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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<body text="#000000" bgcolor="#FFFFFF">
<head>
<title>Object-by-Value status</title>
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</head>
<body>
<center>
<h1><a name="orb"></a>Objects-by-Value</h1>
Point of contact: <a href="mailto: coryan@cs.wustl.edu">Carlos O'Ryan</a>
<p>Last Update: $Date$</p>
</center>
<p>Objects-by-Value (OBV) describes the new type, <CODE>valuetype</CODE>.
It is introduced in CORBA 2.3
<br>(Core:
<a href="http://www.omg.org/docs/ptc/98-12-04.pdf">ptc/98-12-04.pdf
</a> Mapping to C++:
<a href="http://www.omg.org/docs/ptc/98-09-03.pdf">ptc/98-09-03.pdf</a>).
This first implementation was contributed from
<a href="mailto: kuepper2@uni-wuppertal.de">Torsten Kuepper</a>.
<CODDE>Valuetype</CODE>s are similar to IDL <CODE>struct</CODE>s extended with
these features:
<ul>
<li><p>Encapsulate state and operations with local implementation.</p>
</li>
<li><p>Single inheritance from a <CODE>valuetype</CODE> and
multiple inheritance from abstract
<CODE>valuetype</CODE>s. Eventually support of an
interface.</p>
</li>
<li><p>References to other <CODE>valuetype</CODE>s, with the
possibility of NULL references or shared (aliased) references.</p>
</li>
</ul>
<p><CODE>Valuetype</CODE>s will likely be used to implement
abstract datatypes (ADT)s that can be copied to another process.
All processes that use the <CODE>valuetype</CODE> must have an
implementation for it since CORBA does not support code
migration, a la Java RMI.</p>
<a name="current"><h3>Current status:</h3></a>
<ul>
<li><p>The IDL compiler understands <CODE>valuetype</CODE>, if
it was compiled with <CODE>IDL_HAS_VALUETYPE</CODE>
defined. New options of TAO's IDL are: <P>
<UL>
<LI>-Gv Enable OBV support
<LI>-Sv Disable OBV support (default)
<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="http://www.cs.wustl.edu/~schmidt/ACE_wrappers/TAO/examples/OBV/Typed_Events">$TAO_ROOT/TAO/examples/OBV/Typed_Events</A>.
<CODE>Valuetype</CODE>s could even reference other
<CODE>valuetype</CODE> objects (but without sharing).
</p>
</li>
</ul>
<a name="issues"><h3>Known issues:</h3></a>
<ul>
<li><p>No support for <code>TypeCodes</code> of
<code>valuetype</code> objects
</p>
</li>
<li><p><CODE>Valuetype</CODE>s work only in conjunction with
compiled marshalling (<CODE>-Gc</CODE>, currently default for
<CODE>tao_idl</CODE>).
</p>
</li>
<li><p>No support for <code>TypeCodes</code> of
<code>valuetype</code> objects. Hence moving it into an
<code>any</code> is not possible yet.
</p>
</li>
<li><p><CODE>Valuetype</CODE>s can not be contained in other types like
<CODE>struct</CODE>, <CODE>union</CODE>,
<CODE>sequence</CODE> or <CODE>array</CODE>.
</p>
</li>
<li><p>No support for sharing (aliasing).
</p>
</li>
<li><p>Forward and typedef'd declaration does not work.</p>
</li>
<li><p>Inheritance (support) from an ordinary interface
not implemented.</p>
</li>
<li><p>No support for abstract interfaces.
</p>
</li>
<li><p>No support for <CODE>valuebox</CODE>es
</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 not
like in the CORBA specs and needs a change. But the
optimized mode (<CODE>-Wb,obv_opt_accessor</CODE>) should be
unaffected of that.
</p>
</li>
<li><p>Obtaining a repository id of a <CODE>valuetype</CODE>
is currently non-standard. Hashed repository ids are not
implemented.
</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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