std::pointer_traits(3)

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
                   Ptr::element_type if present. Otherwise T if Ptr is a template
   element_type specialization Template<T, Args...>. Otherwise, the pointer_traits
                   specialization is ill-formed
   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...> if
   rebind Ptr is a template specialization Template<T, Args...>

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)
   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
   [static] (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: __cpp_lib_constexpr_memory

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()
 {
     [[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

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)