On Fri, Jun 13, 2025 at 11:31 AM Jonathan Wakely <jwak...@redhat.com> wrote:
> The std::uninitialized_{value,default}_construct{,_n} algorithms should
> be able to create arrays, but that currently fails because when an
> exception happens they clean up using std::_Destroy and in C++17 that
> doesn't support destroying arrays. (For C++20 and later, std::destroy
> does handle destroying arrays.)
>
> This commit adjusts the _UninitDestroyGuard RAII type used by those
> algos so that in C++17 mode it recursively destroys each rank of an
> array type, only using std::_Destroy for the last rank when it's
> destroying non-array objects.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/120397
> * include/bits/stl_uninitialized.h (_UninitDestroyGuard<I,void>):
> Add new member function _S_destroy and call it from the
> destructor (for C++17 only).
> *
> testsuite/20_util/specialized_algorithms/uninitialized_default_construct/120397.cc:
> New test.
> *
> testsuite/20_util/specialized_algorithms/uninitialized_value_construct/120397.cc:
> New test.
> ---
>
> v2: Only define+use _S_destroy for C++17, at Tomasz's request.
>
LGTM. Thanks. The previous version required understanding subtleties of
_Destroy,
that is sometimes forwards to std::destroy_at. Now, I can understand it
just by looking on the file.
>
> Tested x86_64-linux.
>
> libstdc++-v3/include/bits/stl_uninitialized.h | 21 +++++++++++++++++++
> .../uninitialized_default_construct/120397.cc | 19 +++++++++++++++++
> .../uninitialized_value_construct/120397.cc | 19 +++++++++++++++++
> 3 files changed, 59 insertions(+)
> create mode 100644
> libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_default_construct/120397.cc
> create mode 100644
> libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_value_construct/120397.cc
>
> diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h
> b/libstdc++-v3/include/bits/stl_uninitialized.h
> index bde787c2beaa..4b3f81604606 100644
> --- a/libstdc++-v3/include/bits/stl_uninitialized.h
> +++ b/libstdc++-v3/include/bits/stl_uninitialized.h
> @@ -118,7 +118,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> ~_UninitDestroyGuard()
> {
> if (__builtin_expect(_M_cur != 0, 0))
> +#if __cplusplus == 201703L
> + // std::uninitialized_{value,default}{,_n} can construct array
> types,
> + // but std::_Destroy cannot handle them until C++20 (PR 120397).
> + _S_destroy(_M_first, *_M_cur);
> +#else
> std::_Destroy(_M_first, *_M_cur);
> +#endif
> }
>
> _GLIBCXX20_CONSTEXPR
> @@ -129,6 +135,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> private:
> _UninitDestroyGuard(const _UninitDestroyGuard&);
> +
> +#if __cplusplus == 201703L
> + template<typename _Iter>
> + _GLIBCXX20_CONSTEXPR
> + static void
> + _S_destroy(_Iter __first, _Iter __last)
> + {
> + using _ValT = typename iterator_traits<_Iter>::value_type;
> + if constexpr (is_array<_ValT>::value)
> + for (; __first != __last; ++__first)
> + _S_destroy(*__first, *__first + extent<_ValT>::value);
> + else
> + std::_Destroy(__first, __last);
> + }
> +#endif
> };
>
> // This is the default implementation of std::uninitialized_copy.
> diff --git
> a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_default_construct/120397.cc
> b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_default_construct/120397.cc
> new file mode 100644
> index 000000000000..7aa05d7d12f4
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_default_construct/120397.cc
> @@ -0,0 +1,19 @@
> +// { dg-do compile { target c++17 } }
> +
> +#include <memory>
> +
> +// PR libstdc++/120397
> +// std::uninitialized_value_construct cannot create arrays of
> non-trivially
> +// destructible types
> +
> +struct X { X() { } ~X() { } };
> +
> +void def(X (*x)[1])
> +{
> + std::uninitialized_default_construct(x, x+1);
> +}
> +
> +void def_n(X (*x)[1])
> +{
> + std::uninitialized_default_construct_n(x, 1);
> +}
> diff --git
> a/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_value_construct/120397.cc
> b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_value_construct/120397.cc
> new file mode 100644
> index 000000000000..f4d9fce20213
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/20_util/specialized_algorithms/uninitialized_value_construct/120397.cc
> @@ -0,0 +1,19 @@
> +// { dg-do compile { target c++17 } }
> +
> +#include <memory>
> +
> +// PR libstdc++/120397
> +// std::uninitialized_value_construct cannot create arrays of
> non-trivially
> +// destructible types
> +
> +struct X { X() { } ~X() { } };
> +
> +void val(X (*x)[1])
> +{
> + std::uninitialized_value_construct(x, x+1);
> +}
> +
> +void val_n(X (*x)[1])
> +{
> + std::uninitialized_value_construct_n(x, 1);
> +}
> --
> 2.49.0
>
>
