std::pointer_traits

 

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std::pointer_traits

Member functions

pointer_traits::pointer_to (C++11)

pointer_traits::to_address (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

pointer_to [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

pointer_to [static]

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

Optional member functions of program-defined specializations

to_address [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

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