allocator_traits Class
The latest version of this topic can be found at allocator_traits Class.
The template class describes an object that supplements an allocator type. An allocator type is any type that describes an allocator object that is used for managing allocated storage. Specifically, for any allocator type Alloc, you can use allocator_traits<Alloc> to determine all the information that is needed by an allocator-enabled container. For more information, see the default allocator Class.
Syntax
template <class Alloc>
class allocator_traits;
Typedefs
| Name | Description |
|---|---|
allocator_traits::allocator_type |
This type is a synonym for the template parameter Alloc. |
allocator_traits::const_pointer |
This type is Alloc::const_pointer, if that type is well-formed; otherwise, this type is pointer_traits<pointer>::rebind<const value_type>. |
allocator_traits::const_void_pointer |
This type is Alloc::const_void_pointer, if that type is well-formed; otherwise, this type is pointer_traits<pointer>::rebind<const void>. |
allocator_traits::difference_type |
This type is Alloc::difference_type, if that type is well-formed; otherwise, this type is pointer_traits<pointer>::difference_type. |
allocator_traits::pointer |
This type is Alloc::pointer, if that type is well-formed; otherwise, this type is value_type *. |
allocator_traits::propagate_on_container_copy_assignment |
This type is Alloc::propagate_on_container_copy_assignment, if that type is well-formed; otherwise, this type is false_type. |
allocator_traits::propagate_on_container_move_assignment |
This type is Alloc::propagate_on_container_move_assignment, if that type is well-formed; otherwise, this type is false_type. If the type holds true, an allocator-enabled container copies its stored allocator on a move assignment. |
allocator_traits::propagate_on_container_swap |
This type is Alloc::propagate_on_container_swap, if that type is well-formed; otherwise, this type is false_type. If the type holds true, an allocator-enabled container swaps its stored allocator on a swap. |
allocator_traits::size_type |
This type is Alloc::size_type, if that type is well-formed; otherwise, this type is make_unsigned<difference_type>::type. |
allocator_traits::value_type |
This type is a synonym for Alloc::value_type. |
allocator_traits::void_pointer |
This type is Alloc::void_pointer, if that type is well-formed; otherwise, this type is pointer_traits<pointer>::rebind<void>. |
Static Methods
The following static methods call the corresponding method on a given allocator parameter.
| Name | Description |
|---|---|
| allocator_traits::allocate Method | Static method that allocates memory by using the given allocator parameter. |
| allocator_traits::construct Method | Static method that uses a specified allocator to construct an object. |
| allocator_traits::deallocate Method | Static method that uses a specified allocator to deallocate a specified number of objects. |
| allocator_traits::destroy Method | Static method that uses a specified allocator to call the destructor on an object without deallocating its memory. |
| allocator_traits::max_size Method | Static method that uses a specified allocator to determine the maximum number of objects that can be allocated. |
| allocator_traits::select_on_container_copy_construction Method | Static method that calls select_on_container_copy_construction on the specified allocator. |
Requirements
Header: <memory>
Namespace: std
allocator_traits::allocate Method
Static method that allocates memory by using the given allocator parameter.
static pointer allocate(Alloc& al, size_type count);
static pointer allocate(Alloc& al, size_type count,
typename allocator_traits<void>::const_pointer* hint);
Parameters
al
An allocator object.
count
The number of elements to allocate.
hint
A const_pointer that might assist the allocator object in satisfying the request for storage by locating the address of an allocated object prior to the request. A null pointer is treated as no hint.
Return Value
Each method returns a pointer to the allocated object.
The first static method returns al.allocate(count).
The second method returns al.allocate(count, hint), if that expression is well formed; otherwise it returns al.allocate(count).
allocator_traits::construct Method
Static method that uses a specified allocator to construct an object.
template <class Uty, class Types>
static void construct(Alloc& al, Uty* ptr, Types&&... args);
Parameters
al
An allocator object.
ptr
A pointer to the location where the object is to be constructed.
args
A list of arguments that is passed to the object constructor.
Remarks
The static member function calls al.construct(ptr, args...), if that expression is well formed; otherwise it evaluates ::new (static_cast<void *>(ptr)) Uty(std::forward<Types>(args)...).
allocator_traits::deallocate Method
Static method that uses a specified allocator to deallocate a specified number of objects.
static void deallocate(Alloc al,
pointer ptr,
size_type count);
Parameters
al
An allocator object.
ptr
A pointer to the starting location of the objects to be deallocated.
count
The number of objects to deallocate.
Remarks
This method calls al.deallocate(ptr, count).
This method throws nothing.
allocator_traits::destroy Method
Static method that uses a specified allocator to call the destructor on an object without deallocating its memory.
template <class Uty>
static void destroy(Alloc& al, Uty* ptr);
Parameters
al
An allocator object.
ptr
A pointer to the location of the object.
Remarks
This method calls al.destroy(ptr), if that expression is well formed; otherwise it evaluates ptr->~Uty().
allocator_traits::max_size Method
Static method that uses a specified allocator to determine the maximum number of objects that can be allocated.
static size_type max_size(const Alloc& al);
Parameters
al
An allocator object.
Remarks
This method returns al.max_size(), if that expression is well formed; otherwise it returns numeric_limits<size_type>::max().
allocator_traits::select_on_container_copy_construction Method
Static method that calls select_on_container_copy_construction on the specified allocator.
static Alloc select_on_container_copy_construction(const Alloc& al);
Parameters
al
An allocator object.
Return Value
This method returns al.select_on_container_copy_construction(), if that type is well formed; otherwise it returns al.
Remarks
This method is used to specify an allocator when the associated container is copy-constructed.
See Also
<memory>
pointer_traits Struct
scoped_allocator_adaptor Class