[PATCH] libstdc++: Make bitset::reference a proper range proxy.
Tomasz Kaminski
tkaminsk@redhat.com
Thu Jun 25 13:01:41 GMT 2026
On Thu, Jun 25, 2026 at 2:51 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> On Thu, 25 Jun 2026 at 13:31, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
> >
> > This implements remaining changes from P3612R1 Harmonize proxy-reference
> > operations (LWG 3638 and 4187).
> >
> > As the const-assigment operator in bitset::reference (from LWG4187)
> > is used by range concepts (std::indirectly_writable) to recognize
> > iterator returning it by value as proxy iterator (see range_proxy.cc)
> test,
> > this change is backported to C++23. This puts it inline with vector<bool>
> > reference and other proxy type, for which the coresponding assigment
> > operator where added by P2321 in C++20 and implemented in
> r13-2159-g72886f.
> >
> > Similary, a swap overloads accepting bitset::reference by value and
> bool&,
> > are necessary for ranges::swap and ranges::iter_swap to work correctly,
> > so they should also backported. This patch follows the approach from
> > resolution of LWG3638 from r12-5311-g59434931fb658f, and backports the
> > overloads to C++11. This harmonizes the operations on this two proxy
> > types (per paper title). Corresponding test are added to both classes.
> >
> > The deprecation warning for static vector<bool>::swap is added sinc
> C++26.
> > The text in _GLIBCXX26_DEPRECATED_SUGGEST leads to following message:
> > 'static constexpr void std::vector<bool, _Alloc>::swap(reference,
> reference)
> > [with... ]' is deprecated: use 'swap' invoked via ADL or
> 'std::ranges::swap'
> > instead
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/stl_bvector.h: Add _GLIBCXX_RESOLVE_LIB_DEFECTS
> > comment for 3638.
> > (vector<bool, _Alloc>::swap): Add deprected attribute via
> > _GLIBCXX26_DEPRECATED_SUGGEST.
> > * include/std/bitset (bitset::operator=(bool) const)
> > [__cplusplus > 202002]: Define.
> > (bistset::reference::swap) [__cplusplus >= 201103L]: Define.
> > * testsuite/20_util/bitset/access/range_proxy.cc: New test.
> > * testsuite/20_util/bitset/access/ref_swap.cc: New test.
> > *
> testsuite/23_containers/vector/bool/element_access/range_proxy.cc:
> > New test.
> > * testsuite/23_containers/vector/bool/element_access/ref_swap.cc:
> > New test.
> > *
> testsuite/23_containers/vector/bool/modifiers/swap/constexpr.cc:
> > Checkfor deprecated warning on static swap.
> > ---
> > Do you agree with my backport choice for swap? I think the minimum is
> > since C++23.
>
> I don't remember why I chose C++11 for the swap overloads for
> vector<bool>::reference, but being consistent with that makes sense.
>
> >
> > Testing on x86_64-linux. *bitset* and *vector/bool* test already passed
> > in all standard modes. OK for trunk?
> >
> > libstdc++-v3/include/bits/stl_bvector.h | 4 +
> > libstdc++-v3/include/std/bitset | 43 +++++++++
> > .../20_util/bitset/access/range_proxy.cc | 94 +++++++++++++++++++
> > .../20_util/bitset/access/ref_swap.cc | 28 ++++++
> > .../vector/bool/element_access/range_proxy.cc | 62 ++++++++++++
> > .../vector/bool/element_access/ref_swap.cc | 38 ++++++++
> > .../vector/bool/modifiers/swap/constexpr.cc | 2 +-
> > 7 files changed, 270 insertions(+), 1 deletion(-)
> > create mode 100644
> libstdc++-v3/testsuite/20_util/bitset/access/range_proxy.cc
> > create mode 100644
> libstdc++-v3/testsuite/20_util/bitset/access/ref_swap.cc
> > create mode 100644
> libstdc++-v3/testsuite/23_containers/vector/bool/element_access/range_proxy.cc
> > create mode 100644
> libstdc++-v3/testsuite/23_containers/vector/bool/element_access/ref_swap.cc
> >
> > diff --git a/libstdc++-v3/include/bits/stl_bvector.h
> b/libstdc++-v3/include/bits/stl_bvector.h
> > index f190d1be5ce..0637e687cbb 100644
> > --- a/libstdc++-v3/include/bits/stl_bvector.h
> > +++ b/libstdc++-v3/include/bits/stl_bvector.h
> > @@ -149,6 +149,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> > { *_M_p ^= _M_mask; }
> >
> > #if __cplusplus >= 201103L
> > + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> > + // 3638. vector<bool>::swap(reference, reference) is useless
> > +
> > _GLIBCXX20_CONSTEXPR
> > friend void
> > swap(_Bit_reference __x, _Bit_reference __y) noexcept
> > @@ -1266,6 +1269,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> > }
> >
> > // [23.2.5]/1, third-to-last entry in synopsis listing
> > + _GLIBCXX26_DEPRECATED_SUGGEST("swap' invoked via ADL or
> 'std::ranges::swap")
> > _GLIBCXX20_CONSTEXPR
> > static void
> > swap(reference __x, reference __y) _GLIBCXX_NOEXCEPT
> > diff --git a/libstdc++-v3/include/std/bitset
> b/libstdc++-v3/include/std/bitset
> > index a8cb337f4b1..a5822319b90 100644
> > --- a/libstdc++-v3/include/std/bitset
> > +++ b/libstdc++-v3/include/std/bitset
> > @@ -908,6 +908,20 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> > return *this;
> > }
> >
> > +#if __cplusplus > 202002L
>
> Should we use __glibcxx_ranges_zip here? That's the macro for P2321
> which is what made the const-assignable changes that motivate this
> change.
>
This is matching check used for vector<bool>::reference from
r13-2159-g72886f
(__cplusplus > 202002L), so I want for consistency. Could change all of them
as separate commit. On the other hand, this is not strictly related to
zip_view,
as people could write they own references.
I have wondered if the "fixes" for proxies shouldn't actually be backported
to C++20. so we support user-provided zip-views in C++20. But touching a
tuple
is always scary.
> If it's changed here it should be changed in the range_proxy.cc test too.
>
That does not work nice if we ever replace includes with modules, as we
would
need to include <version>.
>
> > + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> > + // 4187. bitset::reference should be const-assignable
> > + constexpr const reference&
> > + operator=(bool __x) const noexcept
> > + {
> > + if (__x)
> > + *_M_wp |= _Base::_S_maskbit(_M_bpos);
> > + else
> > + *_M_wp &= ~_Base::_S_maskbit(_M_bpos);
> > + return *this;
> > + }
> > +#endif // C++23
> > +
> > // For b[i] = b[__j];
> > _GLIBCXX23_CONSTEXPR
> > reference&
> > @@ -939,6 +953,35 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> > *_M_wp ^= _Base::_S_maskbit(_M_bpos);
> > return *this;
> > }
> > +
> > +#if __cplusplus >= 201103L
>
> Using 201103 here matches the condition for the
> vector<bool>::reference swap overloads, good.
>
> > + _GLIBCXX23_CONSTEXPR
> > + friend void
> > + swap(reference __x, reference __y) noexcept
> > + {
> > + bool __tmp = __x;
> > + __x = __y;
> > + __y = __tmp;
> > + }
> > +
> > + _GLIBCXX23_CONSTEXPR
> > + friend void
> > + swap(reference __x, bool& __y) noexcept
> > + {
> > + bool __tmp = __x;
> > + __x = __y;
> > + __y = __tmp;
> > + }
> > +
> > + _GLIBCXX23_CONSTEXPR
> > + friend void
> > + swap(bool& __x, reference __y) noexcept
> > + {
> > + bool __tmp = __x;
> > + __x = __y;
> > + __y = __tmp;
> > + }
> > +#endif
> > };
> > friend class reference;
> >
> > diff --git a/libstdc++-v3/testsuite/20_util/bitset/access/range_proxy.cc
> b/libstdc++-v3/testsuite/20_util/bitset/access/range_proxy.cc
> > new file mode 100644
> > index 00000000000..9eac384ce44
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/20_util/bitset/access/range_proxy.cc
> > @@ -0,0 +1,94 @@
> > +// { dg-do run { target c++20 } }
> > +
> > +#include <bitset>
> > +#include <concepts>
> > +#include <iterator>
> > +#include <testsuite_hooks.h>
> > +
> > +void
> > +test_swap()
> > +{
> > + std::bitset<2> v("01");
> > + std::ranges::swap(v[0], v[1]);
> > + VERIFY( v[0] == false );
> > + VERIFY( v[1] == true );
> > +
> > + bool b = true;
> > + std::ranges::swap(v[0], b);
> > + VERIFY( v[0] == true );
> > + VERIFY( b == false );
> > +
> > + std::ranges::swap(b, v[1]);
> > + VERIFY( v[1] == false );
> > + VERIFY( b == true );
> > +}
> > +
> > +template<size_t N>
> > +struct bitset_iterator
> > +{
> > + using value_type = bool;
> > + using reference = std::bitset<2>::reference;
> > + using iterator_category = std::forward_iterator_tag;
> > + using difference_type = std::ptrdiff_t;
> > +
> > + bitset_iterator() = default;
> > + bitset_iterator(std::bitset<N>& c) noexcept : cont(&c) {}
> > +
> > + reference operator*() const noexcept
> > + { return (*cont)[pos]; }
> > +
> > + bitset_iterator& operator++() {
> > + ++pos;
> > + return *this;
> > + }
> > +
> > + bitset_iterator operator++(int) {
> > + auto tmp = *this;
> > + ++pos;
> > + return *this;
> > + };
> > +
> > + friend bool operator==(bitset_iterator, bitset_iterator) = default;
> > +
> > +private:
> > + std::bitset<N>* cont = nullptr;
> > + size_t pos = 0;
> > +};
> > +
> > +void
> > +test_iter_swap()
> > +{
> > + std::bitset<2> v("01");
> > + auto it1 = bitset_iterator<2>(v);
> > + auto it2 = std::ranges::next(it1);
> > + std::ranges::iter_swap(it1, it2);
> > + VERIFY( v[0] == false );
> > + VERIFY( v[1] == true );
> > +
> > + bool b = true, *p = &b;
> > + std::ranges::iter_swap(it1, p);
> > + VERIFY( v[0] == true );
> > + VERIFY( b == false );
> > +
> > + std::ranges::iter_swap(p, it2);
> > + VERIFY( v[1] == false );
> > + VERIFY( b == true );
> > +}
> > +
> > +#if __cplusplus > 202002L
> > +static_assert( std::indirectly_writable<bitset_iterator<2>, bool> );
> > +static_assert( std::indirectly_writable<bitset_iterator<2>,
> > + std::bitset<2>::reference> );
> > +
> > +#else
> > +static_assert( !std::indirectly_writable<bitset_iterator<2>, bool> );
> > +static_assert( !std::indirectly_writable<bitset_iterator<2>,
> > + std::bitset<2>::reference> );
> > +#endif
> > +
> > +int
> > +main()
> > +{
> > + test_swap();
> > + test_iter_swap();
> > +}
> > diff --git a/libstdc++-v3/testsuite/20_util/bitset/access/ref_swap.cc
> b/libstdc++-v3/testsuite/20_util/bitset/access/ref_swap.cc
> > new file mode 100644
> > index 00000000000..1d8e3c1bf49
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/20_util/bitset/access/ref_swap.cc
> > @@ -0,0 +1,28 @@
> > +// { dg-do run { target c++11 } }
> > +
> > +#include <bitset>
> > +#include <testsuite_hooks.h>
> > +
> > +void
> > +test_friend()
> > +{
> > + std::bitset<2> v("01");
> > + swap(v[0], v[1]);
> > + VERIFY( v[0] == false );
> > + VERIFY( v[1] == true );
> > +
> > + bool b = true;
> > + swap(v[0], b);
> > + VERIFY( v[0] == true );
> > + VERIFY( b == false );
> > +
> > + swap(b, v[1]);
> > + VERIFY( v[1] == false );
> > + VERIFY( b == true );
> > +}
> > +
> > +int
> > +main()
> > +{
> > + test_friend();
> > +}
> > diff --git
> a/libstdc++-v3/testsuite/23_containers/vector/bool/element_access/range_proxy.cc
> b/libstdc++-v3/testsuite/23_containers/vector/bool/element_access/range_proxy.cc
> > new file mode 100644
> > index 00000000000..075132034f1
> > --- /dev/null
> > +++
> b/libstdc++-v3/testsuite/23_containers/vector/bool/element_access/range_proxy.cc
> > @@ -0,0 +1,62 @@
> > +// { dg-do run { target c++20 } }
> > +
> > +#include <vector>
> > +#include <concepts>
> > +#include <iterator>
> > +#include <testsuite_hooks.h>
> > +
> > +void
> > +test_swap()
> > +{
> > + std::vector<bool> v{true, false};
> > + std::ranges::swap(v[0], v[1]);
> > + VERIFY( v[0] == false );
> > + VERIFY( v[1] == true );
> > +
> > + bool b = true;
> > + std::ranges::swap(v[0], b);
> > + VERIFY( v[0] == true );
> > + VERIFY( b == false );
> > +
> > + std::ranges::swap(b, v[1]);
> > + VERIFY( v[1] == false );
> > + VERIFY( b == true );
> > +}
> > +
> > +void
> > +test_iter_swap()
> > +{
> > + std::vector<bool> v{true, false};
> > + auto it1 = v.begin();
> > + auto it2 = std::ranges::next(it1);
> > + std::ranges::iter_swap(it1, it2);
> > + VERIFY( v[0] == false );
> > + VERIFY( v[1] == true );
> > +
> > + bool b = true, *p = &b;
> > + std::ranges::iter_swap(it1, p);
> > + VERIFY( v[0] == true );
> > + VERIFY( b == false );
> > +
> > + std::ranges::iter_swap(p, it2);
> > + VERIFY( v[1] == false );
> > + VERIFY( b == true );
> > +}
> > +
> > +#if __cplusplus > 202002L
> > +static_assert( std::indirectly_writable<std::vector<bool>::iterator,
> bool> );
> > +static_assert( std::indirectly_writable<std::vector<bool>::iterator,
> > + std::vector<bool>::reference> );
> > +
> > +#else
> > +static_assert( !std::indirectly_writable<std::vector<bool>::iterator,
> bool> );
> > +static_assert( !std::indirectly_writable<std::vector<bool>::iterator,
> > + std::vector<bool>::reference> );
> > +#endif
> > +
> > +int
> > +main()
> > +{
> > + test_swap();
> > + test_iter_swap();
> > +}
> > diff --git
> a/libstdc++-v3/testsuite/23_containers/vector/bool/element_access/ref_swap.cc
> b/libstdc++-v3/testsuite/23_containers/vector/bool/element_access/ref_swap.cc
> > new file mode 100644
> > index 00000000000..2221e3b03f0
> > --- /dev/null
> > +++
> b/libstdc++-v3/testsuite/23_containers/vector/bool/element_access/ref_swap.cc
> > @@ -0,0 +1,38 @@
> > +// { dg-do run { target c++11 } }
> > +
> > +#include <vector>
> > +#include <testsuite_hooks.h>
> > +
> > +void
> > +test_friend()
> > +{
> > + std::vector<bool> v{true, false};
> > + swap(v[0], v[1]);
> > + VERIFY( v[0] == false );
> > + VERIFY( v[1] == true );
> > +
> > + bool b = true;
> > + swap(v[0], b);
> > + VERIFY( v[0] == true );
> > + VERIFY( b == false );
> > +
> > + swap(b, v[1]);
> > + VERIFY( v[1] == false );
> > + VERIFY( b == true );
> > +}
> > +
> > +void
> > +test_static()
> > +{
> > + std::vector<bool> v{true, false};
> > + std::vector<bool>::swap(v[0], v[1]); // { dg-warning "deprecated"
> "::swap" { target c++26 } }
> > + VERIFY( v[0] == false );
> > + VERIFY( v[1] == true );
> > +}
> > +
> > +int
> > +main()
> > +{
> > + test_friend();
> > + test_static();
> > +}
> > diff --git
> a/libstdc++-v3/testsuite/23_containers/vector/bool/modifiers/swap/constexpr.cc
> b/libstdc++-v3/testsuite/23_containers/vector/bool/modifiers/swap/constexpr.cc
> > index d2a0218a109..1eb526800f7 100644
> > ---
> a/libstdc++-v3/testsuite/23_containers/vector/bool/modifiers/swap/constexpr.cc
> > +++
> b/libstdc++-v3/testsuite/23_containers/vector/bool/modifiers/swap/constexpr.cc
> > @@ -53,7 +53,7 @@ constexpr bool
> > test_reference_swap()
> > {
> > std::vector<bool> v1 = {true, false, true};
> > - std::vector<bool>::swap(v1[0], v1[1]);
> > + std::vector<bool>::swap(v1[0], v1[1]); // { dg-warning "deprecated"
> "::swap" { target c++26 } }
> >
> > VERIFY(v1[0] == false);
> > VERIFY(v1[1] == true);
> > --
> > 2.54.0
> >
>
>
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