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Showing content from https://en.cppreference.com/w/cpp/language/../memory/weak_ptr/../pointer_traits.html below:

std::pointer_traits - cppreference.com

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

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.

[edit] Member types Type Definition pointer Ptr element_type /*element-type-of*/<Ptr> 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 Ptr is a template specialization Template<T, Args...> [edit] Member functions obtains a dereferenceable pointer to its argument
(public static member function) [edit] 2)

A specialization is provided for pointer types,

T*

, which declares the following members:

[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 obtains a dereferenceable pointer to its argument
(public static member function) [edit] [edit] Optional member functions of program-defined specializations

[static] (C++20)(optional)

obtains a raw pointer from a fancy pointer (inverse of pointer_to)
(public static member function) [edit] [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,

A specialization for user-defined fancy pointer types may provide an additional static member function to_address to customize the behavior of std::to_address.

(since C++20) [edit] Example

#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:

[edit] 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 element_type is invalid made SFINAE-friendly [edit] See also provides information about allocator types
(class template) [edit] obtains actual address of an object, even if the & operator is overloaded
(function template) [edit]

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