Inheritance in C++
CS-1030
Dr. Mark L. Hornick
1
Inheritance Syntax



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
2
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
3
Public/protected/private rules
class Dog : <inheritance specifier> Pet {
private:
bool hasFleas;
public:
int speak();
}
<Inheritance specifier> is one of the following

public




protected




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
All public and protected members of Pet will be considered protected within Dog
What was public in Pet is protected within Dog
Still inherits Pet’s private members, but they’re not accessible from Dog methods
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
Still inherits Pet’s private members, but they’re not accessible from Dog methods
CS-1030
Dr. Mark L. Hornick
4
C++ Construction Order
1.
2.
3.
4.

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…
5
Destruction order

Destructor order (reverse of ctor order)
1.
2.
3.
4.
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
6
Method overrides



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
7
Functional binding
void main() {
Dog spot;
spot.speak();
Dog::speak()
Pet* pet = &spot;
Pet::speak()
pet->speak();
Pet aPet;
Pet::speak()
apet.speak();
}
CS-1030
Dr. Mark L. Hornick
8
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
9
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
10
virtual Functions

Declaration in the base class



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
11
virtual Example
class Pet {
public:
virtual void speak();
…
};
class Dog : public Pet {
public:
void speak();
…
};
virtual is optional here
CS-1030
Dr. Mark L. Hornick
12
Calling virtual Functions
void main() {
Dog spot;
spot.speak(); //virtual
Pet* pet = &spot;
Dog::speak()
Dog::speak()
pet->speak();
Pet aPet;
Pet::speak()
apet.speak();
}
CS-1030
Dr. Mark L. Hornick
13
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
14
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
15
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
16
Upcasting

Inheritance embodies “is a”


Derived objects are base objects



A derived object is a base object
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
17
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
18
virtual Destructors

Allows upcast objects to properly destroy



ALWAYS make base class destructors virtual


Forces use of the most derived destructor
All the destructors are still called
 Most derived  base (in order)
Good habit
Pure virtual destructors must still be defined
CS-1030
Dr. Mark L. Hornick
19
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
20
Polymorphic method Example
int Max(int a, int b)
{ return (a<b) ? b : a;}
double Max(double a, double b)
{ return (a<b) ? b : a;}
Complex Max(const Complex& a,
const Complex& b)
{ return (a<b) ? b : a;}
All the functions are the same!
CS-1030
Dr. Mark L. Hornick
21