| Defined in header <memory> | ||||
|---|---|---|---|---|
| (1) | (since C++11) | ||
| (2) | (since C++11) |
The pointer_traits class template provides the standardized way to access certain properties of pointer-like types. 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:
Member 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 |
Member alias templates
| 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...> |
Member functions
| [static] | obtains a dereferencable pointer to its argument (public static member function) |
2) A specialization is provided for pointer types, 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,
1 2 | typedef std::pointer_traits<std::shared_ptr<int>>::rebind<double> another_pointer;static_assert(std::is_same<another_pointer, std::shared_ptr<double>>::value, ""); |
Example
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 | #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> typedef typename std::pointer_traits<Ptr>::template rebind<block> block_ptr_t; 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:
1 | 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) |
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