allocator - libcxx 源码
来看allocator定义，首先是两个void特化版本
// include/__memory/allocator.h
template <class _Tp> class allocator;
#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_VOID_SPECIALIZATION)
// These specializations shouldn't be marked _LIBCPP_DEPRECATED_IN_CXX17.
// Specializing allocator<void> is deprecated, but not using it.
template <>
class _LIBCPP_TEMPLATE_VIS allocator<void>
{
#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
public:
_LIBCPP_DEPRECATED_IN_CXX17 typedef void* pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef const void* const_pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef void value_type;
template <class _Up> struct _LIBCPP_DEPRECATED_IN_CXX17 rebind {typedef allocator<_Up> other;};
#endif
};
template <>
class _LIBCPP_TEMPLATE_VIS allocator<const void>
{
#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
public:
_LIBCPP_DEPRECATED_IN_CXX17 typedef const void* pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef const void* const_pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef const void value_type;
template <class _Up> struct _LIBCPP_DEPRECATED_IN_CXX17 rebind {typedef allocator<_Up> other;};
#endif
};
#endif
- 在 C++11/C++14，标准对
allocator<void>有一个专门的特化版本，里面定义了pointer、const_pointer、value_type和rebind等成员，以便用来表示“无类型指针”或在容器内部做重绑定。- 从 C++17 开始，标准把对
allocator<void>那些成员的硬性要求去掉了——不再强制要有allocator<void>::pointer、allocator<void>::rebind这样的特化接口。
- 取而代之的是统一使用
allocator_traits<Alloc>来获取一个 allocator 的各种“traits”——包括pointer、void_pointer、以及rebind_alloc<U>等。
也就是说，不再需要你专门写或依赖一个
allocator<void>特化来提供“无类型指针”；只要对任意allocator<T>，你都可以通过：
allocator_traits<allocator<T>>::void_pointer // 用来表示 void* allocator_traits<allocator<T>>::rebind_alloc<U> // 用来得到 allocator<U>
来统一地获取这些信息。这样做的好处是：
- 简化标准，去掉了一个不常用的模板特化；
- 统一接口，所有 allocator 都通过
allocator_traits暴露同一套成员，不再需要特殊分支。
接下来是主类模板
// include/__memory/allocator.h
// allocator
//
// Note: For ABI compatibility between C++20 and previous standards, we make
// allocator<void> trivial in C++20.
template <class _Tp>
class _LIBCPP_TEMPLATE_VIS allocator
: private __non_trivial_if<!is_void<_Tp>::value, allocator<_Tp> >
{
static_assert(!is_volatile<_Tp>::value, "std::allocator does not support volatile types");
public:
typedef size_t size_type;
typedef ptrdiff_t difference_type;
typedef _Tp value_type;
typedef true_type propagate_on_container_move_assignment;
typedef true_type is_always_equal;
_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
allocator() _NOEXCEPT = default;
template <class _Up>
_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
allocator(const allocator<_Up>&) _NOEXCEPT { }
_LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
_Tp* allocate(size_t __n) {
if (__n > allocator_traits<allocator>::max_size(*this))
__throw_bad_array_new_length();
if (__libcpp_is_constant_evaluated()) {
return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)));
} else {
return static_cast<_Tp*>(_VSTD::__libcpp_allocate(__n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp)));
}
}
#if _LIBCPP_STD_VER > 20
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr
allocation_result<_Tp*> allocate_at_least(size_t __n) {
return {allocate(__n), __n};
}
#endif
_LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
void deallocate(_Tp* __p, size_t __n) _NOEXCEPT {
if (__libcpp_is_constant_evaluated()) {
::operator delete(__p);
} else {
_VSTD::__libcpp_deallocate((void*)__p, __n * sizeof(_Tp), _LIBCPP_ALIGNOF(_Tp));
}
}
// C++20 Removed members
#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
_LIBCPP_DEPRECATED_IN_CXX17 typedef _Tp* pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp* const_pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef _Tp& reference;
_LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp& const_reference;
template <class _Up>
struct _LIBCPP_DEPRECATED_IN_CXX17 rebind {
typedef allocator<_Up> other;
};
_LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
pointer address(reference __x) const _NOEXCEPT {
return _VSTD::addressof(__x);
}
_LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
const_pointer address(const_reference __x) const _NOEXCEPT {
return _VSTD::addressof(__x);
}
_LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_INLINE_VISIBILITY _LIBCPP_DEPRECATED_IN_CXX17
_Tp* allocate(size_t __n, const void*) {
return allocate(__n);
}
_LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY size_type max_size() const _NOEXCEPT {
return size_type(~0) / sizeof(_Tp);
}
template <class _Up, class... _Args>
_LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
void construct(_Up* __p, _Args&&... __args) {
::new ((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
}
_LIBCPP_DEPRECATED_IN_CXX17 _LIBCPP_INLINE_VISIBILITY
void destroy(pointer __p) {
__p->~_Tp();
}
#endif
};
在non trivial if中定义non trivial ctor是为了满足cpp 20标准的要求，而使用模板参数使每一个allocator<T>继承各自不同的non trivial if基类(通过模板参数_Unique区分)是为了避免ebo折叠，从而保持未来继承allocator<T>的类的abi
// include/__memory/allocator.h
template <bool _Cond, class _Unique>
struct __non_trivial_if { };
template <class _Unique>
struct __non_trivial_if<true, _Unique> {
_LIBCPP_INLINE_VISIBILITY
_LIBCPP_CONSTEXPR __non_trivial_if() _NOEXCEPT { }
};
所以，根据allocator<T>的定义，得知allocator<int>的定义。
那么__alloc_traits::void_pointer就是allocator_traits<allocator<int>>中的成员void_pointer.
// include/__memory/allocator_traits.h
template <class _Alloc>
struct _LIBCPP_TEMPLATE_VIS allocator_traits
{
using allocator_type = _Alloc;
using value_type = typename allocator_type::value_type;
using pointer = typename __pointer<value_type, allocator_type>::type;
using const_pointer = typename __const_pointer<value_type, pointer, allocator_type>::type;
using void_pointer = typename __void_pointer<pointer, allocator_type>::type;
using const_void_pointer = typename __const_void_pointer<pointer, allocator_type>::type;
using difference_type = typename __alloc_traits_difference_type<allocator_type, pointer>::type;
using size_type = typename __size_type<allocator_type, difference_type>::type;
using propagate_on_container_copy_assignment = typename __propagate_on_container_copy_assignment<allocator_type>::type;
using propagate_on_container_move_assignment = typename __propagate_on_container_move_assignment<allocator_type>::type;
using propagate_on_container_swap = typename __propagate_on_container_swap<allocator_type>::type;
using is_always_equal = typename __is_always_equal<allocator_type>::type;
#ifndef _LIBCPP_CXX03_LANG
template <class _Tp>
using rebind_alloc = __allocator_traits_rebind_t<allocator_type, _Tp>;
template <class _Tp>
using rebind_traits = allocator_traits<rebind_alloc<_Tp> >;
根据
// include/__memory/allocator_traits.h
// __void_pointer
_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_void_pointer, void_pointer);
template <class _Ptr, class _Alloc,
bool = __has_void_pointer<_Alloc>::value>
struct __void_pointer {
using type _LIBCPP_NODEBUG = typename _Alloc::void_pointer;
};
template <class _Ptr, class _Alloc>
struct __void_pointer<_Ptr, _Alloc, false> {
#ifdef _LIBCPP_CXX03_LANG
using type _LIBCPP_NODEBUG = typename pointer_traits<_Ptr>::template rebind<void>::other;
#else
using type _LIBCPP_NODEBUG = typename pointer_traits<_Ptr>::template rebind<void>;
#endif
};
对于一个int，因为在allocator_traits中，_Alloc即allocator<int>，是通过allocator<T>主模板实例化出来的，其没有void_pointer,因此__void_pointer<...>::type是通过第二个__void_pointer特化，实际上是把模板参数_Ptr rebind到void*来获得void_pointer
// include/__memory/allocator_traits.h
// __pointer
_LIBCPP_ALLOCATOR_TRAITS_HAS_XXX(__has_pointer, pointer);
template <class _Tp, class _Alloc,
class _RawAlloc = typename remove_reference<_Alloc>::type,
bool = __has_pointer<_RawAlloc>::value>
struct __pointer {
using type _LIBCPP_NODEBUG = typename _RawAlloc::pointer;
};
template <class _Tp, class _Alloc, class _RawAlloc>
struct __pointer<_Tp, _Alloc, _RawAlloc, false> {
using type _LIBCPP_NODEBUG = _Tp*;
};
这个_Ptr是allocator
即int*
// include/__memory/allocator.h allocator<T>
// C++20 Removed members
#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_ALLOCATOR_MEMBERS)
_LIBCPP_DEPRECATED_IN_CXX17 typedef _Tp* pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp* const_pointer;
_LIBCPP_DEPRECATED_IN_CXX17 typedef _Tp& reference;
_LIBCPP_DEPRECATED_IN_CXX17 typedef const _Tp& const_reference;
本文来自博客园，作者：ijpq，转载请注明原文链接：https://www.cnblogs.com/ijpq/p/18916673
浙公网安备 33010602011771号