Objective c key path operators @avg,@max

arr = [[NSMutableArray alloc]init];
[开发者_开发问答arr addObject:[NSNumber numberWithInt:4]];
[arr addObject:[NSNumber numberWithInt:45]];
[arr addObject:[NSNumber numberWithInt:23]];
[arr addObject:[NSNumber numberWithInt:12]];
NSLog(@"The avg = %@", [arr valueForKeyPath:@"@avg.intValue"]);

This code works fine, but why? valueForKeyPath:@"@avg.intValue" is requesting (int) from each NSNumber, but we are outputting a %@ string in the log. If i try to output a decimal %d i get a number that possibly is a pointer to something. Can somebody explain why the integers become NSNumbers when i call the @avg operator?

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valueForKeyPath: must return an object (its return type is id), so you get an NSNumber or NSValue (I'm not sure which; it depends on whether the intValue gives an int or NSNumber, and (if the former) how the result is bundled back into an object), despite the intValue. It's the same reason you can't store non-objects in collections.

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The @avg operator returns an NSNumber instance. If you use %d, you will print the memory address of the NSNumber instance. When you use %@, on the other hand, the NSNumber instance is sent a description message, and the resulting NSString is printed.

See Set and Array Operators in the Key-Value Coding Programming Guide for more information on @avg and other operators.