std::pointer_traits - cppreference.com (original) (raw)

Defined in header
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

| | 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

#include #include   template 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, block_ptr_t is smart_ptr using block_ptr_t = typename std::pointer_traits::template rebind;   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> bl2; // The type of bl2.free_blocks is // std::shared_ptr<BlockList<std::shared_ptr>::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] |