[fpc-devel] Reference counting interface objects

[Jonas Maebe]

On 16 Oct 2014, at 16:43, Adriaan van Os wrote:

> Jonas Maebe wrote:
>
>> No, it won't:
>> 1) if the external code expects it to be "out" and you declare it  
>> as "var": the caller will decrease the reference count and the  
>> ppEvent that you get will have as initial value "nil". You can just  
>> assign to it like normal.
>> 2) if the external code expects it to be "var" and you declare it  
>> as "out": the caller will not do anything and hence the ppEvent  
>> will not be nil. However, assignments to the "out" parameter still  
>> cause its reference count to be decreased. After all, you can  
>> assign multiple times to an "out" parameter, so the compiler cannot  
>> assume it's already nil. The only problem that could happen here is  
>> if you would explicitly write your code in a way that behaves  
>> differently depending on whether the initial value is nil (e.g.,  
>> assigning something to it only in one branch of an if-statement,  
>> and then later checking whether it's "still" nil to determine  
>> whether you have to assign something else to it).
>
> Sorry, but the above explanation flabberghasts me.

I explained the opposite scenario as you: calling a method implemented  
in Pascal from external Windows code (with mismatched var/out  
modifiers). Rereading your message, that's indeed not what you were  
talking about.

In general, using wrong/mismatched modifiers when importing routines  
virtually always causes problems though (and often much worse than  
just a memory leak), there's nothing special about reference counted  
types here.


Jonas
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