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In section ref(VIRTDES) we learned that bf(C++) supports em(virtual
destructors). Like many other object oriented languages (e.g., bf(Java)),
however, the notion of a emi(virtual constructor) is not supported. Not having
virtual constructors becomes a liability when only base class references or
pointers are available, and a copy of a derived class object is
required. em(Gamma et al.) (1995) hi(Gamma, E.) discuss the
emi(Prototype design pattern) hi(design pattern: Prototype) to deal with this
situation.
According to the em(Prototype Design Pattern) each derived class is given
the responsibility of implementing a member function returning a pointer to a
copy of the object for which the member is called. The usual name for this
function is tt(clone). Separating the user interface from the reimplementation
interface tt(clone) is made part of the interface and tt(newCopy) is defined
in the reimplementation interface. A base class supporting `cloning' defines a
virtual destructor, tt(clone), returning tt(newCopy)'s return value and the
em(virtual copy constructor), a pure virtual function, having the prototype
tt(virtual Base *newCopy() const = 0). As tt(newCopy) is a pure virtual
function all derived classes must now implement their own `virtual
constructor'.
This setup suffices in most situations where we have a pointer or
reference to a base class, but it fails when used with abstract
containers. We can't create a tt(vector<Base>), with tt(Base) featuring the
pure virtual tt(copy) member in its interface, as tt(Base) is called to
initialize new elements of such a vector. This is impossible as tt(newCopy) is a
pure virtual function, so a tt(Base) object can't be constructed.
The intuitive solution, providing tt(newCopy) with a default
implementation, defining it as an ordinary virtual function, fails too as the
container calls tt(Base(Base const &other)), which would have to call
tt(newCopy) to copy tt(other). At this point it is unclear what to do with
that copy, as the new tt(Base) object already exists, and contains no tt(Base)
pointer or reference data member to assign tt(newCopy)'s return value to.
Alternatively (and preferred) the original tt(Base) class (defined as an
i(abstract base class)) is kept as-is and a wrapper class tt(Clonable) is used
to manage the tt(Base) class pointers returned by tt(newCopy). In chapter
ref(NESTEDCLASSES) ways to merge tt(Base) and tt(Clonable) into one class are
discussed, but for now we'll define tt(Base) and tt(Clonable) as separate
classes.
The class tt(Clonable) is a very standard class. It contains a pointer member
so it needs a copy constructor, destructor, and overloaded assignment
operator. It's given at least one non-standard member: tt(Base &base() const),
returning a reference to the derived object to which tt(Clonable)'s tt(Base *)
data member refers. It is also provided with an additional constructor to
initialize its tt(Base *) data member.
Any non-abstract class derived from tt(Base) must implement tt(Base
*newCopy()), returning a pointer to a newly created (allocated) copy of the
object for which tt(newCopy) is called.
Once we have defined a derived class (e.g., tt(Derived1)), we can put our
tt(Clonable) and tt(Base) facilities to good use. In the next example we see
tt(main) defining a tt(vector<Clonable>). An anonymous tt(Derived1)
object is then inserted into the vector using the following steps:
itemization(
it() A new anonymous tt(Derived1) object is created;
it() It initializes a tt(Clonable) using tt(Clonable(Base *bp));
it() The just created tt(Clonable) object is inserted into the vector,
using tt(Clonable)'s move constructor. There are only temporary tt(Derived)
and tt(Clonable) objects at this point, so no copy construction is required.
)
In this sequence, only the tt(Clonable) object containing the tt(Derived1
*) is used. No additional copies need to be made (or destroyed).
Next, the tt(base) member is used in combination with tt(typeid) to show
the actual type of the tt(Base &) object: a tt(Derived1) object.
tt(Main) then contains the interesting definition tt(vector<Clonable>
v2(bv)). Here a copy of tt(bv) is created. This copy construction observes
the actual types of the tt(Base) references, making sure that the appropriate
types appear in the vector's copy.
At the end of the program, we have created two tt(Derived1) objects, which
are correctly deleted by the vector's destructors. Here is the full program,
illustrating the `virtual constructor' concept+footnote(
Jesse van den Kieboom created an alternative implementation of a class
tt(Clonable), implemented as a link(class template)(TEMPCLASS). His
implementation is found in the source archive under
tt(contrib/classtemplates/).):
verbinclude(-a examples/virtcons2.cc)
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