std::pointer_traits

STD：：指针[医]性状

Defined in header
template< class Ptr > struct pointer_traits;	(1)	(since C++11)
template< class T > struct pointer_traits<T*>;	(2)	(since C++11)

大pointer_traits类模板提供了访问类指针类型%28的某些属性的标准化方法。花哨指针，如*进程间：：抵消[医]PTR29%。标准模板std::allocator_traits倚靠pointer_traits来确定各种类型的默认值。Allocator...

1%29非专业pointer_traits声明以下类型：

成员类型

Type	Definition
pointer	Ptr
element_type	Ptr::element_type if present. Otherwise T if Ptr is a template instantiation Template<T, Args...>
difference_type	Ptr::difference_type if present, otherwise std::ptrdiff_t

成员别名模板

Template	Definition
template <class U> using rebind	Ptr::rebind<U> if exists, otherwise Template<U, Args...> if Ptr is a template instantiation Template<T, Args...>

成员函数

pointer_to static	obtains a dereferenceable pointer to its argument (public static member function)

2%29为指针类型提供专门化，T*，它声明了以下类型。

成员类型

Type	Definition
pointer	T*
element_type	T
difference_type	std::ptrdiff_t

成员别名模板

Template	Definition
template< class U > using rebind	U*

成员函数

pointer_to static	obtains a dereferenceable pointer to its argument (public static member function)

注记

重新绑定成员模板别名使得在指针类类型指向T的情况下，可以获得指向U的相同的指针类型--例如，

二次

typedef std::pointer_traits<std::shared_ptr<int>>::rebind<double> another_pointer;
static_assert(std::is_same<another_pointer, std::shared_ptr<double>>::value, ""

二次

例

二次

#include <memory>
#include <iostream>
 
template <class Ptr>
struct BlockList
{
   // Predefine a memory block 
   struct block;
 
   // Define a pointer to a memory block from the kind of pointer Ptr s
   // If Ptr is any kind of T*, block_ptr_t is block*
   // If Ptr is smart_ptr<T>, block_ptr_t is smart_ptr<block>
   typedef typename std::pointer_traits<Ptr>::template rebind<block> block_ptr_t;
 
   struct block
   {
      std::size_t size;
      block_ptr_t next_block;
   }; 
 
   block_ptr_t free_blocks;
}; 
 
int main()
{
    BlockList<int*> bl1;
    // The type of bl1.free_blocks is block*
 
    BlockList<std::shared_ptr<char>> bl2;
    // The type of bl2.free_blocks is std::shared_ptr<block>
    std::cout << bl2.free_blocks.use_count() << '\n';
}

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产出：

二次

另见

allocator_traits (C++11)	provides information about allocator types (class template)
addressof (C++11)	obtains actual address of an object, even if the & operator is overloaded (function template)

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