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<b>Data Structures and Algorithms 
with Object-Oriented Design Patterns in C++</b><br>
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<H2><A NAME="SECTION0062100000000000000000">Ownership of Contained Objects</A></H2>
<A NAME="secadtsowner">&#160;</A>
<P>
A matter that is closely related to the containment of objects
is the <em>ownership</em><A NAME=5167>&#160;</A> of objects.
Ownership is important because it is the owner of an object
that ensures that the object's destructor is called
and that any storage dynamically allocated for the object
is freed when the object is no longer needed.
<P>
The ownership of contained objects is clear
when direct containment is used.
Since the container makes a copy of any object put into the container,
it is the container that is responsible for deleting the copy
when it is longer needed.
In particular, the destructor for the container would normally
delete all contained objects.
<P>
Unfortunately, the issue of ownership is not as clear
when indirect containment is used.
Indirect containment means that when an object is put into a container,
it is a pointer to that object which is recorded in the container.
This implies that the object was initially created outside of the container.
The question then becomes,
should the container delete the object when the time comes to clean up
or should the deletion be done by outside of the container?
<P>
If we assume that it is the responsibility of the container
to delete the contained objects,
then we must make sure that the only objects put into a container
are objects whose storage was dynamically allocated.
This is because as the owner of the object,
the container must delete the object when the time comes to clean up.
But, given a pointer to an object,
it is not possible for the container to know
whether the object it points to has been dynamically allocated
or whether it is actually a statically allocated global variable
or a stack allocated local variable.
<P>
Another consequence of assigning the ownership of objects to a container
is that things become complicated when an object is put into
two or more different containers
as well as when a given object is put into the same container
more than once.
The problem is that an object inserted in more than one container
has more than one owner.
In order to ensure that the object is only deleted once,
we would have to extract the pointer from all but one
of the containers before deleting them.
<P>
On the other hand,
if we assume that it is not the responsibility of the of the container
to delete contained objects,
then the responsibility to clean up falls on the user of the container.
In order to ensure that all contained objects are properly deleted,
it is necessary to extract all of the contained objects
from a container before deleting the container itself.
<P>
The solution to this dilemma is to support both paradigms.
I.e., make it possible for the user of a container
to specify whether that container is to be the owner of its objects.
The <tt>Ownership</tt> class given
in Program&nbsp;<A HREF="page124.html#progownerh" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page124.html#progownerh"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A> does precisely that.
<P>
<P><A NAME="5540">&#160;</A><A NAME="progownerh">&#160;</A> <IMG WIDTH=575 HEIGHT=314 ALIGN=BOTTOM ALT="program5170" SRC="img698.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img698.gif"  ><BR>
<STRONG>Program:</STRONG> <tt>Ownership</tt> Class Definition<BR>
<P>
<P>
The <tt>Ownership</tt> class encapsulates a single Boolean variable,
<tt>isOwner</tt>,
which records whether the container is the owner of the contained objects.
By default, the <tt>isOwner</tt> field is set to <tt>true</tt>
in the constructor.
Two member functions, <tt>AssertOwnership</tt> and <tt>RescindOwnership</tt>,
provide a means for the user of a container to change the state
of the <tt>isOwner</tt> datum.
The <tt>IsOwner</tt> accessor reveals the current ownership status.
<P>
The behavior of the copy constructor is subtle:
It transfers ownership status from the original container to the copy.
This behavior is useful because it simplifies
the task of returning a container as the result of a function.
<P>
In its declaration in Program&nbsp;<A HREF="page117.html#progcontainer1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page117.html#progcontainer1h"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>
we saw that the <tt>Container</tt> class was derived both
from the <tt>Object</tt> base class <em>and</em>
from the <tt>Ownership</tt> class.
Therefore, all containers have ownership associated with them.
<P>
The following code fragment gives the design framework
for the implementation of the <tt>Purge</tt> member function
in some concrete class, say <tt>SomeContainer</tt>,
which is derived from the abstract base class <tt>Container</tt>:
<PRE>void SomeContainer::Purge ()
<P>
    if (IsOwner ())
<P>
        <em>for each</em> Object i <em>in this container</em>
<P>
            delete &i;
<P>
    <em>Now clean up the container itself.</em>
<P>
</PRE>
The container calls its own <tt>IsOwner</tt> member function
to determine whether it is the owner of its contained objects.
Then, and only if it is the owner,
the contained objects are deleted.
Given the <tt>Purge</tt> function,
the implementation of the destructor is trivial:
<PRE>SomeContainer::~SomeContainer ()
    { Purge (); }</PRE>
<P>
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