std::pointer_traits
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pointer_traits
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std::pointer_traits
Defined in header
<memory>
template< class Ptr >
struct pointer_traits;
(1)
(since C++11)
struct pointer_traits;
template< class T >
struct pointer_traits<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,
TifPtris 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.
Contents
[edit] Member types
Type
Definition
pointer
Ptr
element_type
/*element-type-of*/<Ptr>
[edit] 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...>
[edit] Member functions
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
[edit] Optional member functions of program-defined specializations
[static] (C++20)(optional)
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,
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.
| 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:
0
[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 |