property's synthesized getter follows Cocoa naming convention for returning 'owned' objects

You own any object you create

You create an object using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy” (for example, alloc, newObject, or mutableCopy).

一定要注意命名规范，不能以alloc，new，copy，mutableCopy 作为开头命名，比如：newPassword,