[fpc-pascal] Interface, _AddRef, _Release, etc.

Jonas Maebe jonas.maebe at elis.ugent.be
Thu Sep 15 14:18:55 CEST 2011

On 14 Sep 2011, at 23:17, Marcos Douglas wrote:

> BUT, if I create my TFoo without refcount should works, right?

Only in some circumstances. These circumstances depend on when the  
compiler allocates temporary interface instances, and when these are  

> ... but, does not work. In Delphi 7 yes, but no in FPC.

It also "works" in FPC if you remove heaptrc. What happens is
a) the compiler creates a temporary copy of the interface instance at  
some point in the code
b) you free the underlying object and heaptrc overwrites its memory  
with 0xf0f0f0f0
c) at the end of the routine, the compiler tries to finalize the  
temporary interface instance. This crashes because the interface  
instance no longer contains valid data

Without heaptrc, the example happens to work because the freed memory  
of the instance hasn't been reallocated and overwritten yet. It's not  
safe in general though.

The answer is: the hack with returning -1 from _AddRef and _Release,  
while documented by Embarcadero, is unsafe because it depends on  
implementation details. Do not use it. Instead, if you do not want  
reference-counted interfaces, use {$interfaces corba} instead as  
mentioned earlier in this thread.


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