Inheritance and Polymorphism in Programming: Stanford CS106X Course, Study notes of Programming Abstractions

Download Inheritance and Polymorphism in Programming: Stanford CS106X Course and more Study notes Programming Abstractions in PDF only on Docsity! Programming Abstractions Cynthia Lee C S 106X Inheritance Topics Inheritance  The basics › Example: Stanford GObject class  Polymorphism › Example: Expression trees (final project) Polymorphism We will keep working with the Employee class: 5  Employees have a name, years worked, salary, vacation, …  Lawyers know how to sue and get paid 2x as much  Programmers know how to write code and get bigger raises each year  (Code is now on lectures page of website.) Polymorphism A pointer of type T can point to any subclass of T. Employee *neha = new Programmer("Neha", 2); Employee *diane = new Lawyer("Diane", "Stanford", 5); Programmer *cynthia = new Programmer("Cynthia", 10);  Why would you do this? › Handy if you want to have a function that works on any Employee, but takes advantage of custom behavior by specific employee type: void doMonthlyPaycheck(Employee *employee) { cout << "You are now $" << employee->salary()/12 << " wealthier!" << endl; } Polymorphism A pointer of type T can point to any subclass of T. Employee *neha = new Programmer("Neha", 2); Employee *diane = new Lawyer("Diane", "Stanford", 5); Programmer *cynthia = new Programmer("Cynthia", 10);  When a member function is called on diane, it behaves as a Lawyer. › diane->salary(); › (This is because all the employee functions are declared virtual.)  You can not call any Lawyer-only members on diane (e.g. sue). › diane->sue(); // will NOT compile!  You can call any Programmer-only members on cynthia (e.g. code). › cynthia->code("Java"); // ok! Rules for “virtual”: pure virtual If a method of a class looks like this:  virtual returntype method() = 0;  then this method is a called “pure virtual” function  and the class is called an “abstract class”  Abstract classes are like Java interfaces  You cannot do “= new Foo();” if Foo is abstract (just like Java interfaces)  ALSO, you cannot do “= new DerivedFoo();” if DerivedFoo extends Foo and DerivedFoo does not implement all the pure virtual methods of Foo class Mammal { public: virtual void makeSound() = 0; string toString() { return “Mammal”; } }; class Cat : public Mammal { public: virtual void makeSound() { cout << “rawr” << endl; } string toString() { return “Cat”; } }; class Siamese : public Cat { public: virtual void makeSound() { cout << “meow” << endl; } string toString() { return “Siamese”; } virtual void scratchCouch() { cout << “scraaaatch” << endl; } }; What is printed? Siamese * s = new Mammal; cout << s->toString(); (A)“Mammal” (B)“Cat” (C)“Siamese” (D) Gives an error (identify compiler or crash) (E) Other/none/more class Mammal { public: virtual void makeSound() = 0; string toString() { return “Mammal”; } }; class Cat : public Mammal { public: virtual void makeSound() { cout << “rawr” << endl; } string toString() { return “Cat”; } }; class Siamese : public Cat { public: virtual void makeSound() { cout << “meow” << endl; } string toString() { return “Siamese”; } virtual void scratchCouch() { cout << “scraaaatch” << endl; } }; What is printed? Siamese * s = new Siamese; cout << s->toString(); (A)“Mammal” (B)“Cat” (C)“Siamese” (D) Gives an error (identify compiler or crash) (E) Other/none/more class Mammal { public: virtual void makeSound() = 0; string toString() { return “Mammal”; } }; class Cat : public Mammal { public: virtual void makeSound() { cout << “rawr” << endl; } string toString() { return “Cat”; } }; class Siamese : public Cat { public: virtual void makeSound() { cout << “meow” << endl; } string toString() { return “Siamese”; } virtual void scratchCouch() { cout << “scraaaatch” << endl; } }; What is printed? Mammal * m = new Siamese; m->scratchCouch(); (A)“Mammal” (B)“Cat” (C)“Siamese” (D) Gives an error (identify compiler or crash) (E) Other/none/more class Mammal { public: virtual void makeSound() = 0; string toString() { return “Mammal”; } }; class Cat : public Mammal { public: virtual void makeSound() { cout << “rawr” << endl; } string toString() { return “Cat”; } }; class Siamese : public Cat { public: virtual void makeSound() { cout << “meow” << endl; } string toString() { return “Siamese”; } virtual void scratchCouch() { cout << “scraaaatch” << endl; } }; What is printed? Cat * c = new Siamese; c->makeSound(); (A)“rawr” (B)“meow” (C)“Siamese” (D) Gives an error (identify compiler or crash) (E) Other/none/more | Stanford 1-2-3 Walkthrough The Expres Stanford University 4 class CompoundExp : public Expression { public: b [Re we ¥f CompoundExp(const std::string& op, const Expression *lhs, const Expression *rhs); /* Prototypes for the virtual methods overridden by this class */ virtual ~CompoundExp(); irtual d eval (EvaluationContext& cortext) const; virtual std::string toStrimgt) const; virtual ExpressionType getType() const; /* Prototypes of methods specific to this class */ std::string getOperator() const; const Expression *getLHS() const; const Expression *getRHS() const; private: std::string op; /* The operator string (+, -, *, /) */ const/fExpression *lhs Jrhs;: /* The left and right subexpression */ Stanford University Expression (base class) Note: you cannot actually create an Expression object These methods are never implemented (note the “= 0”)  “pure virtual” Expression exists solely to provide a base class to others  “abstract” Another Derived cla eExp class DoubleExp : public Expression public: DoubleExp(double value); /* Prototypes for the virtual methods overridden by this class */ double eval(EvaluationContext& context) const; std::string toString() const; ExpressionType getfype() const; /* Prototypes of methods specific to this class */ double getDoubleValue() const; private: double value; /* The value of the constant */ Be Stanford University