Objects-by-Value

Objects-by-Value (OBV) describes the new type, valuetype, introduced in CORBA 2.3
(Core: formal/02-12-06.pdf; Mapping to C++: formal/03-06-03.pdf).

The original TAO implementation was contributed by Torsten Kuepper, and has subsequently been enhanced and corrected by Jeff Parsons and George Edwards.

Valuetypes are similar to IDL structs extended with these capabilities:

• Encapsulate both state information and operations in an implementation that is guaranteed to be local.

• Can be declared abstract, in which case no state members are defined.

• Can inherit from a single concrete valuetype and multiple abstract valuetypes.

• Can support a single concrete interface, allowing them to be manipulated as either a valuetype or an object reference.

• Can support multiple abstract interfaces, allowing them to be substituted for those interfaces in operation invocations.

• Can hold references to other valuetypes, with the possibility of NULL references or shared (aliased) references.

Valuetypes have the following uses:

• Implement abstract datatypes (ADTs) that can be copied to another process.

• Represent eventtypes in the Event Service.

• Ensure operations are executed locally for increased performance.

• The IDL compiler understands valuetype. Relevant option of TAO's IDL compiler is:

  • -Wb,obv_opt_accessor Make accessor and modifier functions inline. Overriding them is not allowed in this mode.

• Valuetypes can be used as arguments in CORBA invocations. There is an example in $TAO_ROOT/TAO/examples/OBV/Typed_Events. Valuetypes can reference other valuetype objects (but without sharing).

• Support for valuetypes as members of IDL aggregate types has been added.

• Support for inheritance from a concrete interface (supports) has been added. There is an example in $TAO_ROOT/TAO/tests/OBV/Supports.

• Support for inheritance from abstract interfaces has been added. There is an example in $TAO_ROOT/TAO/tests/Abstract_Interface.

• Support for forward declared valuetypes defined in another compilation unit has been added. See $TAO_ROOT/TAO/examples/OBV/Typed_Events.

• Support for recursively-defined valuetypes has been added. See $TAO_ROOT/TAO/tests/OBV/Supports.

• Support for user-declared factories has been added. See $TAO_ROOT/TAO/tests/OBV/Factory.

• Valuetypes work only in conjunction with compiled marshaling (-Gc, currently default for tao_idl).

• No support for sharing (aliasing).

• No support for valueboxes

• No support for valuetypes with cyclic references.

• No support for fragmentation (chunking) of the marshalled valuetype object. Hence no support for truncation or custom marshalling.

• 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 (-Wb,obv_opt_accessor) should be unaffected by that.

• The map of valuetype factories needs some revision to provide proper locking. Currently the registration of factories is best completed before unmarshalling valuetypes. 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.

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