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OliverLetterer/tollfree.m

Forked from Catfish-Man/tollfree.m
Created February 7, 2016 08:14
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Revisions

  1. @Catfish-Man Catfish-Man created this gist Feb 7, 2016.
    31 changes: 31 additions & 0 deletions tollfree.m
    View file
    Original file line number Diff line number Diff line change
    @@ -0,0 +1,31 @@
    //Given this:
    NSArray *objects = @[@1, @2, @3]

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