EXC_BAD_ACCESS when simply casting a pointer in Obj-C

Answer 1

First, if you have a crash, you have a backtrace. Always provide the backtrace with your question (it'll be in the debugger and can be copy/pasted).

As Vojito implied, the most common cause of crashes like these is related to the over-releasing of objects.

In your case, for(;;) and for(... in ...) are not actually exactly the same. The latter is very likely causing the objects within the array to be retained for the duration of iteration or autoreleased upon retrieval (I say "very likely" because I didn't test it -- but it would explain the behavior).

In your code, you are modifying your object graph during iteration with statements like otherBody = contact.bodyB. If any one of those statements happens to cause one of the items in the array being iterated to be released out from under the array, you would see a crash. Similarly, if the modification of the object graph causes either contact.bodyA or contact.bodyB to become a dangling reference, you would see a crash.

All just an educated guess. Post the backtrace and, as Vojito suggested, run under the Allocation instrument in Instruments with zombie detection enabled.

Answer 2

Stack variables, including object references, will not automatically initialize to nil/0/NULL. Try setting otherBody to nil in its initializer:

PhysicsBody *otherBody = nil;

If the debugger is saying that the le assignment is where the exception occurs, it actually may be the statement above it, i.e. [otherBody userData]. Without initializing it, otherBody is a garbage value.