std::pointer_traits
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.
pointer_traits
declares the following types:
Member types
Type | Definition |
---|---|
pointer |
Ptr |
element_type |
Ptr::element_type if present. Otherwise T if Ptr is a template specialization Template<T, Args...> . Otherwise, the pointer_traits specialization is ill-formed |
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) |
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) |
T*
, which declares the following types:
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) |
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 |
(since C++20) |
Feature testing macro: __cpp_lib_constexpr_memory
.
Example
#include <memory> #include <iostream> 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
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) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/memory/pointer_traits