Objects-by-Value

Objects-by-Value (OBV) describes the new type, valuetype. It is introduced in CORBA 2.3
(Core: ptc/98-12-04.pdf Mapping to C++: ptc/98-09-03.pdf). This first implementation was contributed from Torsten Kuepper. Valuetypes are similar to IDL structs extended with these features:

• Encapsulate state and operations with local implementation.

• Single inheritance from a valuetype and multiple inheritance from abstract valuetypes. Eventually support of an interface.

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

Valuetypes will likely be used to implement abstract datatypes (ADT)s that can be copied to another process. All processes that use the valuetype must have an implementation for it since CORBA does not support code migration, a la Java RMI.

• The IDL compiler understands valuetype, if it was compiled with IDL_HAS_VALUETYPE defined. New options of TAO's IDL are:

  • -Gv Enable OBV support
  • -Sv Disable OBV support (default)
  • -Wb,obv_opt_accessor Make accessor and modifier functions inline. Overriding them is not allowed in this mode.

  libTAO must be compiled with TAO_HAS_VALUETYPE.

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

• No support for TypeCodes of valuetype objects

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

• No support for TypeCodes of valuetype objects. Hence moving it into an any is not possible yet.

• Valuetypes can not be contained in other types like struct, union, sequence or array.

• No support for sharing (aliasing).

• Forward and typedef'd declaration does not work.

• Inheritance (support) from an ordinary interface not implemented.

• No support for abstract interfaces.

• No support for valueboxes

• 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 not like in the CORBA specs and needs a change. But the optimized mode (-Wb,obv_opt_accessor) should be unaffected of that.

• Obtaining a repository id of a valuetype is currently non-standard. Hashed repository ids are not implemented.

• 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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