std::pointer_traits

C++

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Technical Specifications

Utilities library

Type support (basic types, RTTI, type traits)
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bitset
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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
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pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
Swap, forward and move
swap
exchange (C++14)
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chars_format (C++17)
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declval (C++11)
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Dynamic memory management

Smart pointers unique_ptr (C++11) shared_ptr (C++11) weak_ptr (C++11) auto_ptr (until C++17) owner_less (C++11) enable_shared_from_this (C++11) bad_weak_ptr (C++11) default_delete (C++11) Allocators allocator allocator_traits (C++11) allocator_arg_t (C++11) allocator_arg (C++11) uses_allocator (C++11) scoped_allocator_adaptor (C++11) pmr::polymorphic_allocator (C++17) Memory resources pmr::memory_resource (C++17) pmr::get_default_resource (C++17) pmr::set_default_resource (C++17) pmr::new_delete_resource (C++17) pmr::null_memory_resource (C++17) pmr::synchronized_pool_resource (C++17) pmr::unsynchronized_pool_resource (C++17) pmr::monotonic_buffer_resource (C++17) pmr::pool_options (C++17)

Uninitialized storage uninitialized_copy uninitialized_fill uninitialized_move (C++17) uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17) uninitialized_copy_n (C++11) uninitialized_fill_n uninitialized_move_n (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17) destroy_at (C++17) raw_storage_iterator (until C++20) get_temporary_buffer (until C++20) return_temporary_buffer (until C++20) Garbage collection support declare_reachable (C++11) undeclare_reachable (C++11) declare_no_pointers (C++11) undeclare_no_pointers (C++11) pointer_safety (C++11) get_pointer_safety (C++11) Miscellaneous pointer_traits (C++11) to_address (C++20) addressof (C++11) align (C++11) C Library malloc aligned_alloc (C++17) calloc realloc free Low level memory management

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:

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

pointer_to [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

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

pointer_to [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

Optional Member functions

to_address [static] (C++20)

obtains a raw pointer from a fancy pointer (inverse of pointer_to) (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,

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

Run this code

#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:

0

See also

allocator_traits (C++11)	provides information about allocator types (class template)
addressof (C++11)	obtains actual address of an object, even if the & operator is overloaded (function template)