On Tue, May 12, 2009 at 6:19 PM, Greg Parker <gpar...@apple.com> wrote: > On May 12, 2009, at 6:43 AM, Michael Ash wrote: >> >> Use -methodForSelector: to get the IMP for a nonexistent method. This >> IMP will be a function pointer straight to the runtime's forwarding >> machinery. Then install that IMP as the implementation for your >> overridden methods. Callers will go straight into the forwarding >> machinery rather than into your overridden methods, which will then >> invoke -forwardInvocation: as usual and you can then do as you like. > > This trick has a few platform- and architecture-specific pitfalls. Here's > the safest way to do it: > > First: don't call that IMP directly. On some platforms it breaks C function > rules, because it assumes it's being called from objc_msgSend. > > Second: don't do this for methods that return a struct. There may be two > distinct forwarding IMPs, with -methodForSelector: returning the non-struct > version. (By "struct return" I mean "method is called via > objc_msgSend_stret()", which differs from "returns C struct" on some > architectures.) > > Someday this will all work seamlessly, but currently all platforms and > architectures suffer from at least one of the above.
Interesting stuff, thanks. I have a couple of questions, though: 1) Won't the inability to call the IMP directly make -methodForSelector: almost completely useless? In theory, *any* method could be implemented using forwarding. You simply can't know in advance. If such an IMP can't be used in confidence for a forwarded method, then -methodForSelector: can't be used to cache an IMP for any method you didn't write yourself. Obviously it could still be used for this case, where the returned IMP is only used for building a class's method tables, but usually the IMP is obtained with the intention of calling it directly. 2) Am I correct in assuming that if you grab the method signature and then conditionally call either class_getMethodImplementation or class_getMethodImplementation_stret that you avoid the problem of getting the non-struct version of the forwarder for struct-returning methods? Mike _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
