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libstdc++: Fix std::allocator<void> for versioned namespace

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Removing the allocator<void> specialization for the versioned namespace
breaks _Extptr_allocator<void> because the allocator<void>
specialization was still declared in <bits/memoryfwd.h>, making it an
incomplete type.  It wrong to remove that specialization anyway, because
it is still needed pre-C++20.

This removes the #if ! _GLIBCXX_INLINE_VERSION check, so that
allocator<void> is still explicitly specialized for the versioned
namespace, consistent with the normal unversioned namespace mode.

To make _Extptr_allocator<void> usable as a ProtoAllocator, this change
adds a default constructor and converting constructor. That is
consistent with std::allocator<void> since C++20 (and harmless to do for
earlier standards).

I'm also explicitly specializing allocator_traits<allocator<void>> so
that it doesn't need to use allocator<void>::construct and destroy.
Doing that allows those members to be removed, further simplifying
allocator<void>.  That new explicit specialization can delete the
allocate, deallocate and max_size members, which are always ill-formed
for allocator<void>.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/bits/alloc_traits.h (allocator_traits): Add explicit
	specialization for allocator<void>. Improve doxygen comments.
	* include/bits/allocator.h (allocator<void>): Restore for the
	versioned namespace.
	(allocator<void>::construct, allocator<void>::destroy): Remove.
	* include/ext/extptr_allocator.h (_Extptr_allocator<void>):
	Add default constructor and converting constructor.
This commit is contained in:
Jonathan Wakely 2021-08-28 11:05:58 +01:00
parent dd3e5859fc
commit 754fca77e8
3 changed files with 120 additions and 27 deletions
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104
libstdc++-v3/include/bits/alloc_traits.h
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@ -559,6 +559,110 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ return __rhs; }
};
/// Explicit specialization for std::allocator<void>.
template<>
struct allocator_traits<allocator<void>>
{
/// The allocator type
using allocator_type = allocator<void>;
/// The allocated type
using value_type = void;
/// The allocator's pointer type.
using pointer = void*;
/// The allocator's const pointer type.
using const_pointer = const void*;
/// The allocator's void pointer type.
using void_pointer = void*;
/// The allocator's const void pointer type.
using const_void_pointer = const void*;
/// The allocator's difference type
using difference_type = std::ptrdiff_t;
/// The allocator's size type
using size_type = std::size_t;
/// How the allocator is propagated on copy assignment
using propagate_on_container_copy_assignment = false_type;
/// How the allocator is propagated on move assignment
using propagate_on_container_move_assignment = true_type;
/// How the allocator is propagated on swap
using propagate_on_container_swap = false_type;
/// Whether all instances of the allocator type compare equal.
using is_always_equal = true_type;
template<typename _Up>
using rebind_alloc = allocator<_Up>;
template<typename _Up>
using rebind_traits = allocator_traits<allocator<_Up>>;
/// allocate is ill-formed for allocator<void>
static void*
allocate(allocator_type&, size_type, const void* = nullptr) = delete;
/// deallocate is ill-formed for allocator<void>
static void
deallocate(allocator_type&, void*, size_type) = delete;
/**
* @brief Construct an object of type `_Up`
* @param __a An allocator.
* @param __p Pointer to memory of suitable size and alignment for
* an object of type `_Up`.
* @param __args Constructor arguments.
*
* Calls `__a.construct(__p, std::forward<_Args>(__args)...)`
* in C++11, C++14 and C++17. Changed in C++20 to call
* `std::construct_at(__p, std::forward<_Args>(__args)...)` instead.
*/
template<typename _Up, typename... _Args>
static _GLIBCXX20_CONSTEXPR void
construct(allocator_type&, _Up* __p, _Args&&... __args)
noexcept(std::is_nothrow_constructible<_Up, _Args...>::value)
{
#if __cplusplus <= 201703L
::new((void *)__p) _Up(std::forward<_Args>(__args)...);
#else
std::construct_at(__p, std::forward<_Args>(__args)...);
#endif
}
/**
* @brief Destroy an object of type `_Up`
* @param __a An allocator.
* @param __p Pointer to the object to destroy
*
* Invokes the destructor for `*__p`.
*/
template<typename _Up>
static _GLIBCXX20_CONSTEXPR void
destroy(allocator_type&, _Up* __p)
noexcept(is_nothrow_destructible<_Up>::value)
{ std::_Destroy(__p); }
/// max_size is ill-formed for allocator<void>
static size_type
max_size(const allocator_type&) = delete;
/**
* @brief Obtain an allocator to use when copying a container.
* @param __rhs An allocator.
* @return `__rhs`
*/
static _GLIBCXX20_CONSTEXPR allocator_type
select_on_container_copy_construction(const allocator_type& __rhs)
{ return __rhs; }
};
#if __cplusplus < 201703L
template<typename _Alloc>
inline void

38
libstdc++-v3/include/bits/allocator.h
View File

@ -61,12 +61,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
*/
// Since C++20 the primary template should be used for allocator<void>,
// but then it would have a non-trivial default ctor and dtor, which
// would be an ABI change. So C++20 still uses the allocator<void> explicit
// specialization, with the historical ABI properties, but with the same
// members that are present in the primary template.
// but then it would have a non-trivial default ctor and dtor for C++20,
// but trivial for C++98-17, which would be an ABI incompatibiliy between
// different standard dialects. So C++20 still uses the allocator<void>
// explicit specialization, with the historical ABI properties, but with
// the same members that are present in the primary template.
#if ! _GLIBCXX_INLINE_VERSION
/// allocator<void> specialization.
template<>
class allocator<void>
@ -77,7 +77,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typedef ptrdiff_t difference_type;
#if __cplusplus <= 201703L
// These were removed for C++20.
// These were removed for C++20, allocator_traits does the right thing.
typedef void* pointer;
typedef const void* const_pointer;
@ -96,7 +96,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
= true_type;
#if __cplusplus >= 202002L
// As noted above, these members are present for C++20 to provide the
// same API as the primary template, but still trivial as in pre-C++20.
allocator() = default;
~allocator() = default;
template<typename _Up>
constexpr
@ -105,28 +108,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// No allocate member because it's ill-formed by LWG 3307.
// No deallocate member because it would be undefined to call it
// with any pointer which wasn't obtained from allocate.
#else // ! C++20
private:
// This uses construct and destroy in C++11/14/17 modes.
friend allocator_traits<allocator<void>>;
template<typename _Up, typename... _Args>
void
construct(_Up* __p, _Args&&... __args)
noexcept(std::is_nothrow_constructible<_Up, _Args...>::value)
{ ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
template<typename _Up>
void
destroy(_Up* __p)
noexcept(std::is_nothrow_destructible<_Up>::value)
{ __p->~_Up(); }
#endif // C++17
#endif // C++20
#endif // C++11
};
#endif // ! _GLIBCXX_INLINE_VERSION
/**
* @brief The @a standard allocator, as per C++03 [20.4.1].
@ -212,7 +196,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return;
}
#endif
__allocator_base<_Tp>::deallocate(__p, __n);
__allocator_base<_Tp>::deallocate(__p, __n);
}
#endif // C++20

5
libstdc++-v3/include/ext/extptr_allocator.h
View File

@ -176,6 +176,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
typedef _Pointer_adapter<_Relative_pointer_impl<const void> >
const_pointer;
_ExtPtr_allocator() { }
template<typename _Up>
_ExtPtr_allocator(const _ExtPtr_allocator<_Up>&) { }
template<typename _Up>
struct rebind
{ typedef _ExtPtr_allocator<_Up> other; };