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<p><a href="../index.html"><img src="title-21.png" width="253" height="39" alt="C++ Portable Types Library (PTypes) Version 2.1" border="0"></a>
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<p class="hpath"><a href="index.html">Top</a>: <a href="basic.html">Basic types</a>:
unknown & component</p>
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<blockquote>
<pre class="lang">#include <ptypes.h>
class unknown {
unknown();
virtual ~unknown();
}
class component {
component();
virtual ~component();
void addnotification(component* c);
void delnotification(component* c);
virtual void freenotify(component* c);
}
component* addref(component*);
bool release(component*);
template <class T> class compref;
int objalloc;</pre>
</blockquote>
<p>The <span class="lang">unknown</span> interface is the base for all interfaces
(classes) in PTypes, <b>except</b> <span class="lang">string</span>, <span class="lang">cset</span>,
<span class="lang">variant</span>, <span class="lang">mutex</span>, <span class="lang">rwlock</span>,
<span class="lang">msgqueue</span> and <span class="lang">tpodlist</span>. The
<span class="lang">unknown</span> class has no semantic or functional meaning,
except that deriving a class from <span class="lang">unknown</span> provides a
simple mechanism of tracking the number of created/destroyed objects in a program.
Historically, <span class="lang">unknown</span> was used in PTypes as a base for
all list item types, however, starting from version 2.0 of the library this requirement
is no longer in place.</p>
<p>The <span class="lang">component</span> class adds reference-counting functionality
to <span class="lang">unknown</span> (see <span class="lang">addref()</span> and
<span class="lang">release()</span> below). PTypes variants require objects to
be derived from <span class="lang">component</span> instead of <span class="lang">unknown</span>
to be able to make assignments and destruction of variants properly. See also
<span class="lang">compref </span>below.</p>
<p>As an alternative to reference-counting <span class="lang">component</span>
also provides so-called 'delete notification' mechanism. If object A holds a reference
to object B, A can add itself to B's notification list to be notified when B is
being destroyed. This allows A to take appropriate actions, e.g. invalidate the
reference to B. A in this case can declare reference to B as a plain pointer.
In other words, A should override its own virtual method <span class="lang">freenotify()</span>,
and it also should call <span class="lang">addnotification(this)</span> for the
object it holds reference to.</p>
<p>All stream objects in PTypes and aslo the <span class="lang">unit</span> class
are derived from <span class="lang">component</span>.</p>
<p>A global variable <span class="lang">objalloc</span> is declared to keep track
of the number of allocated objects (derived from <span class="lang">unknown</span>
or <span class="lang">component</span>) in an application program, which can help
you to find memory leaks or other potential bugs. The allocation counting works
only in <span class="lang">DEBUG</span> mode. If the memory is cleaned up properly,
this value should be zero upon program termination. You can write code, possibly
enclosed within <span class="lang">#ifdef DEBUG ... #endif</span>, which checks
whether this value is zero at the end of <span class="lang">main()</span>.</p>
<p>These interfaces are declared in <a href="include/ptypes.h.html"><ptypes.h></a>.</p>
<p><span class="def">component* addref(component* c)</span> increments the reference
counter for the given component object <span class="lang">c</span>. The return
value is the same as <span class="lang">c</span> and is provided for convenience.</p>
<p><span class="def">bool release(component* c)</span> decrements the reference
counter and destroys the object <span class="lang">c</span> if the counter reached
0. Returns <span class="lang">true</span> if the object has been destroyed. Passing
<span class="lang">NULL</span> to this function is not an error.</p>
<p><span class="def">template <class T> class compref</span> implements
a 'smart' pointer to <span class="lang">component</span> (or any derivative class)
with automatic reference counting. Use this template in place of a plain pointer
declaration (e.g. <span class="lang">compref<myclass></span> instead of
<span class="lang">myclass*</span>) to automatically destroy objects as soon as
there are no more references left. The behavior of <span class="lang">compref</span>
pointers is similar to plain pointers, except that they perform additional actions
when assigning new values to them.</p>
<p><span class="def">virtual component::~component()</span> destroys the component
and calls <span class="lang">freenotify()</span> for each object in the notification
list.</p>
<p><span class="def">component::addnotification(component* c)</span> adds object
<span class="lang">c</span> to the notification list of this object. This object's
destructor will then call <span class="lang">c->freenotify()</span>.</p>
<p><span class="def">component::delnotification(component* c)</span> deletes object
<span class="lang">c</span> from this object's notification list.</p>
<p><span class="def">virtual component::freenotify(component* caller)</span> is
called by the <span class="lang">caller</span>'s destructor for each object
in the notification list. This method should be overriden in classes which add
themselves to other objects' notification lists. The overridden method usually
invalidates (assigns NULL) all references to <span class="lang">caller</span>.</p>
<p class="seealso">See also: <a href="lists.html">Lists</a>, <a href="variant.html">variant</a></p>
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