A polymorphic value-type for C++

ISO/IEC JTC1 SC22 WG21 Programming Language C++

P0201R5

Working Group: Library Evolution, Library

Date: 2019-03-11

Jonathan Coe <jonathanbcoe@gmail.com>

Sean Parent <sparent@adobe.com>

Change history

Changes in P0201R5

Make constructor of default_copy noexcept and constexpr .

Clarifications to formal wording.

Changes in P0201R4

Clarify authors’ agreement with LEWG design changes.

Add wording to clarify meaning of custom copier and deleter.

Make constructors explicit and remove converting assignment.

Add a second template parameter to make_polymorphic_value to facilitate construction of objects of derived classes.

Changes in P0201R3

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:

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).

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:

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].

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:

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 compiler-generated destructor, copy constructor, move constructor, assignment operator and move assignment operator. All of these compiler-generated functions will behave correctly.

Deep copies

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

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

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.

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 and 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 destructor 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 may be used.

A custom copier and deleter are not required, if no custom copier and deleter are provided then the copy constructor and destructor of the managed object 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 (from an early revision of this paper) and modified following advice from LEWG. The authors (who unreservedly agree with the design direction suggested by LEWG) would like to make explicit 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 cast operations like those that are provided by the standard for std::shared_ptr .

No implicit conversions

Following design feedback, polymorphic_value ’s constructors have been made explicit so that surprising implicit conversions cannot take place. Any conversion to a polymorphic_value must be explicitly requested by user-code.

The converting assignment operators that were present in earlier drafts have also been removed.

For a base class, BaseClass , and derived class, DerivedClass , the converting assignment

is no longer valid, the conversion must be made explicit:

The removal of converting assigments makes make_polymorphic_value slightly more verbose to use:

is not longer valid and must be written as

This is somewhat cumbersome so make_polymorphic_value has been modified to take an optional extra template argument allowing users to write

The change from implicit to explicit construction is deliberately conservative. One can change explicit constructors into implicit constructors without breaking code (other than SFINAE checks), the reverse is not true. Similarly, converting assignments could be added non-disruptively but not so readily removed.

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]

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

Effects: Equivalent to: return new T(t);

X.Y Class bad_polymorphic_value_construction [bad_polymorphic_value_construction]

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

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 manages the lifetime of an owned object. A polymorphic_value object may own objects of different types at different points in its lifetime. A polymorphic_value object is empty if it has no owned object. polymorphic_value implements value semantics: the owned object (if any) is copied or destroyed when the polymorphic_value is copied or destroyed. Copying and destruction of the owned object can be customized by supplying a copier and deleter.

The template parameter T of polymorphic_value<T> shall be a non-union class type; otherwise the program is ill-formed. The template parameter T of polymorphic_value<T> may be an incomplete type.

A copier and deleter are said to be present if a polymorphic_value object is constructed from a non-null pointer, or from a polymorphic_value object where a copier and deleter are present.

[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]

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

Ensures: *this is empty.

Let V be remove_cvref_t<U> .

Constraints: V* is convertible to T* .

Effects: Constructs a polymorphic_value which owns an object of type V , direct-non-list-initialized with std::forward<U>(u) .

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

Constraints: U* is convertible to T* .

Expects: C and D meet the Cpp17CopyConstructible and Cpp17Destructible requirements. Move-initialization of objects of type C and D does not exit via an exception. If p is non-null then the expression c(*p) has type U* . The expression d(p) is well formed, has well-defined behavior, and does not throw exceptions.

Effects: If p is null, creates an empty object. If p is non-null creates an object that owns the object *p , with a copier and deleter initialized from std::move(c) and std::move(d) .

Throws: bad_alloc if required storage cannot be obtained; bad_polymorphic_value_construction if is_same_v<C, default_copy<U>> , is_same_v<D, default_delete<U>> and typeid(*p)!=typeid(U) are all true .

Note: A copier and deleter are said to be present in a non-empty object initialized with this constructor.

Constraints: For the second constructor, U* is convertible to T* .

Effects: If pv is empty, constructs an empty object. Otherwise creates an object that owns a copy of the object managed by pv . If a copier and deleter are present in pv then the copy is created by the copier in pv . Otherwise the copy is created by copy construction of the owned object. If a copier and deleter are present in pv then the copier and deleter of the object constructed are copied from those in pv .

Ensures: bool(*this) == bool(pv) .

Throws: Any exception thrown by invocation of the copier, copying the copier and deleter, or bad_alloc if required storage cannot be obtained.

Constraints: For the second constructor, U* is convertible to T* .

Effects: If pv is empty, constructs an empty object. Otherwise the object owned by pv is transferred to the constructed object. If a copier and deleter are present in pv then the copier and deleter are transferred to the constructed object.

Ensures: *this owns the object previously owned by pv (if any). pv is empty.

[Note: This constructor can allow an implementation to avoid the need for dynamic memory allocation.]

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

Effects: If a copier c and a deleter d are present, evaluates d(operator->()) and destroys c and d . Otherwise destroys the owned object (if any).

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

Effects: Equivalent to polymorphic_value(pv).swap(*this) . No effects if an exception is thrown.

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

Returns: *this .

Ensures: The state of *this is as if copy constructed from pv .

Effects: Equivalent to polymorphic_value(pv).swap(*this) .

Returns: *this .

Ensures: The state *this is equivalent to the original state of pv . pv is empty.

[Note: move construction of an owned object may be used by an implementation to avoid the need for use of dynamic memory.]

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

Effects: Exchanges the state of p and *this .

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

Expects: bool(*this) is true .

Returns: A reference to the owned object.

Expects: bool(*this) is true .

Returns: A pointer to the owned object.

Returns: true if the polymorphic_value owns an object, otherwise false .

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

Returns: A polymorphic_value<T> owning an object of type U direct-non-list-initialized with std::forward<Ts>(ts)... .

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

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

Effects: Equivalent to p.swap(u) .

Acknowledgements

The authors would like to thank Maciej Bogus, Matthew Calabrese, Casey Carter, Germán Diago, Louis Dionne, Bengt Gustafsson, Tom Hudson, Stephan T Lavavej, Tomasz Kamiński, David Krauss, Thomas Koeppe, LanguageLawyer, Nevin Liber, Nathan Myers, Roger Orr, Geoff Romer, Patrice Roy, Tony van Eerd and Ville Voutilainen for suggestions and 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