[fpc-devel] Reference counting interface objects

[Jonas Maebe]

On 16 Oct 2014, at 12:35, Adriaan van Os wrote:

> Now, it seems (correct me if I am wrong) that theEvent doesn't need  
> to be manually released, because it has been declared as an OUT  
> parameter rather than a VAR parameter. So, if this is true, this  
> information is crucial when writing interface bindings.

Nothing ever needs to be manually released, regardless of whether it's  
a VAR or OUT parameter. That's the whole point of automatic reference  
counting)

Regarding the effects of the various qualifiers:
* nothing (value): the reference count of the parameter is increased  
(I forgot whether it's the caller or callee that does it) on entry and  
decreased on exit
* OUT: the reference count of the value that's passed in is decreased  
by 1, and the variable that's passed into the procedure is initialized  
to "empty" (nil, but that's an implementation detail)
* VAR: nothing happens to the reference count. A reference to the  
original variable is passed in, and changing it or reading it has  
exactly the same effect as changing/reading the original variable.
* CONST: this is the only tricky one. Again nothing happens to the  
reference count, but because you can pass non-lvalues here. In  
particular, you can pass a class implementing an interface rather than  
the interface itself, which can cause the class to be freed  
unexpectedly. See the example at http://docwiki.embarcadero.com/RADStudio/XE7/en/Using_Reference_Counting 
  . The same can happen with functions returning a class instance  
passed to a function expecting a CONST interface parameter. There is  
an open bug report about adding a warning for it: http://bugs.freepascal.org/view.php?id=19503


Jonas