CODEWITHSUNDEEP
objective-cioscocoaobjective-c-runtime
Oscar
Oscar

Reputation: 1035

How does objective-c handle method resolution at run-time?

I've read here recently that an objective-c object is stored on the heap as a struct. The struct contains the objects iVars, inherited iVars, and the isa pointer.

I'm trying to figure out when I send a message to this object, how does the run-time figure out the code to run?

I know there is a class object for each class. Is this also stored on the heap?

I think the way it works is that the run-time gets the isa pointer from the struct, uses this to call the message on the class object. Is this correct?

Upvotes: 3

Views: 896

Answers (1)

bbum
bbum

Reputation: 162712

In short, every Objective-C instance has a pointer to its class. The class contains an inventory of metadata that includes all the methods that the class implements. When a message is sent to an object -- when a method is called -- the runtime uses the pointer to the class to lookup the method by name and call it, if it can be found. If it isn't found, the runtime looks to the superclass (which is a part of each class's metadata) on up the inheritance chain to NSObject. If the method ultimately can't be found, the runtime goes through a series of last ditch efforts to see if their is an alternative handler and eventually raises an exception, if not.

If you want more detail than that, I wrote up a multipart tour of exactly how Objective-C method dispatch works. It is slightly out of date -- doesn't deal with ARC, tagged pointers or blocks-as-IMP -- but still fully applicable.

Yes, classes are stored in the heap, but generally not in malloc()d memory. Classes are generally loaded as read-only, shared, memory. That is, there will be only one copy of the NSString class in memory for all applications running on the system. You can dynamically create classes on the fly and these will be in the regular heap, but it is atypical.

Upvotes: 11

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