Section
std::pointer_traits
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.
# Declarations
template< class Ptr >
struct pointer_traits;
(since C++11)
template< class T >
struct pointer_traits<T*>;
(since C++11)
# 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.
# Example
#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';
}
# Defect reports
| 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 |