C Inheritance

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C Inheritance

• Gordon College
• CPS212

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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).

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Data Abstraction vs. Inheritance
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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."

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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

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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!)

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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

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What a derived class can add

• New data members
• New member functions (also overwrite existing
  ones)
• New constructors and destructor
• New friends

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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

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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

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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)

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

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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.

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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)

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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

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Initializer lists

• class Coefficient
• public
• Coefficient(void)
• myValue 0 myAccesses 0
• Coefficient(double initval)
• myValue initval myAccesses 0
• // ETC.

class StatusCoefficient public
Coefficient public StatusCoefficient(vo
id) myStatus OverStatus
StatusCoefficient(double initval, Status
initStatus) Coefficient(initval)
myStatus initStatus // ETC.
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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
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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 - baseCLfunction()
• appropriate version of function is determined at
  runtime dynamically
• virtual member functions used to implement
  polymorphism

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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()

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Resources

• Data Structures with C, William Ford and
  William Topp
• Single and Multiple Inheritance in C, Douglas
  Schmidt