std::pointer_traits (3) - Linux Manuals

std::pointer_traits: std::pointer_traits

NAME

std::pointer_traits - std::pointer_traits

Synopsis

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 (fancy_pointers, such as boost::interprocess::offset_ptr). 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:

Member types

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

Member alias templates

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...>

Member functions

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

2) A specialization is provided for pointer types, T*, which declares the following types

Member types

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

Member alias templates

Template Definition
template< class U > using rebind U*

Member functions

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

3) A specialization for user-defined fancy pointer types may define an additional static member function

Optional Member functions

to_address obtains a raw pointer from a fancy pointer (inverse of pointer_to)
                 (public static member function)
[static] (C++20)

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,

  using another_pointer = std::pointer_traits<std::shared_ptr<int>>::rebind<double>;
  static_assert(std::is_same<another_pointer, std::shared_ptr<double>>::value);

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>
     using block_ptr_t = typename std::pointer_traits<Ptr>::template rebind<block>;

     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