std::pointer_traits(3)

NAME
std::pointer_traits - std::pointer_traits

Synopsis
   Defined in header <memory>
   template< class Ptr > (1) (since C++11)
   struct pointer_traits;
   template< class T > (2) (since C++11)
   struct pointer_traits<T*>;
   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 conditionally declares the following members:
   Let /*element-type-of*/<Ptr> be
     * Ptr::element_type if present;
     * otherwise, T if Ptr is a class template specialization Template<T, Args...>,
       where Args... is zero or more type arguments;
     * otherwise, not defined.
   If /*element-type-of*/<Ptr> is not defined, the primary template has no members
   specified in this page.

Member types
   Type Definition
   pointer Ptr
   element_type /*element-type-of*/<Ptr>
   difference_type Ptr::difference_type if present, otherwise std::ptrdiff_t

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

Member functions
   pointer_to obtains a dereferenceable pointer to its argument
   [static] (public static member function)
   2) A specialization is provided for pointer types, T*, which declares the following
   members:

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
   [static] (public static member function)
   Optional member functions of program-defined specializations
   to_address obtains a raw pointer from a fancy pointer (inverse of
   [static] (C++20)(optional) pointer_to)
                        (public static member function)

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);
   A specialization for user-defined fancy pointer types may provide an
   additional static member function to_address to customize the behavior (since C++20)
   of std::to_address.
       Feature-test macro Value Std Feature
   __cpp_lib_constexpr_memory 201811L (C++20) constexpr in std::pointer_traits

Example
// Run this code
 #include <iostream>
 #include <memory>
 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()
 {
     [[maybe_unused]]
     BlockList<int*> bl1;
     // The type of bl1.free_blocks is BlockList<int*>:: block*
     BlockList<std::shared_ptr<char>> bl2;
     // The type of bl2.free_blocks is
     // std::shared_ptr<BlockList<std::shared_ptr<char>>::block>
     std::cout << bl2.free_blocks.use_count() << '\n';
 }

Output:
 0
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3545 C++11 primary template caused hard error when made SFINAE-friendly
                       element_type is invalid

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