garbage collection in objective-c

default, gc is not enabled.

how to enable it:
in Xcode, in the target, in the build panel,
under gcc-4.0,
in Objective-c garbage collection, choose supported( -fobjc-gc)
This means your application can uses gc & retain/release method
If choose required( -fobjc-gc-only) , your application can only use gc
required mode is more recommended.