On 27.09.2014 17:00, Constantine Yannakopoulos wrote:
On Sat, Sep 27, 2014 at 4:36 PM, Sven Barth <pascaldra...@googlemail.com <mailto:pascaldra...@googlemail.com>> wrote:There are however some nasty problems inside constructors and destructors, because "Self" is reference counted as well (and should be after all as we don't want the instance to be destroyed behind our backs suddenly). The parenthesis can happen in Delphi interfaced objects as well. It is perfectly possible to lose the object from under your feet if the code of one of its methods leads to references reaching zero (e.g. removal from a strong-ref'd list). If such a possibility exists the developer needs to manually _AddRef/_Release (e.g. store Self in a local interface variable) if they need to access Self after that point, and that is perfectly OK. The only way to avoid that would be to emit code at a compiler-generated implicit try-finally block to _AddRef/_Release, which is a bad idea IMHO as it adds overhead when it won't be required for 99.99% of cases.
That's what the compiler currently does though, because it treats Self just a like a normal reference counted variable... Will need to check what I need to do do switch that of ;)
As for destructors, the way refcounting is implemented in Delphi it prohibits performing _Addref/_Release (e.g. pass an interface variable of Self to an external method) in them because this will cause infinite recursion.
Huh? Delphi really prohibits such things inside the destructor? Would you mind to share an example (possibly including error messages), please?
This problem has an easy solution: Override BeforeDestruction and set RefCount to a positive value, 1 or MaxInt div 2 (not -1, consider the sequence _AddRef[0], _AddRef[1], _Release[0-bang], _Release[AV]) after calling inherited (which checks for 0). Since the object destruction process has already started due to RefCount reaching zero, refcounting is useless at this point and can be safely disabled. You could consider doing this in your implementation.
I've done a different approach already: the decrease helper sets refcount to -1 directly before it calls the destructor and both the increase and the decrease helper check for -1. Considering that it's currently only a proof of concept I can live with the performance impact that comparison might have.
Regards, Sven _______________________________________________ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
