[2025-02 LWG Motion 3] P3137R3 views::to_input

source/ranges.tex

@@ -551,6 +551,17 @@
   class cache_latest_view;
 
   namespace views { inline constexpr @\unspec@ cache_latest = @\unspec@; }
+
+  // \ref{range.to.input}, to input view
+  template<@\libconcept{input_range}@ V>
+    requires @\libconcept{view}@<V>
+  class to_input_view;
+
+  template<class V>
+    constexpr bool enable_borrowed_range<to_input_view<V>> =
+      enable_borrowed_range<V>;
+
+  namespace views { inline constexpr @\unspec@ to_input = @\unspec@; }
 }
 
 namespace std {
@@ -17088,6 +17099,289 @@
 \tcode{x.\exposid{end_} - y.\exposid{current_}}.
 \end{itemdescr}
 
+\rSec2[range.to.input]{To input view}
+
+\rSec3[range.to.input.overview]{Overview}
+
+\pnum
+\tcode{to_input_view} presents a view of an underlying sequence
+as an input-only non-common range.
+\begin{note}
+This is useful to avoid overhead
+that can be necessary to provide support for the operations
+needed for greater iterator strength.
+\end{note}
+
+\pnum
+The name \tcode{views::to_input} denotes
+a range adaptor object\iref{range.adaptor.object}.
+Let \tcode{E} be an expression and let \tcode{T} be \tcode{decltype((E))}.
+The expression \tcode{views::to_input(E)} is expression-equivalent to:
+\begin{itemize}
+\item
+\tcode{views::all(E)}
+if \tcode{T} models \libconcept{input_range},
+does not satisfy \libconcept{common_range}, and
+does not satisfy \libconcept{forward_range}.
+\item
+Otherwise, \tcode{to_input_view(E)}.
+\end{itemize}
+
+\rSec3[range.to.input.view]{Class template \tcode{to_input_view}}
+
+\begin{codeblock}
+template<@\libconcept{input_range}@ V>
+  requires @\libconcept{view}@<V>
+class to_input_view : public view_interface<to_input_view<V>> {
+  V @\exposid{base_}@ = V();                        // \expos
+
+  template<bool Const> class @\exposid{iterator}@;  // \expos
+
+public:
+  to_input_view() requires @\libconcept{default_initializable}@<V> = default;
+  constexpr explicit to_input_view(V base);
+
+  constexpr V base() const & requires @\libconcept{copy_constructible}@<V> { return @\exposid{base_}@; }
+  constexpr V base() && { return std::move(@\exposid{base_}@); }
+
+  constexpr auto begin() requires (!@\exposconcept{simple-view}@<V>);
+  constexpr auto begin() const requires @\libconcept{range}@<const V>;
+
+  constexpr auto end() requires (!@\exposconcept{simple-view}@<V>);
+  constexpr auto end() const requires @\libconcept{range}@<const V>;
+
+  constexpr auto size() requires @\libconcept{sized_range}@<V>;
+  constexpr auto size() const requires @\libconcept{sized_range}@<const V>;
+};
+
+template<class R>
+  to_input_view(R&&) -> to_input_view<views::all_t<R>>;
+\end{codeblock}
+
+\begin{itemdecl}
+constexpr explicit to_input_view(V base);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Initializes \exposid{base_} with \tcode{std::move(base)}.
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr auto begin() requires (!@\exposconcept{simple-view}@<V>);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{return \exposid{iterator}<false>(ranges::begin(\exposid{base_}));}
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr auto begin() const requires @\libconcept{range}@<const V>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{return \exposid{iterator}<true>(ranges::begin(\exposid{base_}));}
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr auto end() requires (!@\exposconcept{simple-view}@<V>);
+constexpr auto end() const requires @\libconcept{range}@<const V>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{return ranges::end(\exposid{base_});}
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr auto size() requires @\libconcept{sized_range}@<V>;
+constexpr auto size() const requires @\libconcept{sized_range}@<const V>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{return ranges::size(\exposid{base_});}
+\end{itemdescr}
+
+\rSec3[range.to.input.iterator]{Class template \tcode{to_input_view::\exposid{iterator}}}
+
+\begin{codeblock}
+namespace std::ranges {
+  template<@\libconcept{input_range}@ V>
+    requires @\libconcept{view}@<V>
+  template<bool Const>
+  class to_input_view<V>::@\exposid{iterator}@ {
+    using @\exposid{Base}@ = @\exposid{maybe-const}@<Const, V>;                         // \expos
+
+    iterator_t<@\exposid{Base}@> @\exposid{current_}@ = iterator_t<@\exposid{Base}@>();             // \expos
+
+    constexpr explicit @\exposid{iterator}@(iterator_t<@\exposid{Base}@> current);      // \expos
+
+  public:
+    using difference_type = range_difference_t<@\exposid{Base}@>;
+    using value_type = range_value_t<@\exposid{Base}@>;
+    using iterator_concept = input_iterator_tag;
+
+    @\exposid{iterator}@() requires @\libconcept{default_initializable}@<iterator_t<@\exposid{Base}@>> = default;
+
+    @\exposid{iterator}@(@\exposid{iterator}@&&) = default;
+    @\exposid{iterator}@& operator=(@\exposid{iterator}@&&) = default;
+
+    constexpr @\exposid{iterator}@(@\exposid{iterator}@<!Const> i)
+      requires Const && @\libconcept{convertible_to}@<iterator_t<V>, iterator_t<@\exposid{Base}@>>;
+
+    constexpr iterator_t<@\exposid{Base}@> base() &&;
+    constexpr const iterator_t<@\exposid{Base}@>& base() const & noexcept;
+
+    constexpr decltype(auto) operator*() const { return *@\exposid{current_}@; }
+
+    constexpr @\exposid{iterator}@& operator++();
+    constexpr void operator++(int);
+
+    friend constexpr bool operator==(const @\exposid{iterator}@& x, const sentinel_t<@\exposid{Base}@>& y);
+
+    friend constexpr difference_type operator-(const sentinel_t<@\exposid{Base}@>& y, const @\exposid{iterator}@& x)
+      requires @\libconcept{sized_sentinel_for}@<sentinel_t<@\exposid{Base}@>, iterator_t<@\exposid{Base}@>>;
+    friend constexpr difference_type operator-(const @\exposid{iterator}@& x, const sentinel_t<@\exposid{Base}@>& y)
+      requires @\libconcept{sized_sentinel_for}@<sentinel_t<@\exposid{Base}@>, iterator_t<@\exposid{Base}@>>;
+
+    friend constexpr range_rvalue_reference_t<@\exposid{Base}@> iter_move(const @\exposid{iterator}@& i)
+      noexcept(noexcept(ranges::iter_move(i.@\exposid{current_}@)));
+
+    friend constexpr void iter_swap(const @\exposid{iterator}@& x, const @\exposid{iterator}@& y)
+      noexcept(noexcept(ranges::iter_swap(x.@\exposid{current_}@, y.@\exposid{current_}@)))
+      requires @\libconcept{indirectly_swappable}@<iterator_t<@\exposid{Base}@>>;
+  };
+}
+\end{codeblock}
+
+\begin{itemdecl}
+constexpr explicit @\exposid{iterator}@(iterator_t<@\exposid{Base}@> current);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Initializes \exposid{current_} with \tcode{std::move(current)}.
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr @\exposid{iterator}@(@\exposid{iterator}@<!Const> i)
+  requires Const && @\libconcept{convertible_to}@<iterator_t<V>, iterator_t<@\exposid{Base}@>>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Initializes \exposid{current_} with \tcode{std::move(i.\exposid{current_})}.
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr iterator_t<@\exposid{Base}@> base() &&;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{std::move(\exposid{current_)}}.
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr const iterator_t<@\exposid{Base}@>& base() const & noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\exposid{current_}.
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr @\exposid{iterator}@& operator++();
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+++@\exposid{current_}@;
+return *this;
+\end{codeblock}
+\end{itemdescr}
+
+\begin{itemdecl}
+constexpr void operator++(int);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{++*this;}
+\end{itemdescr}
+
+\begin{itemdecl}
+friend constexpr bool operator==(const @\exposid{iterator}@& x, const sentinel_t<@\exposid{Base}@>& y);
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{x.\exposid{current_} == y}.
+\end{itemdescr}
+
+\begin{itemdecl}
+friend constexpr difference_type operator-(const sentinel_t<@\exposid{Base}@>& y, const @\exposid{iterator}@& x)
+  requires @\libconcept{sized_sentinel_for}@<sentinel_t<@\exposid{Base}@>, iterator_t<@\exposid{Base}@>>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{y - x.\exposid{current_}}.
+\end{itemdescr}
+
+\begin{itemdecl}
+friend constexpr difference_type operator-(const @\exposid{iterator}@& x, const sentinel_t<@\exposid{Base}@>& y)
+  requires @\libconcept{sized_sentinel_for}@<sentinel_t<@\exposid{Base}@>, iterator_t<@\exposid{Base}@>>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\returns
+\tcode{x.\exposid{current_} - y}.
+\end{itemdescr}
+
+\begin{itemdecl}
+friend constexpr range_rvalue_reference_t<@\exposid{Base}@> iter_move(const @\exposid{iterator}@& i)
+  noexcept(noexcept(ranges::iter_move(i.@\exposid{current_}@)));
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{return ranges::iter_move(i.\exposid{current_});}
+\end{itemdescr}
+
+\begin{itemdecl}
+friend constexpr void iter_swap(const @\exposid{iterator}@& x, const @\exposid{iterator}@& y)
+  noexcept(noexcept(ranges::iter_swap(x.@\exposid{current_}@, y.@\exposid{current_)}@))
+  requires @\libconcept{indirectly_swappable}@<iterator_t<@\exposid{Base}@>>;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to: \tcode{ranges::iter_swap(x.\exposid{current_}, y.\exposid{current_});}
+\end{itemdescr}
+
 \rSec1[coro.generator]{Range generators}
 
 \rSec2[coroutine.generator.overview]{Overview}

source/support.tex

@@ -765,6 +765,7 @@
 #define @\defnlibxname{cpp_lib_ranges_starts_ends_with}@           202106L // also in \libheader{algorithm}
 #define @\defnlibxname{cpp_lib_ranges_stride}@                     202207L // freestanding, also in \libheader{ranges}
 #define @\defnlibxname{cpp_lib_ranges_to_container}@               202202L // freestanding, also in \libheader{ranges}
+#define @\defnlibxname{cpp_lib_ranges_to_input}@                   202502L // freestanding, also in \libheader{ranges}
 #define @\defnlibxname{cpp_lib_ranges_zip}@                        202110L
   // freestanding, also in \libheader{ranges}, \libheader{tuple}, \libheader{utility}
 #define @\defnlibxname{cpp_lib_ratio}@                             202306L // freestanding, also in \libheader{ratio}