TAO/docs/releasenotes/OBV.html

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   6       <title>Object-by-Value status</title>
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  11       <center>
  12         <h1><a name="orb"></a>Objects-by-Value</h1>
  13         Points of contact: <a href="mailto: parsons@cs.wustl.edu">Jeff Parsons</a>
  14         <a href="mailto: bosk@ipmce.ru">Boris Kolpackov</a>
  15         <a href="mailto:g.edwards@vanderbilt.edu">George Edwards</a>
  16
  17         <p>Last Update: 2004/08/24 </p>
  18       </center>
  19
  20       <p>Objects-by-Value (OBV) describes the new type, <CODE>valuetype</CODE>, introduced in CORBA 2.3
  21         <br>(Core:
  22         <a href="http://www.omg.org/cgi-bin/apps/doc?formal/02-12-06.pdf">formal/02-12-06.pdf</a>; Mapping to C++:
  23         <a href="http://www.omg.org/cgi-bin/apps/doc?formal/03-06-03.pdf">formal/03-06-03.pdf</a>).
  24        <p>The original TAO implementation was contributed by
  25         <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>.
  26       <p><CODE>Valuetype</CODE>s are similar to IDL <CODE>struct</CODE>s extended with
  27         these capabilities:
  28       <ul>
  29         <li><p>Encapsulate both state information and operations in an implementation that is guaranteed to be local.</p>
  30         </li>
  31
  32         <li><p>Can be declared <CODE>abstract</CODE>, in which case no state members are defined.</p>
  33         </li>
  34 <li><p>Can inherit from a single concrete <CODE>valuetype</CODE> and multiple <CODE>abstract valuetype</CODE>s.</p></li>
  35 <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>
  36 <li><p>Can support multiple <CODE>abstract interface</CODE>s, allowing them to be substituted for those interfaces in operation invocations.</p></li>
  37         <li><p>Can hold references to other <CODE>valuetype</CODE>s, with the
  38             possibility of NULL references or shared (aliased) references.</p>
  39         </li>
  40       </ul>
  41
  42 <p>Valuetypes have the following uses:
  43 <UL>
  44 <li><p>Implement abstract datatypes (ADTs) that can be copied to another process.</p></li>
  45 <li><p>Represent <CODE>eventtype</CODE>s in the Event Service.</p></li>
  46 <li><p>Ensure operations are executed locally for increased performance.</p></li>
  47 </UL>
  48       <a name="current"><h3>Current status:</h3></a>
  49
  50       <ul>
  51         <li><p>The IDL compiler understands <CODE>valuetype</CODE>.
  52         Relevant option of TAO's IDL compiler is: <P>
  53             <UL>
  54             <LI>-Wb,obv_opt_accessor Make accessor and modifier functions inline.
  55             Overriding them is not allowed in this mode.</LI>
  56             </UL></P>
  57             <p></p>
  58         </li>
  59         <li><p><CODE>Valuetype</CODE>s can be used as arguments in CORBA invocations.
  60             There is an example in
  61             <A HREF="../../examples/OBV/Typed_Events">$TAO_ROOT/TAO/examples/OBV/Typed_Events</A>.
  62             <CODE>Valuetype</CODE>s can reference other
  63             <CODE>valuetype</CODE> objects (but without sharing).
  64           </p>
  65         </li>
  66         <li>
  67           <p>Support for valuetypes as members of IDL aggregate types has been
  68               added.</p>
  69         </li>
  70         <li>
  71           <p>Support for inheritance from a concrete interface (<CODE>supports</CODE>) has been added. There is an example in <A HREF="../../tests/OBV/Supports">$TAO_ROOT/TAO/tests/OBV/Supports</A>.</p>
  72         </li>
  73 <li>
  74 <p>Support for inheritance from abstract interfaces has been added. There is an example in <A HREF="../../tests/Abstract_Interface">$TAO_ROOT/TAO/tests/Abstract_Interface</A>.</p>
  75         <li>
  76           <p>Support for forward declared valuetypes defined in another compilation
  77               unit has been added. See <A HREF="../../examples/OBV/Typed_Events">$TAO_ROOT/TAO/examples/OBV/Typed_Events</A>.</p>
  78         </li>
  79 <li>
  80 <p>Support for recursively-defined valuetypes has been added. See <A HREF="../../tests/OBV/Supports">$TAO_ROOT/TAO/tests/OBV/Supports</A>.</p>
  81 </li>
  82 <li>
  83 <p>Support for user-declared factories has been added. See <A HREF="../../tests/OBV/Factory">$TAO_ROOT/TAO/tests/OBV/Factory</A>.</p>
  84 </li>
  85       </ul>
  86
  87       <a name="issues"><h3>Known issues:</h3></a>
  88
  89       <ul>
  90         <li><p><CODE>Valuetype</CODE>s work only in conjunction with
  91             compiled marshaling (<CODE>-Gc</CODE>, currently default for
  92             <CODE>tao_idl</CODE>).
  93           </p>
  94         </li>
  95
  96         <li><p>No support for sharing (aliasing).
  97           </p>
  98         </li>
  99
 100         <li><p>No support for <CODE>valuebox</CODE>es
 101           </p>
 102         </li>
 103 <li>
 104 <p>
 105 No support for <CODE>valuetype</CODE>s with cyclic references.</p></li>
 106
 107         <li><p>No support for fragmentation (chunking) of the marshalled
 108             <CODE>valuetype</CODE> object. Hence no support for truncation or
 109             custom marshalling.</p>
 110         </li>
 111
 112         <li><p>The marshal engine accesses the state members directly and
 113             does not utilize the accessor/modifier functions. This is different
 114             from the CORBA specs and needs to be changed. But the
 115             optimized mode  (<CODE>-Wb,obv_opt_accessor</CODE>) should be
 116             unaffected by that.
 117           </p>
 118         </li>
 119
 120         <li><p>The map of <CODE>valuetype</CODE> factories needs some
 121             revision to provide proper locking. Currently the
 122             registration of factories is best completed before
 123             unmarshalling <CODE>valuetype</CODE>s.
 124             There is one map of factories for the whole process. This
 125             will once be changed to conform to the specs, which
 126             suggests one per ORB.
 127           </p>
 128         </li>
 129       </ul>
 130
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