Pointer cast for unique_ptr

• Pointer cast for unique_ptr
  • Introduction
  • Prior Art
  • Motivation
  • Design
  • Technical Specification
  • Implementation Experience
  • Acknowledgements

Introduction

We propose unique_ptr overloads for std::const_pointer_cast and std::dynamic_pointer_cast. For each kind of cast, we allow users to choose between either using the defaulted deleter or preserving the original deleter type for each kind of cast.

The table following illustrates the simpler case of the two where users expect defaulted deleters in the resulting types.

auto GetClient() -> std::unique_ptr<const Client>;

std::unique_ptr<Client> client;
client.reset(const_cast<Client*>(GetClient().release()));

void UseV2Client(std::unique_ptr<Client>&& client)
{
    std::unique_ptr<ClientV2> v2;
    // ...
    v2.reset(dynamic_cast<ClientV2*>(client.release()));

std::unique_ptr<Client> client;
client = const_pointer_cast<Client>(GetClient());

void UseV2Client(std::unique_ptr<Client>&& client)
{
    std::unique_ptr<ClientV2> v2;
    // ...
    v2 = dynamic_pointer_cast<ClientV2>(std::move(client));

Prior Art

Boost.SmartPtr ships all four casts (dynamic_pointer_cast, static_pointer_cast, const_pointer_cast, and reinterpret_pointer_cast) that create std::unique_ptr<T> by releasing std::unique_ptr<U> since 2016.

Motivation

Improve resource safety

The need for pointer casts between unique_ptrs was spotted in code reviews from independent parties. Without coincidence, the conclusions were to use .release(), a resource-unsafe API, as a one-off solution. Such a practice encourages the use of unsafe constructs and potentially breaks future code as a result. The standard C++ should encourage the opposite.

Express the intent of pointer cast via established vocabularies

When people look for pointer casts between smart pointers, they look for std::dynamic_pointer_cast, std::const_pointer_cast, etc. These names resemble dynamic_cast and const_cast and work for std::shared_ptr already. There is no simpler way to express the same intent and no reason to find a different set of names.

Standardize existing practices

The cpplang Slack workspace rediscovers dynamic_pointer_cast(std::unique_ptr<T>&&) on a yearly basis, albeit boost::dynamic_pointer_cast(std::unique_ptr<T>&&) existed before the group's birth. Some of the work supports preserving the incoming deleter type. It's time to consider adopting the working parts from Boost and explore the recurring extension.

Design

unique_ptr differs from shared_ptr in a few significant ways. Obviously, we can only cast from an rvalue of unique_ptr by moving its ownership. The other differences that made impacts on the design are:

1. A shared_ptr<T> carries a type-erased deleter, while unique_ptr<T, D>'s deleter is a part of the type. The seemly intuitive unique_ptr<T> to unique_ptr<U> actually requires replacing std::default_delete<T> with std::default_delete<U>, which may not apply to the customized deleters.
2. A casted shared_ptr<U> is only an alias to the original shared_ptr<T>. The newly created object requires no deleter, and you can expect the original deleter to be able to delete the uncasted pointer. Meanwhile, unique_ptr<U, D>, in general, must deal with the question "whether D can delete the casted pointer."
3. A shared_ptr<T> owns a "real" pointer, while unique_ptr<T, D> can customize its pointer type as indicated by its pointer typedef. Converting between the pointer types may create loopholes as the unique_ptr<T[], D> specializations reused this mechanism.

It turns out that using unique_ptr with a type-erased deleter is not uncommon in the industry. It does not have to be as sophisticated as something that calls into memory_resource. A deleter that takes a pointer to a polymorphic base class is possibly a type-erased deleter. Even &std::free is a legitimate type-erased deleter. So when designing the APIs to cast between unique_ptrs, we would like to support both expectations: one set of APIs to cast unique_ptr<T> to unique_ptr<U> and the other set to cast unique_ptr<T, D> to unique_ptr<U, D>. In other words, one defaults the deleter, and the other one preserves the deleter.

However, the set of casts we can safely perform in practice is not without boundaries with both API styles. According to our preliminary survey using GitHub code search, static_casts between unique_ptrs using the .release() trick almost always perform downcast in the hope of gaining performance over dynamic_cast by sacrificing safety, and reinterpret_casts between unique_ptrs only retrieve byte sequences. The authors consider both use cases require expertise and cannot be a part of the intuitive APIs, which are supposed to be safe by default. On the other hand, type-erased APIs for these types of casts would not only be expert-only but also serve no use case, as we know so far. Therefore, this paper proposes only dynamic_pointer_cast and const_pointer_cast between unique_ptrs.

Attention to safety is also reflected in the proposed APIs. For example, in the API to downcast unique_ptr<T> to unique_ptr<U>, we require U to have a virtual destructor. This is not required in the deleter-preserving API since the behavior of the deleter is unknown. But when the deleter is known to be default_delete<U>, we can prevent undefined behavior ahead of time. In some cases, prior knowledge of the default deleter reduces the amount of checks. For example, unique_ptr<T[], D>::pointer may also be T*, which requires extra checks to prevent accidentally creating unique_ptr that manages new[]-ed resources with a non-array deleter and such. The following chart summarizes the guardrails in the proposed APIs beyond the underlying calls to the unique_ptr constructors.

Technical Specification

template<class T, class U>
constexpr unique_ptr<T> dynamic_pointer_cast(unique_ptr<U>&& r) noexcept;

Constraints: dynamic_cast<T*>((U*)nullptr) is a valid expression.

Mandates: has_virtual_destructor_v<T> is true.

Preconditions: The expression dynamic_cast<T*>(r.get()) has well-defined behavior.

Effects: Equivalent to:

if (auto p = dynamic_cast<T*>(r.get()))
  return (void)r.release(), unique_ptr<T>(p);
else
  return nullptr;

[Note 1: The seemingly equivalent expression unique_ptr<T>(dynamic_cast<T*>(r.get())) can result in undefined behavior, attempting to delete the same object twice. –end note]

template<class T, class D, class U>
constexpr unique_ptr<T, D> dynamic_pointer_cast(unique_ptr<U, D>&& r) noexcept;

Constraints: dynamic_cast<unique_ptr<T, D>::pointer>(declval<typename unique_ptr<U, D>::pointer>())) is a valid expression.

Mandates: Neither T nor U is an array type.

Preconditions: The expression dynamic_cast<unique_ptr<T, D>::pointer>(r.get()) has well-defined behavior.

Effects: Equivalent to:

if (auto p = dynamic_cast<unique_ptr<T, D>::pointer>(r.get()))
  return (void)r.release(), unique_ptr<T, D>(p, std::forward<D>(r.get_deleter()));
else if constexpr (!is_pointer_v<D> && is_default_constructible_v<D>)
  return nullptr;
else if constexpr (is_copy_constructible_v<D>)
  return unique_ptr<T, D>(nullptr, r.get_deleter());

template<class T, class U>
constexpr unique_ptr<T> const_pointer_cast(unique_ptr<U>&& r) noexcept;

Constraints: const_cast<T*>((U*)nullptr) is a valid expression.

Effects: Equivalent to: return unique_ptr<T>(const_cast<T*>(r.release()));

[Note 2: The seemingly equivalent expression unique_ptr<T>(const_cast<T*>(r.get())) can result in undefined behavior, attempting to delete the same object twice. –end note]

template<class T, class D, class U>
constexpr unique_ptr<T, D> const_pointer_cast(unique_ptr<U, D>&& r) noexcept;

Constraints: const_cast<unique_ptr<T, D>::pointer>(declval<typename unique_ptr<U, D>::pointer>()) is a valid expression.

Mandates: is_array_v<T> == is_array_v<U> is true.

Effects: Equivalent to: return unique_ptr<T, D>(const_cast<unique_ptr<T, D>::pointer>(r.release()), std::forward<D>(r.get_deleter()));

Implementation Experience

Here is a full implementation:  n3Kh376hsCompiler Explorer

The snippet below implements the variant of dynamic_pointer_cast that preserves the deleter type (i.e., supports type-erased deleter).

template<class T, class D, class U>
    requires(requires(unique_ptr<U, D>::pointer p) {
        dynamic_cast<unique_ptr<T, D>::pointer>(p);
    })
constexpr auto dynamic_pointer_cast(unique_ptr<U, D>&& r) noexcept
    -> unique_ptr<T, D>
{
    static_assert(!is_array_v<T> && !is_array_v<U>,
                  "don't work with array of polymorphic objects");
    if (auto p = dynamic_cast<unique_ptr<T, D>::pointer>(r.get()))
    {
        r.release();
        return unique_ptr<T, D>(p, std::forward<D>(r.get_deleter()));
    }
    else if constexpr (!is_pointer_v<D> && is_default_constructible_v<D>)
    {
        return {};
    }
    else if constexpr (is_copy_constructible_v<D>)
    {
        return unique_ptr<T, D>(nullptr, r.get_deleter());
    }
    else
    {
        static_assert(false, "unable to create an empty unique_ptr");
    }
}

Acknowledgements

Thank Broadcom Software for supporting the work.