0.1 Basic knowledge

• Basic relationships: Inheritance, Dependency, Association, Composition, Aggregation.
  
• Composition is one part of another class, it has the same life time and exclusive ownership such as People and Heart class.
• Aggregation is a specialized form of Association where all objects have their own life time, and hasownership and one class can’t belongs to another class simultaneously. Such as Teacher and School.
• Association is a relationship where all objects have their own life time and there is no ownership. Let’s take an example of Teacher and Student. Both can create and delete independently.
• Dependency is weaker relationship. And Class just use another class as method parameter or method return. No member in another Class. Such as Teacher and Computer. In GOF, It use dotted line to represent A class create B class.
• Basic Ideas:
  1. Don’t overdesign, Don’t use design pattern just because it is here. Just know some basic design pattern, and follow your real problem, make design fit you problem first, If something changes in the future, you can refactory it later.
  2. Main question in design is: How many classes do you need? Interface of each classes? relationship of Classes(is-a, has-a, and association. )
  3. When you have all these classes from your practical problem(application), then you need to think three questions. 1) If it’s high cohension 2) if it’s loose coupling 3) if there is future change.
  4. Then you should use design pattern to design your class again.
  5. Inheritance has three level knowledges.
     1. The basic knowledge, manager is a person, apple is a fruit, and bla bla bla.
     2. Reused code, pure virtual, virtual, and non virtual, Such as introduced in inheritance section.
     3. Isolate change between Client and Implement. That is the highest level in design pattern.