Objective-C questions

The IMP is an IMplementation Pointer, which is basically the hook that determines what gets run upon receipt of a message (like foo length). You don't usually need them unless you're getting down and dirty; it's usually easier to deal with selectors.

6.1 What is an IMP ?

 It's the C type of a method implementation pointer, a function 

pointer to the function that implements an Objective-C method. It is defined to return id and takes two hidden arguments, self and _cmd :

 typedef id (*IMP)(id self,SEL _cmd,...); 6.2 How do I get an IMP given a SEL ? This can be done by sending a methodFor: message : IMP myImp = [myObject methodFor:mySel]; 6.3 How do I send a message given an IMP ? By dereferencing the function pointer. The following are all equivalent : [myObject myMessage]; or IMP myImp = [myObject methodFor:@selector(myMessage)]; myImp(myObject,@selector(myMessage)); or [myObject perform:@selector(myMessage)]; 

From section 6.1 of the Objective C FAQ.

As for the msgSend, that's the way you invoke a remote message on another object; objc_msgSend(foo,@selector(bar)) is roughly the same as [foo bar]. But these are all low-level implementation details; you rarely (if ever) need to use the expanded calls for Objective C code, since you can do @selector to get hold of a method and performSelector: to invoke it on any object.