Introduction to Inheritance

Objectives

1. Explore the basics of inheritance in c++

Notes

• This is from the book, but also from Gregorie and others.
• This is just an introduction, light on restrictions and such.
  • We will be back and formalize things more later.
• In the real world, we frequently look for "relationships" between things.
  • Two in object oriented design are is-a and has-a
  • A student is a person.
  • A student has a grade point average
• Let's examine the student person relationship a bit
  • For simplicity, a person has a name and a role.
  • On top of that a student has a GPA.
• If we were writing a program in our current style, we could
  • Implement a person class and a student class separately
  • Implement a student class and make everyone a student
• The problem with the first is that they will have the same code, but it will not be shared.
• The problem with he second is that people will have extra properties (GPA)
• Inheritance allows us to solve this problem.
  • A person has properties that ALL people will have, therefore they will form a base class.
  • A student has all of the properties of a person, with some added, so they will form a derived class.
  • The student will inherit all of the properties of the person.
  • The student can override some of the properties if they wish, replacing them with their own implementation.
• To do this
  • Build a base class that contains all the data/methods of ALL things
  • Build (a/some) derived class(es) that modify and add specific things to those classes.
  • This can be done further (ie graduate student derived from student, undergrad student derived from student)
• Syntax:

  • class derivedClassIdentifier: [public/private/protected] baseClassIdentifier{  
        // class members
    } 

• The derived class has access to all public methods/data of the parent/base class.
  • This depends partially on the public/private/protected choice in the class line
  • And we will explore this more in the future.
• The derived class does not have access to the private methods/data of the parent/base class.
• The derived class, however, contains all of the data of the parent/base class.
• Several new things
  • The keyword virtual says that we expect this function to be replaced in derived classes.
  • The keyword override is optional, but says that this function is overriding some function in a parent/base class.
• Take a look at PersonT.[h,cpp] and people.cpp in the code directory.