ISO/IEC JTC1 SC22 WG21 P00211r1

Date: 2016-05-30

To: LEWG

Allocator-aware library wrappers for dynamic allocation

Revision history
Summary
Example use cases
Dynamic allocation wrappers
Unique pointers
Design questions
Proposed wording

Revision history

P00211r0: Initial proposal, pre-Jacksonville.
D00211r1: make pointer in allocate_delete const; delete unnecessary null check from allocate_new; add some more design questions.
P00211r1 (this version): Rename functions as requested by LEWG in Jacksonville. Add conversions to allocation_deleter. Add formal wording.

Summary

Short form: We propose to add library functions that allow the systematic use of allocators as a customisation point for dynamic allocations. The new functions complete the following picture:

	using `operator {new,delete}`	using allocator
Manual	`T * p = new T(args...)`	`auto p = allocator_new<T>(alloc, args...)`
Manual	`delete p`	`allocator_delete(alloc, p)`
Unique pointer	`default_delete<T>`	`allocation_deleter<T>`
Unique pointer	`make_unique<T>(args...)`	`allocate_unique<T>(alloc, args...)`
Shared pointer	`make_shared<T>(args...)`	`allocate_shared<T>(alloc, args...)`

Long form: The standard library rarely uses new/delete directly, but instead allows customisation of dynamic allocation and object construction via allocators. Currently this customisation is only available for container elements and for shared_ptr (as well as for a few other types that require dynamically allocated memory), but not for the top-level objects themselves.

The proposal is to complete the library facilities for allocator-based customisation by providing a direct mechanism for creating and destroying a dynamically stored object through an allocator, as well as a new deleter type for unique_ptr to hold an allocator-managed unique pointee, together with the appropriate factory function.

Example use cases

Consider an arena allocator. A vector may put its elements into the arena, but the vector itself cannot easily be placed in the same arena. The proposed std::allocate_unique<std::vector<T, ScopedArenaAlloc<T>>>(arena_alloc) allows this. (Here we assume the use of the usual alias idiom template <class T> using ScopedArenaAlloc = std::scoped_allocator_adaptor<ArenaAlloc<T>>;).
Consider a shared memory allocator. As in the previous example, containers will put their elements in shared memory, but one process needs to create the container itself and place it in shared memory. This is now possible with auto p = std::allocator_new<std::vector<T, ScopedShmemAlloc<T>>>(shmem_alloc). The returned pointer is presumably an offset pointer, and its offset value needs to be communicated to the other participating processes. The last process to use the container calls allocator_delete(shmem_alloc, p).
Allocator support is frequently requested on Stack Overflow.

Dynamic allocation wrappers

Allocation and object creation

template <class T, class A, class ...Args> auto allocator_new(A& alloc, Args&&... args) { using Traits = allocator_traits<A>; static_assert(std::is_same_v<T, typename Traits::value_type>); auto p = Traits::allocate(alloc, 1); try { Traits::construct(alloc, addressof(*p), std::forward<Args>(args)...); return p; } catch(...) { Traits::deallocate(alloc, p, 1); throw; } }

Object destruction and Deallocation

template <class A> void allocator_delete(A& alloc, typename allocator_traits<A>::const_pointer p) { using Traits = allocator_traits<A>; Traits::destroy(alloc, addressof(*p)); Traits::deallocate(alloc, p, 1); }

Unique pointers

To allow std::unique_ptr to use custom allocators, we first need a deleter template that stores the allocator:

template <class A> struct allocation_deleter { using pointer = typename allocator_traits<A>::pointer; A a_; // exposition only allocation_deleter(const A& a) : a_(a) {} void operator()(pointer p) const { allocator_delete(a_, p); } };

The factory function is:

template <class T, class A, class ...Args> unique_ptr<T, allocation_deleter<A>> allocate_unique(A& alloc, Args&&... args) { return unique_ptr<T, allocator_delete<A>>(allocator_new<T>(alloc, std::forward<Args>(args)...), alloc); }

The type T must not be an array type.

Design questions

Require exact allocator or allow rebind-copying? Rebind-copying would allow taking the original allocator by const-reference, but would incur a local allocator’s destruction (which is not required to be noexcept).

Proposed wording

In 20.9.2, add to the synopsis:

// 20.9.7, uses_allocator template <class T, class Alloc> struct uses_allocator; // 20.9.8, allocator traits template <class Alloc> struct allocator_traits; // 20.9.?, allocation helpers: template <class A, class ...Args> typename allocator_traits<A>::pointer allocator_new(A& alloc, Args&&... args); template <class A> void allocator_delete(A& alloc, typename allocator_traits<A>::const_pointer p); // 20.9.9?, the default allocator: template <class T> class allocator; template <> class allocator<void>; template <class T, class U> bool operator==(const allocator<T>&, const allocator&) noexcept; template <class T, class U> bool operator!=(const allocator<T>&, const allocator&) noexcept;

// 20.10.1 class template unique_ptr: template <class T> struct default_delete; template <class T> struct default_delete<T[]>; template <class A> struct allocation_deleter; template <class T, class D = default_delete<T>> class unique_ptr; template <class T, class D> class unique_ptr<T[], D>; template <class T, class... Args> unique_ptr<T> make_unique(Args&&... args); template <class T> unique_ptr<T> make_unique(size_t n); template <class T, class... Args> unspecified make_unique(Args&&...) = delete; template <class T, class A, class ...Args> unique_ptr<T, allocation_deleter<A>> allocate_unique(A& alloc, Args&&... args)

Insert a new subsection between 20.9.8 (allocator traits) and 20.9.9 (the default allocator):

20.9.? Allocation helpers [allocator.alloc_helpers]

The function templates allocator_new and allocator_delete create and delete objects dynamically using an allocator to provide storage and perform construction (rather than by looking up an allocation function (3.7.4.1)).

template <class T, class A, class ...Args> typename allocator_traits<A>::pointer allocator_new(A& alloc, Args&&... args);

Requires: A shall be an allocator class for type T.

Effects: Obtains storage for one element from alloc and constructs an object of type T from args using alloc. If the construction exits with an exception, the storage is released using alloc.

Returns: A pointer to the obtained storage that holds the constructed object. [Note: This pointer may have a user-defined type. – end note]

Throws: Any exception thrown by the constructor of T.

template <class A> void allocator_delete(A& alloc, typename allocator_traits<A>::const_pointer p);

Requires: p was obtained from an allocator that compares equal to alloc, and p is dereferenceable.

Effects: Uses alloc to destroy the object *p and to release the underlying storage.

Insert a new subsection 20.10.1.1.? after 20.10.1.1.3 (default_delete<T[]>):

20.10.1.1.? `allocation_deleter<A>` [unique.ptr.dltr.alloc]

template <class A> struct allocation_deleter { using pointer = typename allocator_traits<A>::pointer; allocation_deleter(const A& alloc); template <class B> allocation_deleter(const allocation_deleter& other); void operator()(pointer p) const; private: A a_; // exposition only };

allocation_deleter(const A& alloc);

Effects: Initializes a_ with alloc.

template <class B> allocation_deleter(const allocation_deleter& other);

Effects: Initializes a_ with other.a_.

Remarks: This constructor shall not participate in overload resolution unless typename allocator_traits::pointer is implicitly convertible to pointer.

void operator()(pointer p) const;

Effects: Calls allocator_delete(a_, p).

Append a new paragraph to the end of subsection 20.10.1.4 (unique_ptr creation):

template <class T, class A, class ...Args> unique_ptr<T, allocation_deleter<A>> allocate_unique(A& alloc, Args&&... args)

Remarks: This function shall not participate in overload resolution unless T is not an array.

Returns: unique_ptr<T, allocator_delete<A>>(allocator_new<T>(alloc, std::forward<Args>(args)...), alloc).