A: 

What is the call stack on EXC_BAD_ACCESS?

Also, you can just use NSString to get a formatted string, no need to use NSMutableString or cast it...

Brian
for the EXC_BAD_ACCESS, there is no stacktrace on console. It just exits saying "Program received signal: “EXC_BAD_ACCESS”. and about the cast and NSMutableString you are right, i can remove them. but removing them doesnt help to solve the problem.
tak
+1  A: 

I think you want to put in a symbolic breakpoint for "objc_exception_throw" so you can see what happens before the crash. Just go to the Run menu -> Manage Breakpoints -> Add Symbolic Breakpoint, and type in objc_exception_throw.

BTW, you are probably calling an object which is already deleted.

Glenn Howes
A: 
tak