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 declares the following types:
| Type | Definition |
|---|---|
pointer | Ptr |
element_type | Ptr::element_type if present. Otherwise T if Ptr is a template instantiation Template<T, Args...> |
difference_type | Ptr::difference_type if present, otherwise std::ptrdiff_t |
| Template | Definition |
|---|---|
| template <class U> using rebind | Ptr::rebind<U> if exists, otherwise Template<U, Args...> if Ptr is a template instantiation Template<T, Args...> |
|
[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 types.
| Type | Definition |
|---|---|
pointer | T* |
element_type | T |
difference_type | std::ptrdiff_t |
| Template | Definition |
|---|---|
| template< class U > using rebind | U* |
|
[static] | obtains a dereferenceable pointer to its argument (public static member function) |
3) A specialization for user-defined fancy pointer types may define an additional static member function.
|
[static] (C++20) | obtains a raw pointer from a fancy pointer (inverse of pointer_to) (public static member function) |
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);
#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()
{
BlockList<int*> bl1;
// The type of bl1.free_blocks is block*
BlockList<std::shared_ptr<char>> bl2;
// The type of bl2.free_blocks is std::shared_ptr<block>
std::cout << bl2.free_blocks.use_count() << '\n';
}Output:
0
|
(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) |
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