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A polymorphic value-type for C++

ISO/IEC JTC1 SC22 WG21 Programming Language C++

D0201R3

Working Group: Library Evolution

Date: 2017-10-16

Jonathan Coe <[email protected]>

Sean Parent <[email protected]>

Change history

Changes in D0201R3

  • Add rationale for absence of allocator support.

Changes in P0201R2

  • Change name to polymorphic_value.

  • Remove operator <<.

  • Add construction and assignment from values.

  • Use std::default_delete.

  • Rename std::default_copier to std::default_copy.

  • Add notes on empty state and pointer constructor.

  • Add bad_polymorphic_value_construction exception when static and dynamic type of pointee mismatch and no custom copier or deleter are supplied.

  • Add clarifying note to say that a small object optimisation is allowed.

Changes in P0201R1

  • Change name to indirect.

  • Remove static_cast, dynamic_cast and const_cast as polymorphic_value is modelled on a value not a pointer.

  • Add const accessors which return const references/pointers.

  • Remove pointer-accessor get.

  • Remove specialization of propagate_const.

  • Amended authorship and acknowledgements.

  • Added support for custom copiers and custom deleters.

  • Removed hash and comparison operators.

TL;DR

Add a class template, polymorphic_value<T>, to the standard library to support polymorphic objects with value-like semantics.

Introduction

The class template, polymorphic_value, confers value-like semantics on a free-store allocated object. A polymorphic_value<T> may hold an object of a class publicly derived from T, and copying the polymorphic_value<T> will copy the object of the derived type.

Motivation: Composite objects

Use of components in the design of object-oriented class hierarchies can aid modular design as components can be potentially re-used as building-blocks for other composite classes.

We can write a simple composite object formed from two components as follows:

// Simple composite
class CompositeObject_1 {
  Component1 c1_;
  Component2 c2_;

 public:
  CompositeObject_1(const Component1& c1,
                    const Component2& c2) :
                    c1_(c1), c2_(c2) {}

  void foo() { c1_.foo(); }
  void bar() { c2_.bar(); }
};

The composite object can be made more flexible by storing pointers to objects allowing it to take derived components in its constructor. (We store pointers to the components rather than references so that we can take ownership of them).

// Non-copyable composite with polymorphic components (BAD)
class CompositeObject_2 {
  IComponent1* c1_;
  IComponent2* c2_;

 public:
  CompositeObject_2(const IComponent1* c1,
                    const IComponent2* c2) :
                    c1_(c1), c2_(c2) {}

  void foo() { c1_->foo(); }
  void bar() { c2_->bar(); }

  CompositeObject_2(const CompositeObject_2&) = delete;
  CompositeObject_2& operator=(const CompositeObject_2&) = delete;

  CompositeObject_2(CompositeObject_2&& o) : c1_(o.c1_), c2_(o.c2_) {
    o.c1_ = nullptr;
    o.c2_ = nullptr;
  }

  CompositeObject_2& operator=(CompositeObject_2&& o) {
    delete c1_;
    delete c2_;
    c1_ = o.c1_;
    c2_ = o.c2_;
    o.c1_ = nullptr;
    o.c2_ = nullptr;
  }

  ~CompositeObject_2()
  {
    delete c1_;
    delete c2_;
  }
};

CompositeObject_2's constructor API is unclear without knowing that the class takes ownership of the objects. We are forced to explicitly suppress the compiler-generated copy constructor and copy assignment operator to avoid double-deletion of the components c1_ and c2_. We also need to write a move constructor and move assignment operator.

Using unique_ptr makes ownership clear and saves us writing or deleting compiler generated methods:

// Non-copyable composite with polymorphic components
class CompositeObject_3 {
  std::unique_ptr<IComponent1> c1_;
  std::unique_ptr<IComponent2> c2_;

 public:
  CompositeObject_3(std::unique_ptr<IComponent1> c1,
                    std::unique_ptr<IComponent2> c2) :
                    c1_(std::move(c1)), c2_(std::move(c2)) {}

  void foo() { c1_->foo(); }
  void bar() { c2_->bar(); }
};

The design of CompositeObject_3 is good unless we want to copy the object.

We can avoid having to define our own copy constructor by using shared pointers. As shared-ptr's copy constructor is shallow, we need to modify the component pointers to be pointers-to const to avoid introducing shared mutable state [S.Parent].

// Copyable composite with immutable polymorphic components class
class CompositeObject_4 {
  std::shared_ptr<const IComponent1> c1_;
  std::shared_ptr<const IComponent2> c2_;

 public:
  CompositeObject_4(std::shared_ptr<const IComponent1> c1,
                    std::shared_ptr<const IComponent2> c2) :
                    c1_(std::move(c1)), c2_(std::move(c2)) {}

  void foo() { c1_->foo(); }
  void bar() { c2_->bar(); }
};

CompositeObject_4 has polymorphism and compiler-generated destructor, copy, move and assignment operators. As long as the components are not mutated, this design is good. If non-const methods of components are used then this won't compile.

Using polymorphic_value a copyable composite object with polymorphic components can be written as:

// Copyable composite with mutable polymorphic components
class CompositeObject_5 {
  std::polymorphic_value<IComponent1> c1_;
  std::polymorphic_value<IComponent2> c2_;

 public:
  CompositeObject_5(std::polymorphic_value<IComponent1> c1,
                    std::polymorphic_value<IComponent2> c2) :
                    c1_(std::move(c1)), c2_(std::move(c2)) {}

  void foo() { c1_->foo(); }
  void bar() { c2_->bar(); }
};

The component c1_ can be constructed from an instance of any class that inherits from IComponent1. Similarly, c2_ can be constructed from an instance of any class that inherits from IComponent2.

CompositeObject_5 has a (correct) compiler-generated destructor, copy, move, and assignment operators.

Deep copies

To allow correct copying of polymorphic objects, polymorphic_value uses the copy constructor of the derived-type pointee when copying a base type polymorphic_value. Similarly, to allow correct destruction of polymorphic component objects, polymorphic_value uses the destructor of the derived-type pointee in the destructor of a base type polymorphic_value.

The requirements of deep-copying can be illustrated by some simple test code:

// GIVEN base and derived classes.
class Base { virtual void foo() const = 0; };
class Derived : public Base { void foo() const override {} };

// WHEN a polymorphic_value to base is formed from a derived pointer
polymorphic_value<Base> poly(new Derived());
// AND the polymorphic_value to base is copied.
auto poly_copy = poly;

// THEN the copy points to a distinct object
assert(&*poly != &*poly_copy);
// AND the copy points to a derived type.
assert(dynamic_cast<Derived*>(*&poly_copy);

Note that while deep-destruction of a derived class object from a base class pointer can be performed with a virtual destructor, the same is not true for deep-copying. C++ has no concept of a virtual copy constructor and we are not proposing its addition. The class template shared_ptr already implements deep-destruction without needing virtual destructors: deep-destruction and deep-copying can be implemented using type-erasure [Impl].

Pointer constructor

polymorphic_value can be constructed from a pointer and optionally a copier and/or deleter. The polymorphic_value constructed in this manner takes ownership of the pointer. This constructor is potentially dangerous as a mismatch in the dynamic and static type of the pointer will result in incorrectly synthesized copiers and deleters, potentially resulting in slicing when copying and incomplete deletion during destruction.

class Base { /* methods and members */ };
class Derived : public Base { /* methods and members */ };

Derived d = new Derived();
Base* p = d; // static type and dynamic type differ
polymorphic_value<Base> poly(p);

// This copy will have been made using Base's copy constructor.
polymorphic_value<Base> poly_copy = poly;

// Destruction of poly and poly_copy uses Base's destructor.

While this is potentially error prone, we have elected to trust users with the tools they are given. shared_ptr and unique_ptr have similar constructors and issues. There are more constructors for polymorphic_value of a less expert-friendly nature that do not present such dangers including a factory method make_polymorphic_value.

Static analysis tools can be written to find cases where static and dynamic types for pointers passed in to polymorphic_value constructors are not provably identical.

If the user has not supplied a custom copier or deleter, an exception bad_polymorphic_value_construction is thrown from the pointer-constructor if the dynamic and static types of the pointer argument do not agree. In cases where the user has supplied a custom copier or deleter it is assumed that they will do so to avoid slicing and incomplete destruction: a class heirarchy with a custom Clone method and virtual desctructor would make use of Clone in a user-supplied copier.

Empty state

polymorphic_value presents an empty state as it is desirable for it to be cheaply constructed and then later assigned. In addition, it may not be possible to construct the T of a polymorphic_value<T> if it is an abstract class (a common intended use pattern). While permitting an empty state will necessitate occasional checks for null, polymorphic_value is intended to replace uses of pointers or smart pointers where such checks are also necessary. The benefits of default constructability (use in vectors and maps) outweigh the costs of a possible empty state.

Lack of hashing and comparisons

For a given user-defined type, T, there are multiple strategies to make polymorphic_value<T> hashable and comparable. Without requiring additional named member functions on the type, T, or mandating that T has virtual functions and RTTI, the authors do not see how polymorphic_value can generically support hashing or comparisons. Incurring a cost for functionality that is not required goes against the 'pay for what you use' philosophy of C++.

For a given user-defined type T the user is free to specialize std::hash and implement comparison operators for polymorphic_value<T>.

Custom copiers and deleters

The resource management performed by polymorphic_value - copying and destruction of the managed object - can be customized by supplying a copier and deleter. If no copier or deleter is supplied then a default copier or deleter will be used.

The default deleter is already defined by the standard library and used by unique_ptr.

We define the default copier in technical specifications below.

Allocator Support

The design of polymorphic_value is similar to that of std::any which does not have support for allocators.

polymorphic_value, like std::any and std::function is implemented in terms of type-erasure. There are technical issues with storing an allocator in a type-erased context and recovering it later for allocations needed during copy assignment [P0302r1].

Until such technical obstacles can be overcome, polymorphic_value will follow the design of std::any and std::function (post C++17) and will not support allocators.

Design changes from cloned_ptr

The design of polymorphic_value is based upon cloned_ptr after advice from LEWG. The authors would like to make LEWG explicitly aware of the cost of these design changes.

polymorphic_value<T> has value-like semantics: copies are deep and const is propagated to the owned object. The first revision of this paper presented cloned_ptr<T> which had mixed pointer/value semantics: copies are deep but const is not propagated to the owned object. polymorphic_value can be built from cloned_ptr and propagate_const but there is no way to remove const propagation from polymorphic_value.

As polymorphic_value is a value, dynamic_pointer_cast, static_pointer_cast and const_pointer_cast are not provided. If a polymorphic_value is constructed with a custom copier or deleter, then there is no way for a user to implement the cast operations provided for cloned_ptr.

[Should we be standardizing vocabulary types (optional, variant and polymorphic_value) or components through which vocabulary types can be trivially composed (propagate_const, cloned_ptr)?]

Impact on the standard

This proposal is a pure library extension. It requires additions to be made to the standard library header <memory>.

Technical specifications

X.X Class template default_copy [default.copy]

namespace std {
template <class T> struct default_copy {
  T* operator()(const T& t) const;
};

} // namespace std

The class template default_copy serves as the default copier for the class template polymorphic_value.

The template parameter T of default_copy may be an incomplete type.

T* operator()(const T& t) const;
  • Returns: new T(t).

X.Y Class bad_polymorphic_value_construction [bad_polymorphic_value_construction]

namespace std {
class bad_polymorphic_value_construction : std::exception
{
  public:
    bad_polymorphic_value_construction() noexcept;

    const char* what() const noexcept override;
};
}

Objects of type bad_polymorphic_value_construction are thrown to report invalid construction of a polymorphic_value from a pointer argument.

bad_polymorphic_value_construction() noexcept;
  • Constructs a bad_polymorphic_value_construction object.
const char* what() const noexcept override;
  • Returns: An implementation-defined ntbs.

X.Z Class template polymorphic_value [polymorphic_value]

X.Z.1 Class template polymorphic_value general [polymorphic_value.general]

A polymorphic_value is an object that owns another object and manages that other object through a pointer. More precisely, a polymorphic value is an object v that stores a pointer to a second object p and will dispose of p when v is itself destroyed (e.g., when leaving block scope (9.7)). In this context, v is said to own p.

A polymorphic_value object is empty if it does not own a pointer.

Copying a non-empty polymorphic_value will copy the owned object so that the copied polymorphic_value will have its own unique copy of the owned object.

Copying from an empty polymorphic_value produces another empty polymorphic_value.

Copying and disposal of the owned object can be customised by supplying a copier and deleter.

The template parameter T of polymorphic_value must be a non-union class type.

The template parameter T of polymorphic_value may be an incomplete type.

[Note: Implementations are encouraged to avoid the use of dynamic memory for ownership of small objects.]

X.Z.2 Class template polymorphic_value synopsis [polymorphic_value.synopsis]

namespace std {
template <class T> class polymorphic_value {
 public:
  using element_type = T;

  // Constructors
  constexpr polymorphic_value() noexcept;

  template <class U, class C=default_copy<U>, class D=default_delete<U>>
    explicit polymorphic_value(U* p, C c=C{}, D d=D{});

  polymorphic_value(const polymorphic_value& p);
  template <class U> polymorphic_value(const polymorphic_value<U>& p);
  polymorphic_value(polymorphic_value&& p) noexcept;
  template <class U> polymorphic_value(polymorphic_value<U>&& p);

  template <class U> polymorphic_value(U&& u);

  // Destructor
  ~polymorphic_value();

  // Assignment
  polymorphic_value& operator=(const polymorphic_value& p);
  template <class U>
    polymorphic_value& operator=(const polymorphic_value<U>& p);
  polymorphic_value& operator=(polymorphic_value&& p) noexcept;
  template <class U>
    polymorphic_value& operator=(polymorphic_value<U>&& p);

  template <class U>
    polymorphic_value& operator=(U&& u);


  // Modifiers
  void swap(polymorphic_value<T>& p) noexcept;

  // Observers
  T& operator*();
  T* operator->();
  const T& operator*() const;
  const T* operator->() const;
  explicit operator bool() const noexcept;
};

// polymorphic_value creation
template <class T, class ...Ts> polymorphic_value<T>
  make_polymorphic_value(Ts&& ...ts);

// polymorphic_value specialized algorithms
template<class T>
  void swap(polymorphic_value<T>& p, polymorphic_value<T>& u) noexcept;

} // end namespace std

X.Z.3 Class template polymorphic_value constructors [polymorphic_value.ctor]

constexpr polymorphic_value() noexcept;

  • Effects: Constructs an empty polymorphic_value.

  • Postconditions: bool(*this) == false

template <class U, class C=default_copy<U>, class D=default_delete<U>>
  explicit polymorphic_value(U* p, C c=C{}, D d=D{});

  • Effects: Creates a polymorphic_value object that owns the pointer p. If p is non-null then the copier and deleter of the polymorphic_value constructed is moved from c and d.

  • Requires: C and D satisfy the requirements of CopyConstructible. If p is non-null then the expression c(*p) returns an object of type U*. The expression d(p) is well formed, has well defined behavior, and does not throw exceptions. Either U and T must be the same type, or the dynamic and static type of U must be the same.

  • Throws: bad_polymorphic_value_construction if std::is_same<C, default_copy<U>>::value, std::is_same<D, default_delete<U>>::value and typeid(*u)!=typeid(U); bad_alloc if required storage cannot be obtained.

  • Postconditions: bool(*this) == bool(p).

  • Remarks: This constructor shall not participate in overload resolution unless U* is convertible to T*. A custom copier and deleter are said to be 'present' in a polymorphic_value initialised with this constructor.

polymorphic_value(const polymorphic_value& p);
template <class U> polymorphic_value(const polymorphic_value<U>& p);

  • Remarks: The second constructor shall not participate in overload resolution unless U* is convertible to T*.

  • Effects: Creates a polymorphic_value object that owns a copy of the object managed by p. The copy is created by the copier in p. If p has a custom copier and deleter then the custom copier and deleter of the polymorphic_value constructed are copied from those in p.

  • Throws: Any exception thrown by the copier or bad_alloc if required storage cannot be obtained.

  • Postconditions: bool(*this) == bool(p).

polymorphic_value(polymorphic_value&& p) noexcept;
template <class U> polymorphic_value(polymorphic_value<U>&& p);

  • Remarks: The second constructor shall not participate in overload resolution unless U* is convertible to T*.

  • Effects: Move-constructs a polymorphic_value instance from p. If p has a custom copier and deleter then the copier and deleter of the polymorphic_value constructed are the same as those in p.

  • Throws: bad_alloc if required storage cannot be obtained.

  • Postconditions: *this contains the old value of p. p is empty.

template <class U> polymorphic_value(U&& u);

  • Remarks: Let V be std::remove_cv_t<std::remove_reference_t<U>>. This constructor shall not participate in overload resolution unless V* is convertible to T*.

  • Effects: Constructs a polymorphic_value whose owned object is initialised with V(std::forward<U>(u)).

  • Throws: Any exception thrown by the selected constructor of V or bad_alloc if required storage cannot be obtained.

X.Z.4 Class template polymorphic_value destructor [polymorphic_value.dtor]

~polymorphic_value();
  • Effects: If get() == nullptr there are no effects. If a custom deleter d is present then d(p) is called and the copier and deleter are destroyed. Otherwise the destructor of the managed object is called.

X.Z.5 Class template polymorphic_value assignment [polymorphic_value.assignment]

polymorphic_value& operator=(const polymorphic_value& p);
template <class U> polymorphic_value& operator=(const polymorphic_value<U>& p);

  • Remarks: The second function shall not participate in overload resolution unless U* is convertible to T*.

  • Effects: *this owns a copy of the resource managed by p. If p has a custom copier and deleter then the copy is created by the copier in p, and the copier and deleter of *this are copied from those in p. Otherwise the resource managed by *this is initialised by the copy constructor of the resource managed by p.

  • Throws: Any exception thrown by the copier or bad_alloc if required storage cannot be obtained.

  • Returns: *this.

  • Postconditions: bool(*this) == bool(p).

template <class U> polymorphic_value& operator=(U&& u);

  • Remarks: Let V be std::remove_cv_t<std::remove_reference_t<U>>. This function shall not participate in overload resolution unless V> is not a specialization of polymorphic_value and V* is convertible to T*.

  • Effects: the owned object of *this is initialised with V(std::forward<U>(u)).

  • Throws: Any exception thrown by the selected constructor of V or bad_alloc if required storage cannot be obtained.

  • Returns: *this.

  • Postconditions: bool(*this) == bool(p).

polymorphic_value& operator=(polymorphic_value&& p) noexcept;
template <class U> polymorphic_value& operator=(polymorphic_value<U>&& p);

  • Remarks: The second constructor shall not participate in overload resolution unless U* is convertible to T*.

  • Effects: Ownership of the resource managed by p is transferred to this. If p has a custom copier and deleter then the copier and deleter of *this is the same as those in p.

  • Throws: bad_alloc if required storage cannot be obtained.

  • Returns: *this.

  • Postconditions: *this contains the old value of p. p is empty.

X.Z.6 Class template polymorphic_value modifiers [polymorphic_value.modifiers]

void swap(polymorphic_value<T>& p) noexcept;
  • Effects: Exchanges the contents of p and *this.

X.Z.7 Class template polymorphic_value observers [polymorphic_value.observers]

const T& operator*() const;
T& operator*();

  • Requires: bool(*this).

  • Returns: A reference to the owned object.

const T* operator->() const;
T* operator->();

  • Requires: bool(*this).

  • Returns: A pointer to the owned object.

explicit operator bool() const noexcept;
  • Returns: false if the polymorphic_value is empty, otherwise true.

X.Z.8 Class template polymorphic_value creation [polymorphic_value.creation]

template <class T, class ...Ts> polymorphic_value<T>
  make_polymorphic_value(Ts&& ...ts);
  • Returns: A polymorphic_value<T> owning an object initialised with T(std::forward<Ts>(ts)...).

[Note: Implementations are encouraged to avoid multiple allocations.]

X.Z.9 Class template polymorphic_value specialized algorithms [polymorphic_value.spec]

template <typename T>
void swap(polymorphic_value<T>& p, polymorphic_value<T>& u) noexcept;
  • Effects: Equivalent to p.swap(u).

Acknowledgements

The authors would like to thank Maciej Bogus, Matthew Calbrese, Germán Diago, Louis Dionne, Bengt Gustafsson, Tomasz Kamiński, David Krauss, Thomas Koeppe, Nevin Liber, Nathan Meyers, Roger Orr, Patrice Roy, Tony van Eerd and Ville Voutilainen for useful discussion.

References

[N3339] "A Preliminary Proposal for a Deep-Copying Smart Pointer", W.E.Brown, 2012 <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3339.pdf>

[S.Parent] "C++ Seasoning", Sean Parent, 2013 <https://github.com/sean-parent/sean-parent.github.io/wiki/Papers-and-Presentations>

[Impl] Reference implementation: polymorphic_value, J.B.Coe <https://github.com/jbcoe/polymorphic_value>

[P0302r1] "Removing Allocator support in std::function", Jonathan Wakely <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0302r1.html>