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Hi Jolyon
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<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">I
spotted that they fixed that a while ago – I remember having to fix the
issue myself many years ago so was quite pleased to see that it was now
taken care of in TInterfaceObject as a matter of course. For some
reason I never noticed the omission of the same facility in the
destructor. And yes, it’s a [potentially] big problem.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">I
need to think about this tho... setting a fake ref count during
execution of the constructor is safe enough as you know precisely when
construction is done and to restore the ref count back to zero.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Setting
a fake ref count during destruction strikes me as more problematic and
makes me nervous, but I can’t quite put my finger on why. It might be
nothing. That doesn’t mean it can’t be fixed, only that the solution
put in place for construction might not work for destruction and it
wasn’t felt necessary to do any extra work for a more comprehensive
solution.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p><br>
</o:p></span></p>
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</blockquote>
I fixed it like this<br>
<br>
<small>procedure TamObject.BeforeDestruction;<br>
begin<br>
//add a reference count, in case an interface is acquired and
released during destruction<br>
InterlockedIncrement(FCount);<br>
inherited BeforeDestruction;<br>
end; <br>
<br>
procedure TamObject.FreeInstance;<br>
begin<br>
//remove the reference count added in </small><small>BeforeDestruction</small><br>
<small> InterlockedDecrement(FCount);<br>
assert(FCount = 0,'Destroying object with non-zero reference count');<br>
inherited FreeInstance;<br>
end;</small><br>
<br>
Of course, an interface can still not be referenced in descendant
BeforeDestruction methods, (since the inherited method is usually
called at the end), but can be safely referenced in the destructor.<br>
<br>
Todd.<br>
<blockquote cite="mid:007501cb8b8c$dcc659a0$96530ce0$@co.nz" type="cite">
<div class="WordSection1">
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Certainly
in the case of my code where I fixed this I had specific “constructing”
/ “destructing” state markers (it wasn’t a general purpose
interfacedobject class but a base class in a far richer framework that
happened to also implement its own version of IUnknown) – I know I
didn’t rely on side effects of a faked ref count.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;"
lang="EN-US">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;"
lang="EN-US"> <a class="moz-txt-link-abbreviated" href="mailto:delphi-bounces@delphi.org.nz">delphi-bounces@delphi.org.nz</a>
[<a class="moz-txt-link-freetext" href="mailto:delphi-bounces@delphi.org.nz">mailto:delphi-bounces@delphi.org.nz</a>] <b>On Behalf Of </b>Todd<br>
<b>Sent:</b> Wednesday, 24 November 2010 16:55<br>
<b>To:</b> NZ Borland Developers Group - Delphi List<br>
<b>Subject:</b> [DUG] How's this for inconsistent<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The Delphi developer who implemented
TInterfacedObject obviously considered the case when an interface
reference is grabbed during construction......<br>
<br>
<span style="font-size: 10pt;">// Set an implicit refcount so that
refcounting<br>
// during construction won't destroy the object.<br>
class function TInterfacedObject.NewInstance: TObject;<br>
begin<br>
Result := inherited NewInstance;<br>
TInterfacedObject(Result).FRefCount := 1;<br>
end;<br>
<br>
procedure TInterfacedObject.AfterConstruction;<br>
begin<br>
// Release the constructor's implicit refcount<br>
InterlockedDecrement(FRefCount);<br>
end;</span><br>
<br>
but didn't consider applying the same logic during destruction. So
grabing an interface reference during destruction causes all hell to
break loose, as the _Release method tries to free the object again and
again recursively.<br>
<br>
<span style="font-size: 10pt;">procedure
TInterfacedObject.BeforeDestruction;<br>
begin<br>
if RefCount <> 0 then<br>
Error(reInvalidPtr);<br>
end;</span><br>
<br>
<span style="font-size: 10pt;">function TInterfacedObject._Release:
Integer;<br>
begin<br>
Result := InterlockedDecrement(FRefCount);<br>
if Result = 0 then<br>
Destroy;<br>
end;</span><br>
<br>
Todd.<o:p></o:p></p>
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