Pablo Halpern <phalpern@halpernwightsoftware.com>
2022-10-10 15:39 EDT
Target audience: LEWG/LWG

Abstract

The allocator_traits class template was introduced in C++11 with two goals in mind: 1) Provide default implementations for allocator types and operations, thus minimizing the requirements on allocators [allocator.requirements], and 2) provide a mechanism by which future standards could extend the allocator interface without changing allocator requirements and thus obsoleting existing allocators. The latter goal is undermined, however, by the standard currently allowing user-defined specializations of std::allocator_traits. Although the standard requires that any such specialization conform to the standard interface, it is not practical to change the standard interface – even by extending it – without breaking any existing user specializations. Indeed, the Sep 2022 C++23 CD, N4919 contains an extension, allocate_at_least, that logically belongs in std:::allocator_traits, but is expressed as an unrelated function because of the problem of potential user-defined specializations.

This paper proposes two possible solutions to this problem: 1) remove the user’s latitude for specializing std::allocator_traits or 2) deprecate std::allocator_traits entirely in favor of an “unbundled” set of allocator-interface functions.

This paper is the proposed resolution to a US NB comment having the same title.

Motivation

The minimal interface for a type conforming to the allocator requirements is that it have a value_type type, allocate and deallocate member functions, and equality comparison operators. The allocator_traits class template provides many other types and functions such as pointer, rebind, and construct. Generic types that use allocators are required to access the allocator through std::allocator_traits. The latter requirement was intended to allow the allocator interface to be extended without necessarily changing every existing allocator.

For example, C++03 allocators did not have a void_pointer member, but such a member is provided automatically through allocator_traits; an allocator class can override the default provided by allocator_traits, but is not required to do so.

The Standard description for each trait X in std::allocator_traits<A> typically follows the form, “a.X if that expression is well-formed; otherwise some default.” There is never any reason to specialize std::allocator_traits because any trait can be overridden simply by defining the appropriate member within the specific allocator class template. Unfortunately, the standard allows such user specialization and even implies that it is a reasonable thing to do. This allowance prevents the Standards Committee from adding new members to std::allocator_traits without breaking existing user specializations.

In P0401R1, allocate_at_least was proposed as a static member of std::allocator_traits but it was changed to a free function in P0401R2 following a poll in LEWG in Cologne after it was pointed out that, because std::allocator_traits can be specialized and that existing specializations would not have the allocate_at_least member. It is this free function that is now in the September 2022 C++23 CD, N4919. The current state of affairs, then, is that accessing an allocator is starting to become a hodgepodge of std::allocator_traits member-function calls and free-function calls. Before we standardize C++23, we should make an attempt to prevent this divergence.

Proposed resolution 1 of 2

This proposed resolution would disallow user specializations of std::allocator_traits. This change would be a breaking one, as existing specializations would become non-conforming. However, with the exception of the new allocate_at_least feature, existing code should continue to work for the time being. It is expected that specializations of std::allocator_traits are very rare, so the amount of potential breakage should be quite limited.

Wording for PR 1:

Modify section 16.4.4.6.1 [allocator.requirements.general], paragraph 3 as follows:

The class template allocator_traits (20.2.8) supplies a uniform interface to all allocator types. This subclause describes the requirements on allocator types and thus on types used to instantiate allocator_traits. A requirement is optional if a default for a given type or expression is specified. Within the standard library allocator_traits template, an optional requirement that is not supplied by an allocator is replaced by the specified default type or expression. A user specialization of allocator_traits may provide different defaults and may provide defaults for different requirements than the primary template. If a program declares an explicit or partial specialization of allocator_traits, the program is ill-formed, no diagnostic required.

And Paragraph 46 as follows:

Remarks: An allocator need not support allocate_at_least, but no default is provided in allocator_traits. If an allocator has an allocate_at_least member, it shall satisfy the requirements. Default: {a.allocate(n), n}.

In section 20.2.2 [memory.syn], remove the non-member declaration for allocate_at_least:

template<class Allocator>
  [[nodiscard]] constexpr allocation_result<typename allocator_traits<Allocator>::pointer>
    allocate_at_least(Allocator& a, size_t n);                   // freestanding

In section 20.2.9.1 [allocator.traits.general], add a new member in allocator_traits, probably immediately before deallocate:

template<class Allocator>
  [[nodiscard]] static constexpr allocation_result<pointer>
    allocate_at_least(Allocator& a, size_t n);

And in section 20.2.9.3 [allocator.traits.members], add its definition

template<class Allocator>
  [[nodiscard]] static constexpr allocation_result<pointer>
    allocate_at_least(Allocator& a, size_t n);

Returns: a.allocate_at_least(n) if that expression is well-formed; otherwise, {a.allocate(n), n}.

Finally, in section 20.2.9.4 [allocator.traits.other] paragraph 2, remove the definition of non-member allocate_at_least:

template<class Allocator>
  [[nodiscard]] constexpr allocation_result<typename allocator_traits<Allocator>::pointer>
    allocate_at_least(Allocator& a, size_t n);

Returns: a.allocate_at_least(n) if that expression is well-formed; otherwise, {a.allocate(n), n}.

Proposed resolution 2 of 2

Another possible resolution is to move everything in the direction of allocate_at_least, i.e., deprecate allocator_traits and replace it by a set of namespace-scoped alias templates and free function templates. Such a change is too large for C++23, but if we can agree on this direction now, it can be made as a non-breaking change in C++26.

Complete proposed wording is not supplied herein, but the general idea is that each type in allocator_traits would be replaced by a type trait or free function, similar to the following:

Type: Alloc::pointer if the qualified-id Alloc::pointer is valid and denotes a type (13.10.3); otherwise, allocator_value_type<Alloc>*.

template <class Alloc, class T>
  constexpr void allocator_destroy(Alloc& a, T* p);

Effects: Calls a.destroy(p) if that call is well-formed; otherwise, invokes destroy_at(p).

Repeat for each trait in allocator_traits change the description of how allocator-aware containers use allocators to use these new versions. The existing allocator_traits template would be moved into Appendix D and each member would be specified in terms of the new traits.

Conclusion

The status quo would have allocator operations specified as a mixture of allocator_traits members and namespace-scope traits. We should decide which is our preferred method of specification. If we prefer a set of loosely related namespace-scope traits, then nothing needs to be done today except deciding this direction for the future. If, however, we prefer to use allocator_traits, then user specializations must be disallowed in C++23, before the standard starts drifting in the other direction.

References

N4919: Programming Languages – C++, Committee Draft, 2022-09-05.

P0401R6: Providing size feedback in the Allocator interface, Jonathan Wakely and Chris Kennelly, 2021-01-22