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[2026-03 LWG Motion 13] P3725R3 Filter View Extensions for Safer Use, Rev 3 #8913

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[2026-03 LWG Motion 13] P3725R3 Filter View Extensions for Safer Use, Rev 3 #8913

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3b957b0
P3725R3 Filter View Extensions for Safer Use, Rev 3
jensmaurer Apr 3, 2026
1eb9d37
[range.filter.iterator,range.filter.sentinel] Replace 'filter_view' w…
jensmaurer Apr 3, 2026
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174 changes: 124 additions & 50 deletions source/ranges.tex
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -4796,10 +4796,10 @@
@\exposidnc{movable-box}@<Pred> @\exposid{pred_}@; // \expos

// \ref{range.filter.iterator}, class \tcode{filter_view::\exposid{iterator}}
class @\exposid{iterator}@; // \expos
template<bool> class @\exposid{iterator}@; // \expos

// \ref{range.filter.sentinel}, class \tcode{filter_view::\exposid{sentinel}}
class @\exposid{sentinel}@; // \expos
template<bool> class @\exposid{sentinel}@; // \expos

public:
filter_view() requires @\libconcept{default_initializable}@<V> && @\libconcept{default_initializable}@<Pred> = default;
Expand All @@ -4810,12 +4810,21 @@

constexpr const Pred& pred() const;

constexpr @\exposid{iterator}@ begin();
constexpr @\exposid{iterator}@<false> begin();
constexpr @\exposid{iterator}@<true> begin() const
requires (@\libconcept{input_range}@<const V> && !@\libconcept{forward_range}@<const V> &&
@\libconcept{indirect_unary_predicate}@<const Pred, iterator_t<const V>>);

constexpr auto end() {
if constexpr (@\libconcept{common_range}@<V>)
return @\exposid{iterator}@{*this, ranges::end(@\exposid{base_}@)};
return @\exposid{iterator}@<false>{*this, ranges::end(@\exposid{base_}@)};
else
return @\exposid{sentinel}@{*this};
return @\exposid{sentinel}@<false>{*this};
}
constexpr @\exposid{sentinel}@<true> end() const
requires (@\libconcept{input_range}@<const V> && !@\libconcept{forward_range}@<const V> &&
@\libconcept{indirect_unary_predicate}@<const Pred, iterator_t<const V>>) {
return @\exposid{sentinel}@<true>{*this};
}
};

Expand Down Expand Up @@ -4849,7 +4858,7 @@

\indexlibrarymember{begin}{filter_view}%
\begin{itemdecl}
constexpr @\exposid{iterator}@ begin();
constexpr @\exposid{iterator}@<false> begin();
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -4870,63 +4879,95 @@
\tcode{filter_view} for use on subsequent calls.
\end{itemdescr}

\rSec3[range.filter.iterator]{Class \tcode{filter_view::\exposid{iterator}}}
\indexlibrarymember{begin}{filter_view}%
\begin{itemdecl}
constexpr @\exposid{iterator}@<true> begin() const
requires (@\libconcept{input_range}@<const V> && !@\libconcept{forward_range}@<const V> &&
@\libconcept{indirect_unary_predicate}@<const Pred, iterator_t<const V>>);
\end{itemdecl}

\begin{itemdescr}
\pnum
\expects
\tcode{\exposid{pred_}.has_value()} is \tcode{true}.

\pnum
\returns
\tcode{\{*this, ranges::find_if(\exposid{base_}, ref(*\exposid{pred_}))\}}.
\begin{note}
This function does not cache the result within the \tcode{filter_view}.
\end{note}
\end{itemdescr}

\rSec3[range.filter.iterator]{Class template \tcode{filter_view::\exposid{iterator}}}

\indexlibraryglobal{filter_view::\exposid{iterator}}%
\indexlibrarymember{iterator}{filter_view}%
\begin{codeblock}
namespace std::ranges {
template<@\libconcept{input_range}@ V, @\libconcept{indirect_unary_predicate}@<iterator_t<V>> Pred>
requires @\libconcept{view}@<V> && is_object_v<Pred>
template<bool Const>
class filter_view<V, Pred>::@\exposid{iterator}@ {
private:
iterator_t<V> @\exposid{current_}@ = iterator_t<V>(); // \expos
filter_view* @\exposid{parent_}@ = nullptr; // \expos
using @\exposid{Parent}@ = @\exposid{maybe-const}@<Const, filter_view>; // \expos
using @\exposid{Base}@ = @\exposid{maybe-const}@<Const, V>; // \expos
iterator_t<Base> @\exposid{current_}@ = iterator_t<Base>(); // \expos
@\exposid{Parent}@* @\exposid{parent_}@ = nullptr; // \expos

public:
using iterator_concept = @\seebelownc@;
using iterator_category = @\seebelownc@; // not always present
using value_type = range_value_t<V>;
using difference_type = range_difference_t<V>;
using value_type = range_value_t<Base>;
using difference_type = range_difference_t<Base>;

@\exposid{iterator}@() requires @\libconcept{default_initializable}@<iterator_t<V>> = default;
constexpr @\exposid{iterator}@(filter_view& parent, iterator_t<V> current);
@\exposid{iterator}@() requires @\libconcept{default_initializable}@<iterator_t<Base>> = default;
constexpr @\exposid{iterator}@(@\exposid{Parent}@& parent, iterator_t<Base> current);
constexpr @\exposid{iterator}@(iterator<!Const> i)
requires Const && @\libconcept{convertible_to}@<iterator_t<V>, iterator_t<Base>>;

constexpr const iterator_t<V>& base() const & noexcept;
constexpr iterator_t<V> base() &&;
constexpr range_reference_t<V> operator*() const;
constexpr iterator_t<V> operator->() const
requires @\exposconcept{has-arrow}@<iterator_t<V>> && @\libconcept{copyable}@<iterator_t<V>>;
constexpr const iterator_t<Base>& base() const & noexcept;
constexpr iterator_t<Base> base() &&;
constexpr range_reference_t<Base> operator*() const;
constexpr iterator_t<Base> operator->() const
requires @\exposconcept{has-arrow}@<iterator_t<Base>> && @\libconcept{copyable}@<iterator_t<Base>>;

constexpr @\exposid{iterator}@& operator++();
constexpr void operator++(int);
constexpr @\exposid{iterator}@ operator++(int) requires @\libconcept{forward_range}@<V>;
constexpr @\exposid{iterator}@ operator++(int) requires @\libconcept{forward_range}@<Base>;

constexpr @\exposid{iterator}@& operator--() requires @\libconcept{bidirectional_range}@<V>;
constexpr @\exposid{iterator}@ operator--(int) requires @\libconcept{bidirectional_range}@<V>;
constexpr @\exposid{iterator}@& operator--() requires @\libconcept{bidirectional_range}@<Base>;
constexpr @\exposid{iterator}@ operator--(int) requires @\libconcept{bidirectional_range}@<Base>;

friend constexpr bool operator==(const @\exposid{iterator}@& x, const @\exposid{iterator}@& y)
requires @\libconcept{equality_comparable}@<iterator_t<V>>;
requires @\libconcept{equality_comparable}@<iterator_t<Base>>;

friend constexpr range_rvalue_reference_t<V> iter_move(const @\exposid{iterator}@& i)
friend constexpr range_rvalue_reference_t<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<V>>;
requires @\libconcept{indirectly_swappable}@<iterator_t<Base>>;
};
}
\end{codeblock}

\pnum
Modification of the element a \tcode{filter_view::\exposid{iterator}} denotes is
permitted, but results in undefined behavior if the resulting value does not
satisfy the filter predicate.
\begin{note}
Modification of the element a \tcode{filter_view::\exposid{iterator}} denotes
can result in undefined behavior
if the underlying range is a \libconcept{forward_range} and
the resulting value does not satisfy the filter predicate
when the predicate is next evaluated for that element\iref{concepts.equality}.
\end{note}

\pnum
\tcode{\exposid{iterator}::iterator_concept} is defined as follows:
\begin{itemize}
\item If \tcode{V} models \libconcept{bidirectional_range}, then
\item If \tcode{Const} is \tcode{true},
then \tcode{iterator_concept} denotes \tcode{input_iterator_tag}.

\item Otherwise, if \tcode{V} models \libconcept{bidirectional_range}, then
\tcode{iterator_concept} denotes \tcode{bidirectional_iterator_tag}.

\item Otherwise, if \tcode{V} models \libconcept{forward_range}, then
Expand All @@ -4937,12 +4978,12 @@

\pnum
The member \grammarterm{typedef-name} \tcode{iterator_category} is declared
if and only if \tcode{V} models \libconcept{forward_range}.
if and only if \tcode{Base} models \libconcept{forward_range}.
In that case,
\tcode{\exposid{iterator}::iterator_category} is defined as follows:
\begin{itemize}
\item Let \tcode{C} denote the type
\tcode{iterator_traits<iterator_t<V>>::iterator_category}.
\tcode{iterator_traits<iterator_t<Base>>::iterator_category}.

\item If \tcode{C} models
\tcode{\libconcept{derived_from}<bidirectional_iterator_tag>},
Expand All @@ -4957,7 +4998,7 @@

\indexlibraryctor{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr @\exposid{iterator}@(filter_view& parent, iterator_t<V> current);
constexpr @\exposid{iterator}@(@\exposid{Parent}@& parent, iterator_t<Base> current);
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -4967,9 +5008,22 @@
\exposid{parent_} with \tcode{addressof(parent)}.
\end{itemdescr}

\indexlibraryctor{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr @\exposid{iterator}@(@\exposid{iterator}@<!Const> i)
requires Const && @\libconcept{convertible_to}@<iterator_t<V>, iterator_t<Base>>;
\end{itemdecl}

\begin{itemdescr}
\pnum
\effects
Initializes \exposid{parent_} with \tcode{i.\exposid{parent_}} and
\exposid{current_} with \tcode{std::move(i.\exposid{current_})}.
\end{itemdescr}

\indexlibrarymember{base}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr const iterator_t<V>& base() const & noexcept;
constexpr const iterator_t<Base>& base() const & noexcept;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -4980,7 +5034,7 @@

\indexlibrarymember{base}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr iterator_t<V> base() &&;
constexpr iterator_t<Base> base() &&;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -4991,7 +5045,7 @@

\indexlibrarymember{operator*}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr range_reference_t<V> operator*() const;
constexpr range_reference_t<Base> operator*() const;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5002,8 +5056,8 @@

\indexlibrarymember{operator->}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr iterator_t<V> operator->() const
requires @\exposconcept{has-arrow}@<iterator_t<V>> && @\libconcept{copyable}@<iterator_t<V>>;
constexpr iterator_t<Base> operator->() const
requires @\exposconcept{has-arrow}@<iterator_t<Base>> && @\libconcept{copyable}@<iterator_t<Base>>;
\end{itemdecl}

\begin{itemdescr}
Expand Down Expand Up @@ -5041,7 +5095,7 @@

\indexlibrarymember{operator++}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr @\exposid{iterator}@ operator++(int) requires @\libconcept{forward_range}@<V>;
constexpr @\exposid{iterator}@ operator++(int) requires @\libconcept{forward_range}@<Base>;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5057,7 +5111,7 @@

\indexlibrarymember{operator--}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr @\exposid{iterator}@& operator--() requires @\libconcept{bidirectional_range}@<V>;
constexpr @\exposid{iterator}@& operator--() requires @\libconcept{bidirectional_range}@<Base>;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5074,7 +5128,7 @@

\indexlibrarymember{operator--}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
constexpr @\exposid{iterator}@ operator--(int) requires @\libconcept{bidirectional_range}@<V>;
constexpr @\exposid{iterator}@ operator--(int) requires @\libconcept{bidirectional_range}@<Base>;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5091,7 +5145,7 @@
\indexlibrarymember{operator==}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
friend constexpr bool operator==(const @\exposid{iterator}@& x, const @\exposid{iterator}@& y)
requires @\libconcept{equality_comparable}@<iterator_t<V>>;
requires @\libconcept{equality_comparable}@<iterator_t<Base>>;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5102,7 +5156,7 @@

\indexlibrarymember{iter_move}{filter_view::\exposid{iterator}}%
\begin{itemdecl}
friend constexpr range_rvalue_reference_t<V> iter_move(const @\exposid{iterator}@& i)
friend constexpr range_rvalue_reference_t<Base> iter_move(const @\exposid{iterator}@& i)
noexcept(noexcept(ranges::iter_move(i.@\exposid{current_}@)));
\end{itemdecl}

Expand All @@ -5116,7 +5170,7 @@
\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<V>>;
requires @\libconcept{indirectly_swappable}@<iterator_t<Base>>;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5125,32 +5179,50 @@
Equivalent to \tcode{ranges::iter_swap(x.\exposid{current_}, y.\exposid{current_})}.
\end{itemdescr}

\rSec3[range.filter.sentinel]{Class \tcode{filter_view::\exposid{sentinel}}}
\rSec3[range.filter.sentinel]{Class template \tcode{filter_view::\exposid{sentinel}}}

\indexlibraryglobal{filter_view::\exposid{sentinel}}%
\indexlibrarymember{sentinel}{filter_view}%
\begin{codeblock}
namespace std::ranges {
template<@\libconcept{input_range}@ V, @\libconcept{indirect_unary_predicate}@<iterator_t<V>> Pred>
requires @\libconcept{view}@<V> && is_object_v<Pred>
template<bool Const>
class filter_view<V, Pred>::@\exposid{sentinel}@ {
private:
sentinel_t<V> @\exposid{end_}@ = sentinel_t<V>(); // \expos
using @\exposid{Base}@ = @\exposid{maybe-const}@<Const, V>; // \expos
sentinel_t<Base> @\exposid{end_}@ = sentinel_t<Base>(); // \expos

public:
@\exposid{sentinel}@() = default;
constexpr explicit @\exposid{sentinel}@(filter_view& parent);
constexpr @\exposid{sentinel}@(@\exposid{sentinel}@<!Const> other)
requires Const && @\libconcept{convertible_to}@<sentinel_t<V>, sentinel_t<Base>>
constexpr explicit @\exposid{sentinel}@(@\exposid{Parent}@& parent);

constexpr sentinel_t<V> base() const;
constexpr sentinel_t<Base> base() const;

friend constexpr bool operator==(const @\exposid{iterator}@& x, const @\exposid{sentinel}@& y);
template<bool OtherConst>
requires @\libconcept{sentinel_for}@<sentinel_t<Base>, iterator_t<@\exposid{maybe-const}@<OtherConst, V>>>
friend constexpr bool operator==(const @\exposid{iterator}@<OtherConst>& x, const @\exposid{sentinel}@& y);
};
}
\end{codeblock}

\indexlibraryctor{filter_view::\exposid{sentinel}}%
\begin{itemdecl}
constexpr explicit @\exposid{sentinel}@(filter_view& parent);
constexpr @\exposid{sentinel}@(@\exposid{sentinel}@<!Const> other)
requires Const && @\libconcept{convertible_to}@<sentinel_t<V>, sentinel_t<Base>>;
\end{itemdecl}

\begin{itemdescr}
\pnum
\effects
Initializes \exposid{end_} with \tcode{std::move(other.\exposid{end_})}.
\end{itemdescr}

\indexlibraryctor{filter_view::\exposid{sentinel}}%
\begin{itemdecl}
constexpr explicit @\exposid{sentinel}@(@\exposid{Parent}@& parent);
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5161,7 +5233,7 @@

\indexlibrarymember{base}{filter_view::\exposid{sentinel}}%
\begin{itemdecl}
constexpr sentinel_t<V> base() const;
constexpr sentinel_t<Base> base() const;
\end{itemdecl}

\begin{itemdescr}
Expand All @@ -5172,7 +5244,9 @@

\indexlibrarymember{operator==}{filter_view::\exposid{sentinel}}%
\begin{itemdecl}
friend constexpr bool operator==(const @\exposid{iterator}@& x, const @\exposid{sentinel}@& y);
template<bool OtherConst>
requires @\libconcept{sentinel_for}@<sentinel_t<Base>, iterator_t<@\exposid{maybe-const}@ <OtherConst, V>>>
friend constexpr bool operator==(const @\exposid{iterator}@<OtherConst>& x, const @\exposid{sentinel}@& y);
\end{itemdecl}

\begin{itemdescr}
Expand Down
1 change: 1 addition & 0 deletions source/support.tex
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -797,6 +797,7 @@
#define @\defnlibxname{cpp_lib_ranges_concat}@ 202403L // freestanding, also in \libheader{ranges}
#define @\defnlibxname{cpp_lib_ranges_contains}@ 202207L // freestanding, also in \libheader{algorithm}
#define @\defnlibxname{cpp_lib_ranges_enumerate}@ 202302L // freestanding, also in \libheader{ranges}
#define @\defnlibxname{cpp_lib_ranges_filter}@ 202603L // freestanding, also in \libheader{ranges}
#define @\defnlibxname{cpp_lib_ranges_find_last}@ 202207L // freestanding, also in \libheader{algorithm}
#define @\defnlibxname{cpp_lib_ranges_fold}@ 202207L // freestanding, also in \libheader{algorithm}
#define @\defnlibxname{cpp_lib_ranges_generate_random}@ 202403L // also in \libheader{random}
Expand Down
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