[fpc-pascal] Interface performance

Tony Whyman tony.whyman at mccallumwhyman.com
Fri Nov 11 11:44:42 CET 2016

On 11/11/16 10:25, Ryan Joseph wrote:
>> On Nov 11, 2016, at 4:56 PM, Tony Whyman <tony.whyman at mccallumwhyman.com> wrote:
>> 3. To get an object back from an interface you must use the "as" operator.
> Another point to do with CORBA. The manual says they are not reference counted and programmer needs to do book keeping. The crash I was getting was in line with accessing memory that has been deallocated so perhaps because I didn’t do my “book keeping” it was freed as it passed scopes. Not sure how to manage reference counting for interfaces though because the docs didn’t actually explain this.
> Maybe the better question is should I ditch CORBA in favor or COM?
> Regards,
> 	Ryan Joseph
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If last question doesn't send the list into overdrive then nothing will;)

With CORBA you are responsible for freeing the objects that provide an 
interface in the same way that you are always responsible for freeing 
the objects that you create. If you free an object before you finish 
using it then it's a bug and using interfaces does not change that.

With COM, once you assign the object providing the interface to a 
variable with an interface type then you are no longer responsible for 
freeing the object. The object is now managed by the system and if you 
do free it then you get an exception. It's really the same principle as 
AnsiStrings and dynamic arrays, which are also reference counted objects.

The classic newbie problem with COM comes when you have an interface 
with a method that returns another interface. If the underlying objects 
"know" about each other and reference each other's data then the order 
in which they are freed becomes important and automatic reference 
counting does not always give the right answer. There are design 
techniques to manage this, such as objects keeping interface references 
to other objects - but then you have to be careful to avoid circular 
references that prevent the objects ever being released.

I would start working with interfaces by using CORBA. It's more obvious 
what is going on and fewer traps for the unwary. However, if you are 
designing a standard package providing an API realised as a Pascal 
Interface then I would argue that a properly designed COM based 
interface is easier for the API user, in the same way that AnsiStrings 
just "work".

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