[fpc-devel] Managed types and reference counts revisited

[Jonas Maebe]

Adriaan van Os wrote on Wed, 17 Aug 2016:

> Either I am looking wrong (and missing the point) or the text at  
> <http://www.freepascal.org/docs-html/ref/refse91.html> (a follow-up  
> on <http://bugs.freepascal.org/view.php?id=26874>) is incorrect.
>
> Doesn't the call to DoIt1(y,7) in procedure Test3 cause the  
> reference count to be increased (from 0 to 1) and then decreased  
> (from 1 to 0 and thus disposed) ? And doesn't DoIt1 use value  
> parameters ?

Yes.

> Therefore, doesn't the comment on  the const case (on the top of the  
> page) belong to the value-parameter (aka "nothing") case ?

It actually applies to both.

> I would say, If const parameters, like var parameters, do nothing,  
> there is no reference count decrease, therefore no dispose of  
> classes (implementing an interface) even when the reference count is  
> 0 ?

It's about when you pass reference counted non-lvalues to parameters,  
e.g. the result of a function or the expression "ClassInstance as  
InterfaceType". In that case, the compiler will create a temp (on the  
caller side) to store that function/expression result, which results  
the reference count of the interface instance to be increased. After  
the function call to which this interface instance has been passed as  
parameter returns, the reference count will be decreased again because  
the temp gets freed. If the reference count becomes 0 at that point,  
the instance gets freed too. As a result, in this case even with a  
"const" parameter you still have an interface instance that gets freed  
at some point after the call (since otherwise there would be a  
memory/reference count leak).

Passing a class instance directly to a const interface parameter does  
not result in the creation of a temp and hence does not result in  
reference count increases/decreases. In case of a value parameter,  
there is reference count on the callee side regardless of what you  
passed in.


Jonas