1. Inheritance in C++
CS-1030
Dr. Mark L. Hornick

2. Inheritance Syntax Example Program…
class Pet { protected:        string name;   public: int speak();     int eat(); }
class Dog : public Pet {// Java uses extends private:     bool hasFleas; public:     int speak(); // override }
class Cat : public Pet { private:     bool isDeclawed; public:     int purr(); }
Example Program…
CS-1030
Dr. Mark L. Hornick

3. C++ Inheritance rules
Protected and public data and methods in the base class are inherited by the derived class
With some exceptions:
Constructors
Destructors
operator=
Are accessible in the derived class.
CS-1030
Dr. Mark L. Hornick

4. Public/protected/private rules
class Dog : <inheritance specifier> Pet { private:     bool hasFleas; public:     int speak(); }
<Inheritance specifier> is one of the following
public
All public and protected members of Pet will be considered public or protected within Dog
Still inherits Pet’s private members, but they’re not accessible from Dog methods
Also inherits Pet’s protected members, and they’re accessible from Dog methods
protected
All public and protected members of Pet will be considered protected within Dog
What was public in Pet is protected within Dog
private
All public and protected members of Pet will be considered private within Dog
What was public or protected in Pet is private within Dog
CS-1030
Dr. Mark L. Hornick

5. C++ Construction Order
Base class data members are constructed in order of declaration
Base class constructor is called
Derived class data members are constructed in order
Derived class constructor is executed
This is much different from Java, where super() has to be invoked from the derived class constructor to call the base class constructor
CS-1030
Dr. Mark L. Hornick
Example Program…

6. Destruction order Destructor order (reverse of ctor order)
Derived class destructor is executed
Derived class data members are destroyed
The base class destructor is called
Base class data members are destroyed
CS-1030
Dr. Mark L. Hornick

7. Method overrides Example Program…
class Pet { protected:        string name;   public:     int eat(); }
class Dog : public Pet { private:     bool hasFleas; public: int eat(); // override replaces the inherited version
    int speak(); }
class Cat : public Pet { private:     bool isDeclawed; public:     int purr(); }
Example Program…
CS-1030
Dr. Mark L. Hornick

8. Functional binding Dog::speak() Pet::speak() Pet::speak()
void main() {
Dog spot;
spot.speak();
Pet* pet = &spot;
pet->speak();
Pet aPet;
apet.speak(); }
Dog::speak()
Pet::speak()
Pet::speak()
CS-1030
Dr. Mark L. Hornick

9. Functional Binding Variant behavior
Derived object – calls derived version
Base pointer – calls base version
This is inconsistent and not like Java
This is called Early Binding
Functional version is chosen at compile time
CS-1030
Dr. Mark L. Hornick

10. Polymorphism The solution to variant behavior
Version is chosen at run time
Delay the choice until we know the real type
Even when upcast!
CS-1030
Dr. Mark L. Hornick

11. virtual Functions Declaration in the base class Implementation
Forces use of late binding for a function
Keyword: “virtual”
Implementation
Compiler inserts code to ask the object before calling a virtual function
CS-1030
Dr. Mark L. Hornick

12. virtual is optional here
virtual Example
class Pet {
public:
virtual void speak(); … };
class Dog : public Pet {
void speak();
virtual is optional here
CS-1030
Dr. Mark L. Hornick

13. Calling virtual Functions
void main() {
Dog spot;
spot.speak(); //virtual
Pet* pet = &spot;
pet->speak();
Pet aPet;
apet.speak(); }
Dog::speak()
Pet::speak()
CS-1030
Dr. Mark L. Hornick

14. General Overriding Rules
If you want variant behavior …
Don’t declare base class functions virtual
Don’t override in derived classes
invariant over specialization
Create virtual base class function(s)
Overrides expected, but not required in derived classes
CS-1030
Dr. Mark L. Hornick

15. Abstract Base Classes Remember Java Abstract classes and Interfaces?
Describes a set form/template
Never intend to create an object
Must have classes that implement the interface or abstract methods
Compiler will not allow abstact classes/interfaces to be created
References are OK
(In C++, Pointers are OK too)
CS-1030
Dr. Mark L. Hornick

16. The C++ equivalent of Java abstract classes and interfaces
Done in C++ by declaring pure virtual methods
Language syntax
virtual void speak()=0; // “0=pure virtual”
Not all methods have to be declared pure virtual
But having even one pure virtual method makes the entire class abstract
CS-1030
Dr. Mark L. Hornick

17. Upcasting Inheritance embodies “is a” Derived objects are base objects
A derived object is a base object
Derived objects are base objects
Dog spot;
Pet* pPet = &spot;
Always use pointers or references to do this
Otherwise the extra is “sliced” off:
Pet aPet = spot; // ouch! Pet.speak(); // calls Pet::speak()
CS-1030
Dr. Mark L. Hornick

18. Object Slicing Upcasting must be done carefully
Always use references or pointers
So you have the original object
Base copy constructors and operator=
Cannot fully copy a derived object
Slices off the “new” and overridden stuff
CS-1030
Dr. Mark L. Hornick

19. virtual Destructors Allows upcast objects to properly destroy
Forces use of the most derived destructor
All the destructors are still called
Most derived  base (in order)
ALWAYS make base class destructors virtual
Good habit
Pure virtual destructors must still be defined
CS-1030
Dr. Mark L. Hornick

20. CS-183
4/21/2017
Polymorphic methods
Polymorphism is multiple versions of an object that:
Change their behavior based on context
Adapt to the needs of the situation
Primary example – upcast objects accessed via base class pointers
Secondary example
Operators: + addition, concatenation (later)
Functions: Multiple constructors
CS-1030
Dr. Mark L. Hornick
Dr. Mark L. Hornick

21. Polymorphic method Example
int Max(int a, int b)
{ return (a<b) ? b : a;}
double Max(double a, double b)
Complex Max(const Complex& a,
const Complex& b)
All the functions are the same!
CS-1030
Dr. Mark L. Hornick