std::pointer_traits - cppreference.com [proxy]

std::pointer_traits

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 conditionally declares the following members:

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

[static]

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)

Feature-test macro	Value	Std	Feature
`__cpp_lib_constexpr_memory`	`201811L`	(C++20)	constexpr in `std::pointer_traits`

[edit] 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:

[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

(C++11)	provides information about allocator types (class template) [edit]
(C++11)	obtains actual address of an object, even if the `&` operator is overloaded (function template) [edit]