Array of polymorphic objects

Question

I commonly come across the need to create arrays or vectors of polymorphic objects. I'd usually prefer to use references, rather than smart pointers, to the base class because they tend to be simpler.

Arrays and vectors are forbidden from containing raw references, and so I've tended to use smart pointers to the base classes instead. However, there is also the option to use std::reference_wrapper instead: https://en.cppreference.com/w/cpp/utility/functional/reference_wrapper

From what I can tell from the documentation, this is what one of its intended uses is, but when the topic of arrays containing polymorphic objects comes up, the common advice seems to be to use smart pointers rather than std::reference_wrapper.

My only thought is that smart pointers may be able to handle the lifetime of the object a little neater?

TL:DR; Why are smart pointers, such as std::unique_ptr seemingly preferred over std::reference_wrapper when creating arrays of polymorphic objects?

Answer 1

If you are sufficiently motiviated, you can write a poly_any<Base> type.

A poly_any<Base> is an any restricted to only storing objects that derive from Base, and provides a .base() method that returns a Base& to the underlying object.

A very incomplete sketch:

template<class Base>
struct poly_any:private std::any
{
  using std::any::reset;
  using std::any::has_value;
  using std::any::type;

  poly_any( poly_any const& ) = default;
  poly_any& operator=( poly_any const& ) = default;

  Base& base() { return get_base(*this); }
  Base const& base() const { return const_cast<Base const&>(get_base(const_cast<poly_any&>(*this))); }

  template< class ValueType,
    std::enable_if_t< /* todo */, bool > =true
  >
  poly_any( ValueType&& value ); // todo

  // TODO: sfinae on ValueType?
  template< class ValueType, class... Args >
  explicit poly_any( std::in_place_type_t<ValueType>, Args&&... args );  // todo

  // TODO: sfinae on ValueType?
  template< class ValueType, class U, class... Args >
  explicit poly_any( std::in_place_type_t<ValueType>, std::initializer_list<U> il,
          Args&&... args ); // todo

  void swap( poly_any& other ) {
    static_cast<std::any&>(*this).swap(other);
    std::swap( get_base, other.get_base );
  }

  poly_any( poly_any&& o ); // todo
  poly_any& operator=( poly_any&& o ); // todo

  template<class ValueType, class...Ts>
  std::decay_t<ValueType>& emplace( Ts&&... ); // todo
  template<class ValueType, class U, class...Ts>
  std::decay_t<ValueType>& emplace( std::initializer_list<U>, Ts&&... ); // todo
private:
  using to_base = Base&(*)(std::any&);
  to_base get_base = 0;
};

Then you just have to intercept every means of putting stuff into the poly_any<Base> and store a get_base function pointer:

template<class Base, class Derived>
auto any_to_base = +[](std::any& in)->Base& {
  return std::any_cast<Derived&>(in);
};

Once you have done this, you can create a std::vector<poly_any<Base>> and it is a vector of value types that are polymorphically descended from Base.

Note that std::any usually uses the small buffer optimization to store small objects internally, and larger objects on the heap. But that is an implementation detail.