On Thu, 16 Oct 2014, Adriaan van Os wrote:
Section 7.6 in the FreePascal Language Reference discusses automatic
reference counting for COM interfaces. It details assignments to variables
and temporary variables. But the section doesn't mention parameter passing.
What happens when interfaces are passed to functions, procedures and methods
?
From reading Delphi documentation and by experimenting a bit, there seems to
be a great difference between value and CONST parameters and between VAR and
OUT parameters. I suggest to extend the manual to explain this in detail.
That will require the compiler devs explaining in detail how it works,
because I haven't got a clue. AFAIK var, const and out behave the same
where ref counting is concerned, with 'value' parameters being the only
difference.
For example, I have an interface IMFMediaEventGenerator (from Windows
MediaFoundation) with a method GetEvent
type
IMFMediaEventGenerator = interface( IUnknown) ...
....
function GetEvent( dwFlags: DWORD; out ppEvent: IMFMediaEvent):
HResult; stdcall;
Now, suppose I am calling GetEvent in a loop
theEvent := nil;
theEventType := MEUnknown;
theResult := S_OK;
while ( theEventType <> MESessionClosed) and ( theResult >= S_OK) do
begin
theResult := theSession.GetEvent( 0, theEvent);
...
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.
Indeed.
In general, I am not a proponent of "compiler magic" because it confuses
things, puts it under the table. I would certainly welcome a compiler option
to NOT auto-refcount.
Talk about going against the spirit of the times :-)
Michael.
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