ISO/IEC JTC1 SC22 WG21 P0211R3

Date: 2020-01-14


Thomas Köppe <>

Allocator-aware library wrappers for dynamic allocation


  1. Revision history
  2. Summary
  3. Example use cases
  4. Dynamic allocation wrappers
  5. Unique pointers
  6. Design questions
  7. Acknowledgements
  8. Proposed wording

Revision history


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...)
delete p allocator_delete(alloc, p)
Unique pointer default_delete<T> allocation_deleter<T>
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

Dynamic allocation wrappers

Allocation and object creation

template <class T, class A, class ...Args> auto allocator_new(A& alloc, Args&&... args) {   using TTraits = typeame allocator_traits<A>::template rebind_traits<T>;   using TAlloc = typename allocator_traits<A>::template rebind_alloc<T>;   auto a = TAlloc(alloc);   auto p = TTraits::allocate(a, 1);   try {     TTraits::construct(a, to_address(p), std::forward<Args>(args)...);     return p;   } catch(...) {     TTraits::deallocate(a, p, 1);     throw;   } }

Object destruction and Deallocation

template <class A, class P> void allocator_delete(A& alloc, P p) {   using Elem = typename pointer_traits<P>::element_type;   using Traits = typename allocator_traits<A>::template rebind_traits<Elem>;   Traits::destroy(alloc, to_address(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) noexcept : a_(a) {}   void operator()(pointer p) {     allocator_delete(a_, p);   } };

The factory function is:

template <class T, class A, class ...Args> auto allocate_unique(A& alloc, Args&&... args) {   using TAlloc = typename allocator_traits<A>::rebind_alloc<T>;   return unique_ptr<T, allocation_deleter<TAlloc>>(allocator_new<T>(alloc, std::forward<Args>(args)...), alloc); }

The type T must not be an array type. The pathological case where the template argument of allocation_deleter<A> is equal (up to qualification) to allocation_deleter<A> has to be taken into account for the purpose of the constructor.

Design questions


The name for the deleter class: “allocation_deleter” or “allocation_delete”? (We had previously used “allocate_delete” for both the function and the class, but that would have been very confusing.)

Default initialization

In C++20, the creation functions make_unique, make_shared, and allocate_shared were augmented by make_unique_default_init, make_shared_default_init, and allocate_shared_default_init. Whereas the former (called with no arguments) value-initialize an object (e.g. ::new T()), the latter default-initialize instead (e.g. ::new T).

One might ask whether there should not also be analogous default-initializing versions of the allocation helpers that are proposed here. However, this is a false analogy, and the answer is no: The proposed allocation helpers are meant to be a symmetric pair of allocation/construction and destruction/deallocation. Note that allocator_delete and allocation_deleter::operator() both call the allocator’s destroy function, which therefore must be matched with initialization by construct. This in turn precludes any notion of default-initialization, which the allocator API does not allow.

By contrast, make_unique and make_shared and their default-initializing versions do not use allocators and can thus correctly call delete or delete[] regardless of how the initialization was performed, and allocate_shared/allocate_shared_default_init use different dynamic deleters so that the allocator is used for the former, and raw delete is used for the latter.


Many thanks to Daniel Krügler for a thorough review and invaluable corrections of the first version from 2016, to the members of LEWG who reviewed the draft and provided many valuable improvements in 2018, and to Alisdair Meredith for pointing out default-initialization.

Proposed wording

In [memory.syn, 20.10.2], add to the synopsis:

// 20.10.8, uses_allocator template <class T, class Alloc> struct uses_allocator; […] // 20.10.9, allocator traits template <class Alloc> struct allocator_traits; // 20.10.?, 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>::pointer p); // 20.10.10?, the default allocator: template <class T> class allocator; template <> class allocator<void>; template <class T, class U> bool operator==(const allocator<T>&, const allocator<U>&) noexcept; template <class T, class U> bool operator!=(const allocator<T>&, const allocator<U>&) noexcept;

// 20.11.1 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 20.9.9 (allocator traits) and 20.9.10 (the default allocator):

20.10.? 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 ([,])).

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

Requires: A shall satisfy the Cpp17Allocator requirements ([allocator.requirements,]).

Effects: Let TTraits be the type allocator_traits<A>::rebind_traits<T>. Obtains storage for one element from a copy a of alloc rebound for type T by calling TTraits::allocate(a, 1) and constructs an object by calling TTraits::construct(a, to_address(p), std::forward<Args>(args)...), where p is the pointer obtained from the allocation. If the construction exits with an exception, the storage is released using TTraits::deallocate(a, p, 1).

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 TTraits::allocate or by TTraits::construct.

template <class A, class P>   void allocator_delete(A& alloc, P p);

Requires: A shall satisfy the Cpp17Allocator requirements ([allocator.requirements,]). P shall satisfy the Cpp17NullablePointer requirements ([nullablepointer.requirements,]). The pointer p was obtained from an allocator that compares equal to alloc, and p is dereferenceable.

Effects: Let TTraits be the type allocator_traits<A>::rebind_traits<pointer_traits<P>::element_type>. Uses a copy a of alloc rebound to the TTraits::value_type to destroy the object *p by calling TTraits::destroy(a, to_address(p)) and to release the underlying storage by calling TTraits::deallocate(a, p, 1).

Insert a new subsection after (default_delete<T[]>): 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) noexcept;   template <class B>   allocation_deleter(const allocation_deleter<B>& other) noexcept;   void operator()(pointer p); private:   [[no_unique_address]] A a_;  // exposition only };

allocation_deleter(const A& alloc) noexcept;

Effects: Initializes a_ with alloc.

Constraints: A, ignoring qualifications, is not allocation_deleter<A>.

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

Effects: Initializes a_ with other.a_.

Constraints: typename allocator_traits<B>::pointer is implicitly convertible to pointer.

void operator()(pointer p);

Effects: Calls allocator_delete(a_, p).

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

template <class T, class A>   unique_ptr<T, allocation_deleter<allocator_traits<A>::rebind_alloc<T>>> allocate_unique_default_init(A& alloc)

Constraints: T is not an array.

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

Throws: Any exception thrown by allocator_new<T>.