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Ingo Klöckl
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Programming CORBA with Orbacus on Linux

Important methods in generated classes

There are some important methods in the classes generated by the IDL compiler.

Narrowing references to a special interface

All CORBA objects are derived from CORBA::Object, and in many cases references are initially returned as objects of this general type. You have to downcast them before using with a special static method: 

X X::_narrow(CORBA::Object_ptr)
This method gets a reference to a CORBA objects (this can already be a derived one) and returns a reference to an object of the desired type or nil if the object does not support the desired interface. Before use you should therefore test the return value with is_nil(). The method calls _duplicate so you have to release the returned reference by yourself! A typical usage is the following: 
CORBA::Object_ptr obj = <get reference from somewhere>
X_ptr xp = X::_narrow(obj);
CORBA::release(obj);  // no longer needed
if (CORBA::is_nil(xp)){
  // interface not supported by object
} else {
  <use object>
  CORBA::release(xp);
}
Note that the initial reference obj is released immediately after the narrowing because we do not longer need this initial reference. This must not be forgotten! After checking the narrowed reference for non-nil, we use it and release it after use.

Of course, the usage of _var variables can make things easier: 

CORBA::Object_var obj = <get reference from somewhere>
X_var xv = X::_narrow(obj);
if (CORBA::is_nil(xv)){
  // interface not supported by object
} else {
  <use object>
}
If these references goes out of scope, they are automatically be released.

Checking for interfaces

The method 

_is_a(<typeid>)
checks wether the reference on which it is invoked is of specified type. It may throw an exception if the type specification is incorrect. It throws a system exception like TRANSIENT because for type checking, the server object may be contacted, resulting in CORBA communication failures.

Checking for existence

The method 

_non_existent()
allows you to check if the reference on which it is invoked points to an existing object or not. The result is definitive true, if the reference dangles. A result of false indicate either that the server object does not exist or cannot be contacted to check the existence! For that, the method may throw system exceptions. A typical usage is the following: 
X_ptr xp = <get reference from somewhere>
try {
  if (xp->non_existent()){
    // definitive answer, release resources
    CORBA::release(xp);  
  } else {
    // try to use reference
    xp->f();
  }  
} catch (const CORBA::TRANSIENT&) {
} catch (const CORBA::SystemException&) {
}

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