std::pointer_traits
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                    | Defined in header  <memory> | ||
| template< class Ptr > struct pointer_traits; | (1) | (since C++11) | 
| template< class T > struct pointer_traits<T*>; | (2) | (since C++11) | 
The pointer_traits class template provides the standardized way to access certain properties of pointer-like types. The standard template std::allocator_traits relies on pointer_traits to determine the defaults for various typedefs required by Allocator.
1) The non-specialized pointer_traits declares the following types:
| Contents | 
[edit] Member types
| Type | Definition | 
| pointer | Ptr | 
| element_type | Ptr::element_type if present. Otherwise TifPtris a template instantiation Template<T, Args...> | 
| difference_type | Ptr::difference_type if present, otherwise std::ptrdiff_t | 
[edit] Member alias templates
| Template | Definition | 
| template <class U> using rebind | Ptr::rebind<U> if exists, otherwise Template<U, Args...> if Ptris a template instantiation Template<T, Args...> | 
[edit] Member functions
| [static] | obtains a dereferencable pointer to its argument (public static member function) | 
2) A specialization is provided for pointer types, T*, which declares the following types
[edit] Member types
| Type | Definition | 
| pointer | T* | 
| element_type | T | 
| difference_type | std::ptrdiff_t | 
[edit] Member alias templates
| Template | Definition | 
| template< class U > using rebind | U* | 
[edit] Member functions
| [static] | obtains a dereferenceable pointer to its argument (public static member function) | 
[edit] Notes
The rebind member template alias makes it possible, given a pointer-like type that points to T, to obtain the same pointer-like type that points to U. For example,
typedef std::pointer_traits<std::shared_ptr<int>>::rebind<double> another_pointer; static_assert(std::is_same<another_pointer, std::shared_ptr<double>>::value, "");
[edit] Example
Run this code
#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'; }
Output:
0
[edit] See also
| (C++11) | provides information about allocator types (class template) | 
| (C++11) | obtains actual address of an object, even if the & operator is overloaded (function template) | 


