IMP self _cmd

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The only way to circumvent dynamic binding is to get the address of a method and call it directly as if it were a function. This might be appropriate on the rare occasions when a particular method will be performed many times in succession and you want to avoid the overhead of messaging each time the method is performed.

With a method defined in the NSObject class, methodForSelector:, you can ask for a pointer to the procedure that implements a method, then use the pointer to call the procedure. The pointer that methodForSelector: returns must be carefully cast to the proper function type. Both return and argument types should be included in the cast.

The example below shows how the procedure that implements the setFilled: method might be called:

void (*setter)(id, SEL, BOOL);

int i;

setter = (void (*)(id, SEL, BOOL))[target

    methodForSelector:@selector(setFilled:)];

for ( i = 0 ; i < 1000 ; i++ )

    setter(targetList[i], @selector(setFilled:), YES);

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.

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.

Note that methodForSelector: is provided by the Cocoa runtime system; it’s not a feature of the Objective-C language itself.

IMP self _cmd

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原文地址：http://www.cnblogs.com/feng9exe/p/7131445.html