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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<html> <head>
<title>Implementing and Using Local Objects</title>
</head>
<body text="#000000" bgcolor="#FFFFFF">
<h1>Implementing Local Objects</h1>
<p>The new addition of "<code>local</code>" interfaces in CCM define a
standard behavior of locality constraint object. There are some
subtle differences in implementing and using local interfaces compared
to regular (remote) interfaces. This page tries to collect
information on things to notice and common pitfalls.</p>
<ul>
<li><p><b>Local or not Local?</b> - A local interface is local.
Any types defined withing the interface are also local. They
include "struct, union, enum, sequence, exceptions, typedef..."
For constructed types, if a constructed type contains local
elements, then it also becomes local even if the type is not
defined within a local interface.</p>
<li><p>You can not put objects of local types into Any's or try to use
it regular CORBA invocations.</p>
<li><p><b>Implementation class</b> - Instead of inheriting from
<code>POA_FOOBAR</code> class, the implementation of local
interface inherit from both the C++ object mapping class (the
client side class) and the <code>CORBA::LocalObject</code>. For
example, when <code>FOOBAR</code> is a regular interface, the
declaration of the implementation class is something like this:
<pre>
class FOOBAR_i : public POA_FOOBAR
{
. . .
};
</pre>
However, if <code>FOOBAR</code> is defined as a local interface,
the implementation class is defined as:
<pre>
class FOOBAR_i : public FOOBAR,
public CORBA::LocalObject
{
. . .
};
</pre></p>
<li><p><b>Reference Counting and Object Reference Lifecycle</b> -
Regular CORBA object references use reference counting to manage
lifecycle of object references. Local object references
<b>may</b> also use reference counting for lifecycle
management.</p>
<p>There are <code>_add_ref ()</code> and <code>_remove_ref
()</code> operations defined in all local object and the ORB
uses these operations to increase/decrease the reference count
when <code>_duplicate ()</code> or <code>CORBA::release
()</code> are invoked on an ojbect reference. <b>However,
notice that the default implementation of <code>_add_ref
()</code> and <code>_remove_ref ()</code> for local objects are
no-ops.</b> Therefore, if you wish to do reference counting on
your local object instances, you must overwrite <code>_add_ref
()</code> and <code>_remove_ref ()</code> in your implementation
for that local object.</p>
<p>By leaving <code>_add_ref ()</code> and <code>_remove_ref
()</code> as no-ops, you assume the responsibility of managing
the local object instance. Objects that have the same lifetime
as the ORB may want to use this strategy and let the ORB to
manage these instances of local objects. This prevents user
errors from crashing the server process. However, in this case,
the object needs to be <code>delete</code>'d explicitly (as
<code>CORBA::release ()</code> basically doesn't do anything in
this case.</p>
<p>TAO developers can get reference counting by using the mixin class
<code>TAO_Local_Ref_Counted_Object ()</code> as:
<pre>
class FOOBAR_i : public FOOBAR,
public TAO_Local_Ref_Counted_Object
{
. . .
};
</pre>
if you wish to use reference counting. However,
this is not portable and should be used with care by the applications.</p>
</ul>
<hr>
<address></address>
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Last modified: Wed Apr 26 02:21:35 Central Daylight Time 2000
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</body> </html>
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