std:: pointer_traits

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C++
Dynamic memory management
Uninitialized memory algorithms
Constrained uninitialized memory algorithms
Allocators
Garbage collection support
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)
(C++11) (until C++23)



std::pointer_traits
Member functions
(C++11)
(C++20) (optional)
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:

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 rebind Ptr :: rebind < U > if exists, otherwise Template < U, Args... > if Ptr is a template specialization Template < T, Args... >

Member functions

[static]
obtains a dereferenceable pointer to its argument
(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

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

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)

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 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

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 element_type is invalid made SFINAE-friendly

See also

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