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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<title>Object-by-Value status</title>
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<center>
<h1><a name="orb"></a>Objects-by-Value</h1>
Points of contact: <a href="mailto: parsons@cs.wustl.edu">Jeff Parsons</a>
<a href="mailto: bosk@ipmce.ru">Boris Kolpackov</a>
<a href="mailto:g.edwards@vanderbilt.edu">George Edwards</a>
<p>Last Update: 2003/06/30 </p>
</center>
<p>Objects-by-Value (OBV) describes the new type, <CODE>valuetype</CODE>, introduced in CORBA 2.3
<br>(Core:
<a href="http://www.omg.org/cgi-bin/apps/doc?formal/02-12-06.pdf">formal/02-12-06.pdf</a>; Mapping to C++:
<a href="http://www.omg.org/cgi-bin/apps/doc?formal/03-06-03.pdf">formal/03-06-03.pdf</a>).
<p>The original TAO implementation was contributed by
<a href="mailto: kuepper2@uni-wuppertal.de">Torsten Kuepper</a>, and has subsequently been enhanced and corrected by <a href="mailto: parsons@cs.wustl.edu">Jeff Parsons</a> and <a href="mailto:g.edwards@vanderbilt.edu">George Edwards</a>.
<p><CODE>Valuetype</CODE>s are similar to IDL <CODE>struct</CODE>s extended with
these capabilities:
<ul>
<li><p>Encapsulate both state information and operations in an implementation that is guaranteed to be local.</p>
</li>
<li><p>Can be declared <CODE>abstract</CODE>, in which case no state members are defined.</p>
</li>
<li><p>Can inherit from a single concrete <CODE>valuetype</CODE> and multiple <CODE>abstract valuetype</CODE>s.</p></li>
<li><p>Can support a single concrete <CODE>interface</CODE>, allowing them to be manipulated as either a valuetype or an object reference.</p></li>
<li><p>Can support multiple <CODE>abstract interface</CODE>s, allowing them to be substituted for those interfaces in operation invocations.</p></li>
<li><p>Can hold references to other <CODE>valuetype</CODE>s, with the
possibility of NULL references or shared (aliased) references.</p>
</li>
</ul>
<p>Valuetypes have the following uses:
<UL>
<li><p>Implement abstract datatypes (ADTs) that can be copied to another process.</p></li>
<li><p>Represent <CODE>eventtype</CODE>s in the Event Service.</p></li>
<li><p>Ensure operations are executed locally for increased performance.</p></li>
</UL>
<a name="current"><h3>Current status:</h3></a>
<ul>
<li><p>The IDL compiler understands <CODE>valuetype</CODE>.
Relevant options of TAO's IDL compiler are: <P>
<UL>
<LI>-Gv Enable OBV support (default)
<LI>-Sv Disable OBV support
<LI>-Wb,obv_opt_accessor Make accessor and modifier functions inline.
Overriding them is not allowed in this mode.
</UL>
<p><CODE>libTAO</CODE> must be compiled with <CODE>TAO_HAS_VALUETYPE</CODE>.</p>
</li>
<li><p><CODE>Valuetype</CODE>s can be used as arguments in CORBA invocations.
There is an example in
<A HREF="../../examples/OBV/Typed_Events">$TAO_ROOT/TAO/examples/OBV/Typed_Events</A>.
<CODE>Valuetype</CODE>s can reference other
<CODE>valuetype</CODE> objects (but without sharing).
</p>
</li>
<li>
<p>Support for valuetypes as members of IDL aggregate types has been
added.</p>
</li>
<li>
<p>Support for inheritance from a concrete interface (<CODE>supports</CODE>) has been added. There is an example in <A HREF="../../tests/OBV/Supports">$TAO_ROOT/TAO/tests/OBV/Supports</A>.</p>
</li>
<li>
<p>Support for inheritance from abstract interfaces has been added. There is an example in <A HREF="../../tests/Abstract_Interface">$TAO_ROOT/TAO/tests/Abstract_Interface</A>.</p>
<li>
<p>Support for forward declared valuetypes defined in another compilation
unit has been added. See <A HREF="../../examples/OBV/Typed_Events">$TAO_ROOT/TAO/examples/OBV/Typed_Events</A>.</p>
</li>
<li>
<p>Support for recursively-defined valuetypes has been added. See <A HREF="../../tests/OBV/Supports">$TAO_ROOT/TAO/tests/OBV/Supports</A>.</p>
</li>
<li>
<p>Support for user-declared factories has been added. See <A HREF="../../tests/OBV/Factory">$TAO_ROOT/TAO/tests/OBV/Factory</A>.</p>
</li>
</ul>
<a name="issues"><h3>Known issues:</h3></a>
<ul>
<li><p><CODE>Valuetype</CODE>s work only in conjunction with
compiled marshaling (<CODE>-Gc</CODE>, currently default for
<CODE>tao_idl</CODE>).
</p>
</li>
<li><p>No support for sharing (aliasing).
</p>
</li>
<li><p>No support for <CODE>valuebox</CODE>es
</p>
</li>
<li>
<p>
No support for <CODE>valuetype</CODE>s with cyclic references.</p></li>
<li><p>No support for fragmentation (chunking) of the marshalled
<CODE>valuetype</CODE> object. Hence no support for truncation or
custom marshalling.</p>
</li>
<li><p>The marshal engine accesses the state members directly and
does not utilize the accessor/modifier functions. This is different
from the CORBA specs and needs to be changed. But the
optimized mode (<CODE>-Wb,obv_opt_accessor</CODE>) should be
unaffected by that.
</p>
</li>
<li><p>The map of <CODE>valuetype</CODE> factories needs some
revision to provide proper locking. Currently the
registration of factories is best completed before
unmarshalling <CODE>valuetype</CODE>s.
There is one map of factories for the whole process. This
will once be changed to conform to the specs, which
suggests one per ORB.
</p>
</li>
</ul>
<P><HR><P>
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