Here are a couple of Stack Overflow answers that illustrate how you do it in Objective-C:

http://stackoverflow.com/a/2573949

http://stackoverflow.com/a/18689338

If you're interested in the nitty-gritty of how objc_msgSend works, bbum from Apple wrote up a great low-level walkthrough a few years back: http://www.friday.com/bbum/2009/12/18/objc_msgsend-part-1-th...