Re: [Pyobjc-dev] HelloWorld.py segfaults

[Jack J. <Jac...@cw...>]

On Monday, Jun 30, 2003, at 15:42 Europe/Amsterdam, Chris Ryland wrote:

>>> If you change the code to:
>>>   dgate = AppDelegate.alloc().init()
>>>       NSApp.setDelegate_(dgate)
>>>
>>> instead of 'NSApp.setDelegate_(AppDelegate.alloc().init())' the
>>> example works as expected.
> Uh-oh--can you explain to the uninitiates why these two code sequences 
> aren't identical?

It's a refcounting issue. The variation with the "dgate" helper 
variable will keep the AppDelegate alive at least as long as the 
"dgate" variable exists, which is until the end of the program.

The other variation would be identical if setDelegate_ was normal 
Python code (the refcount of the AppDelegate would be one lower, but 
still greater than zero). But ObjC refcounting works different than 
Python refcounting, and while usually the differences are not 
noticeable there are some exceptions. Apparently delegates are one such 
exceptions: the receiving class (NSApp) does not increment the refcount 
on its delegate. Therefore, when setDelegate_() returns the refcount on 
the AppDelegate will be decremented to zero, and the object will be 
garbage collected.
--
Jack Jansen, <Jac...@cw...>, http://www.cwi.nl/~jack
If I can't dance I don't want to be part of your revolution -- Emma 
Goldman