Objective C: Two implicit parameters passed to every method called by an object

Question

What are the two implicit parameters passed to every method by an object in Objective-c? Is it _cmd and id?

Answer 1

That is a good question, the short answer is: yes, two parameters are being implicitly passed. self of type id, and _cmd of type SEL.

to better understand it, here is what happens behind the method call.

@interface Item:NSObject
-(void)dummyMethod;
@end
@implementation Item
-(void)dummyMethod{
     NSLog(@"dummyPrint");
}
@end

a normal method call would be like:

Item* objectOne = [[Item alloc]init;
[objectOne dummyMethod];

that call gets translated/complied to:

objc_msgSend(objectOne,@selector(dummyMethod)); //since the dummy method takes no parameters and returns void

to try it/use it your self. 1. #import<objc/message.h 2. cast the objc_msgSend method to an appropriate function pointer type before being used, like that.

void (*objc_msgSendPointer)(id self, SEL _cmd) = (void*)objc_msgSend;
objc_msgSendPointer(one, @selector(dummyMethod)); 

here is the function declaration

 id objc_msgSend(id self, SEL op, ...);

take a look at this documentation for further info.. objc_msgSend. take a look also at this topic using objc_msgSend to call a Objective C function with named arguments for more info regarding other methods, specially Ken's answer.

I hope that was helpful!