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Free bug?

Posted on 2002-04-04
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Last Modified: 2010-04-04
I had this very strange bug the last day. It's probally easiest if I show you the bug in code.
I have a radiobutton on a form.

formcreate
var
  r: tregistry;
begin
  r.free; //if I don't comment this out rad is destroyed and is nil
  if radiobutton.checked then ...
 
end;
 

is this a bug in delphi ? I thought that a call to free would always be safe even when it's wrong.
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Question by:bnz
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by:DragonSlayer
ID: 6920210
a safer alternative would be to use

FreeAndNil
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robert_marquardt earned 100 total points
ID: 6920232
This is wrong code.
A local variable resides on the stack and is NOT initialized to nil.
r.Free only checks for nil and does not call the destructor then. This is a bit of compiler magic.
Since r contains a random value the destructor is called and tries to deallocate data which has never been allocated.
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LVL 44

Expert Comment

by:CrazyOne
ID: 6922392
Well I suspect you already now this but before an object can be freed it has to exist first. In this case the object was never created so it can't be freed.

r := TRegistry.Create;


The Crazy One
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