tollfree.m

//Given this:
NSArray *objects = @[@1, @2, @3]

//These are equivalent in behavior
NSNumber *first = [objects objectAtIndex:0]
NSNumber *second = (NSNumber *)CFArrayGetValueAtIndex(objects, 0)

//But is the code that runs the same? Not so much… in the first one we do…
objc_msgSend(objects, <selector reference>, 0)
-> http://sealiesoftware.com/msg/x86-mavericks.html
-> -[__NSArrayI objectAtIndex:]
-> check if index is in bounds
-> return _list[0]

//In the second one we do…
CFArrayGetValueAtIndex()
-> CFArrayGetTypeID()
-> dispatch_once()
-> grab __kCFArrayTypeID
-> CF_IS_OBJC() (see http://opensource.apple.com/source/CF/CF-744.19/CFInternal.h)
-> check if objects is a tagged pointer (if (objects & 1))
-> check if objects->isa is 0
-> check if objects->isa is the constant string class
-> check if __kCFArrayTypeID is small enough to be in the bridged classes array
-> grab the class for __kCFArrayTypeID from the bridged classes array
-> check if the class we grabbed is the same as objects->isa
-> objc_msgSend(objects, <selector reference>, 0)
-> http://sealiesoftware.com/msg/x86-mavericks.html
-> -[__NSArrayI objectAtIndex:]
-> check if index is in bounds
-> return _list[0]