> On Apr 21, 2015, at 12:08 PM, Jens Alfke <[email protected]> wrote: > > Under ARC, what’s the recommended/supported way to message an object, given > the object reference and a selector? I’m implementing an instance of the > target-action pattern that’s all over Cocoa, but running into roadblocks: > > * If I try to use -[NSObject performSelector:withObject:] I get a compiler > error “PerformSelector may cause a leak because its selector is unknown”. > > I’m aware that _in general_, dynamic dispatch can cause bad side effects > under ARC. But all I want to do is send a message that takes an NSObject > parameter and returns void, which is safe.
It is in fact *not* safe to use -performSelector: to do that. ARC may attempt to perform the caller's side of the return-autoreleased optimization on the result of -performSelector:. That goes poorly if the return value is uninitialized. > * If I try to call objc_msgSend directly, it doesn’t work because > <objc/message.h> has purposely obfuscated the real signature of the function > to hide its parameters. (Unless I redefine OBJC_OLD_DISPATCH_PROTOTYPES as 1, > but even if I do that it doesn’t take effect; I’m suspecting message.h gets > imported during my prefix header or something.) You can cast objc_msgSend to the appropriate function pointer type and call that. The new declaration in objc/message.h is intended to force you to add the cast. Adapting your example: // [self.delegate performSelector: action withObject: newWidget]; void (*msgSend_action)(id, SEL, Widget*) = (typeof(msgSend_action))objc_msgSend; msgSend_action(self.delegate, action, newWidget); -- Greg Parker [email protected] Runtime Wrangler _______________________________________________ Do not post admin requests to the list. They will be ignored. Objc-language mailing list ([email protected]) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/objc-language/archive%40mail-archive.com This email sent to [email protected]
