Inheritance: when multiple classes share the same instance variables and methods, you must consider using inheritance.
Use of inheritance: write multiple identical instance variables and methods into a common parent class
Error cause: the current system version is earlier than the system version of the project to be created
Change this to a version lower than your current version.
Error: duplicate instance variables
Solution: delete instance variables
Loop import problems
Unknown type name "woman"
Unrecognized type woman
Import # The import header file will assign a value to the content in the header file. When woman is used, the class is not defined, so it cannot be identified. A typical cyclic Import
This class cannot be used as a class. in the H file, use @ class woman; to declare woman as a string and then use it. However, it does not have content in the class. import # import "woman" to the M file"
Error: Repeated Import
Solution: Change the header file (. h file) to @ Class "woman", and import the # import "woman" file in the. M file (implementation file ).
Inheritance error (OC)
