Word of Runtime 1

This chapter describes how the message expressions are converted into objc_msgSend function calls, and how you can refer to methods by name.

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It then explains how you can take advantage of objc_msgSend, and how—if you need to—you can circumvent dynamic binding.

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In Objective-C, messages aren’t bound to method implementations until runtime.

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This function takes the receiver and the name of the method mentioned in the message—that is, the method selector—as its two principal parameters.

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Any arguments passed in the message are also handed to objc_msgSend

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It first finds the procedure (method implementation) that the selector refers to

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Since the same method can be implemented differently by separate classes, the precise procedure that it finds depends on the class of the receiver

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The key to messaging lies in the structures that the compiler builds for each class and object

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Every class structure includes these two essential elements

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This table has entries that associate method selectors with the class-specific addresses of the methods they identify

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This pointer, called isa, gives the object access to its class and, through the class, to all the classes it inherits from

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These elements of class and object structure are illustrated in Figure 3-1

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Successive failures cause objc_msgSend to climb the class hierarchy until it reaches the NSObject class

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Caches grow dynamically to accommodate new messages as the program runs

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These arguments give every method implementation explicit information about the two halves of the message expression that invoked it

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The pointer that methodForSelector: returns must be carefully cast to the proper function type

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The first two arguments passed to the procedure are the receiving object (self) and the method selector (_cmd). These arguments are hidden in method syntax but must be made explicit when the method is called as a function

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Using methodForSelector: to circumvent dynamic binding saves most of the time required by messaging. However, the savings will be significant only where a particular message is repeated many times, as in the for loop shown above

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