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Using Hidden Arguments

When the objc_msgSend finds the procedure that implements a method, it calls the procedure and passes it all the arguments in the message. It also passes the procedure two hidden arguments:

• The receiving object

• The selector for the method

These arguments give every method implementation explicit information about the two halves of the message expression that invoked it. They’re said to be “hidden” because they aren’t declared in the source code that defines the method. They’re inserted into the implementation when the code is compiled.

Although these arguments aren’t explicitly declared, source code can still refer to them (just as it can refer to the receiving object’s instance variables). A method refers to the receiving object as self, and to its own selector as _cmd. In the example below, _cmd refers to the selector for the strange method and self to the object that receives a strange message.

- strange

{

    id  target = getTheReceiver();

    SEL method = getTheMethod();

    if ( target == self || method == _cmd )

        return nil;

    return [target performSelector:method];

}

self is the more useful of the two arguments. It is, in fact, the way the receiving object’s instance variables are made available to the method definition.

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