Can I add the UIBannerViewDelegate protocol to my UIViewController subclass while remaining compatible with pre-iOS 4 devices?
This is NOT a duplicate, the question is specifically related to the delegate protocol.
If you weak-link the iAd framework, you will encounter no issues with a controller class that conforms to the ADBannerViewDelegate protocol. You will, of course, need to add the ADBannerView programmatically (if it exists on the running OS) or otherwise load a different Nib file for 3.x vs. 4.x.
Even though there is no ADBannerViewDelegate protocol in iPhone OS 3.x, I just tested and no errors are thrown for missing symbols if a class that conforms to it is loaded in that older OS.
来源：https://stackoverflow.com/questions/3128457/are-apps-using-iad-compatible-with-older-ios