[omniORB] strange behavior of '_this()' method

[Duncan Grisby]

On Wednesday 1 April, Michael wrote:

> Michael Kilburn wrote:
> > On Tue, Mar 31, 2009 at 4:59 AM, Michael <omniorb at bindone.de> wrote:
> > 
> >>  PortableServer::Servant servant = _poa->reference_to_servant(obj);
> >>                servant->_remove_ref();
> > 
> > Just being pedantic... this is better:
> > 
> > PortableServer::Servant_var servant = _poa->reference_to_servant(obj);
> 
> Sorry, but no :) PortableServer::Servant is a pointer to (not a copy of)
>  the incarnating servant in the active object map. You don't want the
> _var class to clean that up ever (and btw it won't compile anyway
> because PortableServer::Servant is a local corba pseudo class thingy).

You're both wrong, just in different ways :-)

The code with Servant_var won't compile, but that's because Servant_var
is a template, not because it's wrong to use a _var type with a
servant. This code is completely valid and does the right thing with
_remove_ref:

  PortableServer::ServantBase_var servant = _poa->reference_to_servant(obj);


> You're confused about memory management rules here (which brings me back
> to my previous argument about CORBA complexity and developers getting
> confused about it :).
> 
> So you understand the context:
> try
> {
>   PortableServer::Servant servant = _poa->reference_to_servant(obj);
>   servant->_remove_ref();

That is a dangerous thing to do. By calling _remove_ref there, you are
assuming that the POA is going to keep holding its reference to the
servant. If another thread deactivates the object, you might find the
servant being deleted from under you. It's much safer to call
_remove_ref once you've finished using the servant, and the easiest way
to do that is to use ServantBase_var.

>   IRObject_impl* impl = dynamic_cast<IRObject_impl*>(servant);
>   if (impl)
>   {
>     // do something
>   }
>   // there is no delete statement here, try this to crash it:
>   // delete servant;

A delete of the servant there is wrong because the POA still holds a
reference to the servant.

Cheers,

Duncan.

-- 
 -- Duncan Grisby         --
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   -- http://www.grisby.org --