Jonas Maebe wrote:
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)
I agree for pure Pascal code, where a parameter can be OUT or VAR and automatic release will work
in both cases. However, I disagree for interfacing with external interfaces. It must be clear for
the bindings-writer that VAR in the bindings instead of (what should have been) OUT can cause a
memory leak. This is tricky enough that it deserves an explanation in the manual. For example, I
think (from Windows MediaFoundation)
type
IMFMediaEventGenerator = interface( IUnknown) ...
....
function GetEvent( dwFlags: DWORD; OUT ppEvent: IMFMediaEvent):
HResult; stdcall;
is correct, whereas
function GetEvent( dwFlags: DWORD; VAR ppEvent: IMFMediaEvent):
HResult; stdcall;
will leak memory.
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
Thanks for the explanation. This clarifies things.
Regards,
Adriaan van Os
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