The current deployment target does not support automated __weak references
iOS developers will face this error when they start developing with ARC enabled for the first time.
This happens because the least version required for using weak keyword is not set in the deployment target of the Target of the project. ARC is available in iOS 4.0 but weak references are not supported in iOS 4.0 , but from iOS 5.0 weak references are also supported.
1. If you do intend to support the versions below 5.0 from iOS 4.0, then use unsafe_unretained instead of weak
2. If you do not want to support the versions below 5.0, then change the Deployment Target in your targets to 5.0 or above.
In addition to this there are several classes which can not use weak references: