On Mon, 14 Nov 2022, Jonathan Wakely wrote:

> On Mon, 14 Nov 2022 at 10:17, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
> >
> > Am Mo., 14. Nov. 2022 um 11:09 Uhr schrieb Jonathan Wakely via
> > Libstdc++ <libstdc++@gcc.gnu.org>:
> > >
> > > On Mon, 14 Nov 2022 at 04:52, Patrick Palka via Libstdc++
> > > <libstdc++@gcc.gnu.org> wrote:
> > > >
> > > > Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
> > > >
> > > > libstdc++-v3/ChangeLog:
> > > >
> > > >         * include/bits/ranges_algo.h (out_value_result): Define.
> > > >         (iota_result): Define.
> > > >         (__iota_fn, iota): Define.
> > > >         * testsuite/25_algorithms/iota/1.cc: New test.
> > > > ---
> > > >  libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
> > > >  .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
> > > >  2 files changed, 77 insertions(+)
> > > >  create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc
> > > >
> > > > diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> > > > index da0ca981dc3..f003117c569 100644
> > > > --- a/libstdc++-v3/include/bits/ranges_algo.h
> > > > +++ b/libstdc++-v3/include/bits/ranges_algo.h
> > > > @@ -3517,6 +3517,54 @@ namespace ranges
> > > >    };
> > > >
> > > >    inline constexpr __contains_subrange_fn contains_subrange{};
> > > > +
> > > > +  template<typename _Out, typename _Tp>
> > > > +    struct out_value_result
> > > > +    {
> > > > +      [[no_unique_address]] _Out out;
> > > > +      [[no_unique_address]] _Tp value;
> > > > +
> > > > +      template<typename _Out2, typename _Tp2>
> > > > +       requires convertible_to<const _Out&, _Out2>
> > > > +         && convertible_to<const _Tp&, _Tp2>
> > > > +       constexpr
> > > > +       operator out_value_result<_Out2, _Tp2>() const &
> > > > +       { return {out, value}; }
> > > > +
> > > > +      template<typename _Out2, typename _Tp2>
> > > > +       requires convertible_to<_Out, _Out2>
> > > > +         && convertible_to<_Tp, _Tp2>
> > > > +       constexpr
> > > > +       operator out_value_result<_Out2, _Tp2>() &&
> > > > +       { return {std::move(out), std::move(value)}; }
> > > > +    };
> > > > +
> > > > +  template<typename _Out, typename _Tp>
> > > > +    using iota_result = out_value_result<_Out, _Tp>;
> > > > +
> > > > +  struct __iota_fn
> > > > +  {
> > > > +    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
> > > > +      requires indirectly_writable<_Out, const _Tp&>
> > > > +      constexpr iota_result<_Out, _Tp>
> > > > +      operator()(_Out __first, _Sent __last, _Tp __value) const
> > > > +      {
> > > > +       while (__first != __last)
> > > > +         {
> > > > +           *__first = static_cast<add_const_t<_Tp>&>(__value);
> > >
> > > Is this any different to const_cast<const _Tp&>(__value) ?
> >
> > I think it is. const_cast<const _Tp&> can potentially mean the removal
> > of volatile,
> 
> True.
> 
> > so I would always look with suspicion on const_cast<const
> > _Tp&>, while static_cast is clearer. Alternatively, as_const could be
> > used, which does add_const_t.
> 
> Which means evaluating the add_const trait *and* overload resolution
> for as_const* *and* a runtime function call.
> 
> Let's go with static_cast<const _Tp&>.

Sounds good, like so?

-- >8 --


Subject: [PATCH 2/3] libstdc++: Implement ranges::iota from P2440R1

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (out_value_result): Define.
	(iota_result): Define.
	(__iota_fn, iota): Define.
	* testsuite/25_algorithms/iota/1.cc: New test.
---
 libstdc++-v3/include/bits/ranges_algo.h       | 48 +++++++++++++++++++
 .../testsuite/25_algorithms/iota/1.cc         | 29 +++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 libstdc++-v3/testsuite/25_algorithms/iota/1.cc

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index 11206bdbcaa..f75735f02cb 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -3517,6 +3517,54 @@ namespace ranges
   };
 
   inline constexpr __contains_subrange_fn contains_subrange{};
+
+  template<typename _Out, typename _Tp>
+    struct out_value_result
+    {
+      [[no_unique_address]] _Out out;
+      [[no_unique_address]] _Tp value;
+
+      template<typename _Out2, typename _Tp2>
+	requires convertible_to<const _Out&, _Out2>
+	  && convertible_to<const _Tp&, _Tp2>
+	constexpr
+	operator out_value_result<_Out2, _Tp2>() const &
+	{ return {out, value}; }
+
+      template<typename _Out2, typename _Tp2>
+	requires convertible_to<_Out, _Out2>
+	  && convertible_to<_Tp, _Tp2>
+	constexpr
+	operator out_value_result<_Out2, _Tp2>() &&
+	{ return {std::move(out), std::move(value)}; }
+    };
+
+  template<typename _Out, typename _Tp>
+    using iota_result = out_value_result<_Out, _Tp>;
+
+  struct __iota_fn
+  {
+    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
+      requires indirectly_writable<_Out, const _Tp&>
+      constexpr iota_result<_Out, _Tp>
+      operator()(_Out __first, _Sent __last, _Tp __value) const
+      {
+	while (__first != __last)
+	  {
+	    *__first = static_cast<const _Tp&>(__value);
+	    ++__first;
+	    ++__value;
+	  }
+	return {std::move(__first), std::move(__value)};
+      }
+
+    template<weakly_incrementable _Tp, output_range<const _Tp&> _Range>
+      constexpr iota_result<borrowed_iterator_t<_Range>, _Tp>
+      operator()(_Range&& __r, _Tp __value) const
+      { return (*this)(ranges::begin(__r), ranges::end(__r), std::move(__value)); }
+  };
+
+  inline constexpr __iota_fn iota{};
 #endif // C++23
 } // namespace ranges
 
diff --git a/libstdc++-v3/testsuite/25_algorithms/iota/1.cc b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
new file mode 100644
index 00000000000..ad2bf08adf5
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/iota/1.cc
@@ -0,0 +1,29 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do run { target c++23 } }
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+namespace ranges = std::ranges;
+
+void
+test01()
+{
+  int x[3] = {};
+  __gnu_test::test_output_range<int> rx(x);
+  auto r0 = ranges::iota(rx, 0);
+  VERIFY( r0.out.ptr == x+3 );
+  VERIFY( r0.value == 3 );
+  VERIFY( ranges::equal(x, (int[]){0,1,2}) );
+  auto r1 = ranges::iota(x, x+2, 5);
+  VERIFY( r1.out == x+2 );
+  VERIFY( r1.value == 7 );
+  VERIFY( ranges::equal(x, (int[]){5,6,2}) );
+}
+
+int
+main()
+{
+  test01();
+}
-- 
2.38.1.420.g319605f8f0