NAME¶

std::pointer_traits - std::pointer_traits

Synopsis¶

Defined in header <memory>
template< class Ptr > (1) (since C++11)
struct pointer_traits;
template< class T > (2) (since C++11)
struct pointer_traits<T*>;

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

Member functions¶

pointer_to obtains a dereferenceable pointer to its argument
[static] (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 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)

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 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 made SFINAE-friendly
element_type is invalid

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)