@;       // \expos
@@ -4959,6 +4958,9 @@
                constructible_from<Pattern, single_view<range_value_t<R>>>
     constexpr split_view(R&& r, range_value_t<R> e);
 
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
+
     constexpr auto begin() {
       if constexpr (forward_range<V>)
         return outer_iterator<@\placeholder{simple-view}@<V>>{*this, ranges::begin(base_)};
@@ -5459,6 +5461,10 @@
 \rSec3[range.common.view]{Class template \tcode{common_view}}
 
 \indexlibraryglobal{common_view}%
+\indexlibrarymember{base}{common_view}%
+\indexlibrarymember{size}{common_view}%
+\indexlibrarymember{begin}{common_view}%
+\indexlibrarymember{end}{common_view}%
 \begin{codeblock}
 namespace std::ranges {
   template<view V>
@@ -5475,7 +5481,8 @@
       requires (!common_range<R> && constructible_from<V, all_view<R>>)
     constexpr explicit common_view(R&& r);
 
-    constexpr V base() const;
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
 
     constexpr auto size() requires sized_range<V> {
       return ranges::size(base_);
@@ -5542,17 +5549,6 @@
 Initializes \tcode{base_} with \tcode{views::all(std::forward<R>(r))}.
 \end{itemdescr}
 
-\indexlibrarymember{base}{common_view}%
-\begin{itemdecl}
-constexpr V base() const;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to: \tcode{return base_;}
-\end{itemdescr}
-
 \rSec3[range.common.adaptor]{\tcode{views::common}}
 
 \pnum
@@ -5589,6 +5585,8 @@
 \rSec3[range.reverse.view]{Class template \tcode{reverse_view}}
 
 \indexlibraryglobal{reverse_view}%
+\indexlibrarymember{base}{reverse_view}%
+\indexlibrarymember{size}{reverse_view}%
 \begin{codeblock}
 namespace std::ranges {
   template<view V>
@@ -5605,7 +5603,8 @@
       requires bidirectional_range<R> && constructible_from<V, all_view<R>>
     constexpr explicit reverse_view(R&& r);
 
-    constexpr V base() const;
+    constexpr V base() const& requires copy_constructible<V> { return base_; }
+    constexpr V base() && { return std::move(base_); }
 
     constexpr reverse_iterator<iterator_t<V>> begin();
     constexpr reverse_iterator<iterator_t<V>> begin() requires common_range<V>;
@@ -5653,17 +5652,6 @@
 Initializes \tcode{base_} with \tcode{views::all(std::forward<R>(r))}.
 \end{itemdescr}
 
-\indexlibrarymember{base}{reverse_view}%
-\begin{itemdecl}
-constexpr V base() const;
-\end{itemdecl}
-
-\begin{itemdescr}
-\pnum
-\effects
-Equivalent to: \tcode{return base_;}
-\end{itemdescr}
-
 \indexlibrarymember{begin}{reverse_view}%
 \begin{itemdecl}
 constexpr reverse_iterator<iterator_t<V>> begin();