1. C++ Inheritance and a
QuickTime™
decompressor
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Gordon College
CPS212

2. Basics
 OO-programming can be defined as a
combination of Abstract Data Types (ADTs)
with Inheritance and Dynamic Binding.
 Encapsulation, inheritance and polymorphism
 Each practice handles a different aspect of
system composition:
 Encapsulation can be seen as a 2D component - public and
private interface
 Inheritance adds an additional dimension - the ADT picks up
the characteristics of another component.
 Polymorphism adds to inheritance - postponing
implementation decisions until later (perhaps even run-time).

3. Data Abstraction vs. Inheritance

4. Basics
 Recall that inheritance is a means of
specifying hierarchical relationships between
types
 C++ classes can inherit both data and
function members from other (parent) classes
 Terminology: "the child (derived or subclass)
type inherits (or is derived from) the parent
(base or superclass) type."

5. The derived type is just the base type
plus:
 Added specializations
 Change implementation
without changing the
base class interface
 Added Generalizations
/Extensions
 new operations and/or
data

6. What a derived class inherits
 Every data member defined in the parent
class (although such members may not
always be accessible in the derived class!)
 Every ordinary member function of the parent
class (although such members may not
always be accessible in the derived class!)

7. What a derived class doesn't inherit
 The base class's constructors and
destructor
 The base class's assignment operator
 The base class's friends

8. What a derived class can add
 New data members
 New member functions (also overwrite existing
ones)
 New constructors and destructor
 New friends

9. When a derived-class object is created & destroyed
 Space is allocated (on the stack or the heap) for the full object
(that is, enough space to store the data members inherited from
the base class plus the data members defined in the derived
class itself)
 The base class's constructor is called to initialize the data
members inherited from the base class
 The derived class's constructor is then called to initialize the
data members added in the derived class
 The derived-class object is then usable
 When the object is destroyed (goes out of scope or is deleted)
the derived class's destructor is called on the object first
 Then the base class's destructor is called on the object
 Finally the allocated space for the full object is reclaimed

10. Inheritance in C++
 The class header is modified to allow a
derivation list consisting of base classes
(C++ allows multiple inheritance)
class Foo { };
class Bar : public Foo { };
class More : public Foo, public Bar { };

11. Key Properties of C++ Inheritance
 The “is-a” relationship is maintained
 A pointer to the base type may point to a derived
type object
 The above relationship combined with
dynamic binding - promotes a type-secure,
polymorphic style of programming
 The programmer need not know the actual type of
an object at compile-time (dynamic-binding)

12. Simple Base Class (Screen class) Derived from Screen (Window
class)

13. Derived from Window (Menu
class)
Inheritance Hierarchy
Multiple Levels of Derivation
A pointer to a derived class can
be assigned to a pointer of any of
its public base classes without
requiring an explicit cast: Menu m;
Window &w = m;
Screen *ps1 = &w;
Screen *ps2 = &m;

14. Public vs private inheritance
 The public keyword in the inheritance syntax
means that publicly accessible members
inherited from the base class stay publicly
accessible in the derived class
 But sometimes its preferable to inherit the
public members of a parent in such a way
that they become private in the child

15. Private inheritance
 Public members of base class become
private members of derived class
 Public and protected members are only
available to derived-class member
functions - not to a derived object.

16. Protected inheritance
 private members of the base class are not
accessible in the derived class (to preserve
encapsulation)
 Protected qualification allows encapsulated
data members which are not publicly
accessible to be accessible by derived
classes
 ("Protected" members are not accessible
from outside the class, except in derived
classes)

17. Initializer lists
 Derived class constructor automatically
calls the "no argument" base class
constructor(s)
 But what if a base class has a non-void
constructor which needs to be called?
 specify which base-class constructor gets
called, as part of the definition of the
derived-class constructor

18. Initializer lists
class Coefficient
{ class StatusCoefficient : public Coefficient
public: {
Coefficient(void) public:
StatusCoefficient(void)
{ myValue = 0; myAccesses = 0; } { myStatus = OverStatus; }
Coefficient(double initval) StatusCoefficient(double initval,
{ myValue = initval; myAccesses = 0; Status initStatus)
} : Coefficient(initval)
{ myStatus = initStatus; }
// ETC. // ETC.
}; };

19. Using Initializer lists to initialize
values
 Initializer lists can also be used to initialize
class members with specific value (instead of
assigning to them in the body of the constructor)
class First
{
public:
First() : _foo( "initialize foo first" ), _bar( "then bar" ) { }
private:
string _foo;
string _bar;
};

20. Polymorphism
 Meaning: some code or operations or objects
behave differently in different contexts
Example: the + operation behaves differently depending on the
operands (overloading)
 member function with the same name can me
implemented in different classes
 Use :: to refer to a function in the base class with the same
name - baseCL::function()
 appropriate version of function is determined
at runtime – dynamically
 virtual member functions used to implement
polymorphism

21. Polymorphism
 Dynamic binding:
 At runtime the C++ program selects which
member function to execute - if the function
is virtual and exists at the different levels
within a inheritance hierarchy
 Contrasts with Static Binding
emp.displayEmployeeInfo();

22. Resources
 Data Structures with C++, William Ford and William
Topp
 Single and Multiple Inheritance in C++, Douglas
Schmidt