CIS162AB C

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CIS162AB - C

• Inheritance Juan Marquez
• 12_inheritance.ppt

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Overview of Topics

• Inheritance
• Base and Derived Classes
• Constructor Overloading
• Base Initialization List
• Constructors in Derived Class
• Function Redefinition vs Overloading
• Function Signatures

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Inheritance

• Inheritance is the process by which a new class
  is created from another class, and the new class
  has additional member variables and/or functions.

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Inheritance Terminology

• Base Class
• Parent Class
• Super Class
• Employee

• Derived Class
• Child Class
• Sub Class
• SalariedEmployee
• HourlyEmployee

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Labels

• class Employee
• private
• protected \\use protected instead of private
• \\ in base class
• public

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protected

• Use protected when you might expect private.
• A protected member is the same as a private
  member to any other class except a class derived
  from the base class, or derived from a class
  derived from the base class.
• In the derived class it is private.

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Base Class - Employee

• class Employee
• protected
• string name, ssn
• double gross
• public
• Employee()
• Employee(string newName, string newSSN)
• void displayGross()
• void calcPay()

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Constructor Overloading

• Overloading is the same function name, but
  different types or number of parameters.
• Constructors can be overloaded as well.
• Default constructor has no parameters by
  definition.

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Constructor with Parameters

• Constructors with parameters is allowed.
• Values to be assigned to members can be passed
  when an object is declared.
• Employee emp1 //no arguments default
• Employee emp2 (Joe Cool, 123-45-6789)

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Derived Class - SalariedEmployee

• class SalariedEmployee public Employee
• private
• double salary
• public
• SalariedEmployee()
• void calcPay()

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Access Specifier

• The access specifier options on the inheritance
  directive are public, protected, private.
• The access specifier, public, in front of the
  base class Employee means that the public,
  protected, and private members in the base class
  are inherited the same way in the derived class.

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Derived Class

• All member variables and functions defined in
  Employee are included in the class
  SalariedEmployee.
• New variable (salary) added.
• calcPay() is listed again, so it will redefine
  the calcPay() in Employee.

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Derived Class - HourlyEmployee

• class HourlyEmployee public Employee
• private
• double rate
• int hours
• public
• HourlyEmployee()
• void calcPay()

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Derived Class

• All member variables and functions defined in
  Employee are included in the class
  HourlyEmployee.
• calcPay() is listed again, so it will redefine
  the calcPay() in Employee.
• New variables (rate, hours) added.

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Functions Employee

• EmployeeEmployee()
• cout ltlt Enter name
• cin gtgt name
• cout ltlt Enter SSN
• cin gtgt ssn
• gross 0
• EmployeeEmployee(string na, string sn)
• name na ssn sn
• gross 0

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• void EmployeedisplayGross()
• cout ltlt Gross ltlt gross
• void EmployeecalcPay()
• cout ltlt Error calcPay() called for an
• ltlt undifferentiated employee

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Base Initialization List

• We need to initialize the inherited member
  variables of the derived class as well as the
  members defined directly in the derived class.
• When defining a constructor you can initialize
  member variables in a Base Initialize List. This
  initialization section is part of the heading for
  the function.

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• Base Initialization List Option
• EmployeeEmployee(string na, string sn)
  name(na), ssn(sn), gross(0)
• //empty body
• This function definition would replace the
  Employee constructor with parameters previously
  defined as
• EmployeeEmployee(string na, string sn)
• name na ssn sn gross 0

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Default Values

• Initialize member variables using base
  initialization list.
• Must be used for const members.
• class Employee
• double TAX_RATE .05 // illegal in class
  definition
• EmployeeEmployee () TAX_RATE(.05)
• //empty body -- default constructor

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Constructors in Derived Class

• A constructor for a derived class should begin
  with an invocation of a constructor for the base
  class.
• List constructor in base initialization list.
• SalariedEmployeeSalariedEmployee()
• Employee()

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Functions SalariedEmployee

• SalariedEmployeeSalariedEmployee() Employee()
• cout ltlt Enter salary
• cin gtgt salary
• void SalariedEmployeecalcPay()
• gross salary

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Reusable Code

• Inheritance allows you to reuse code.
• Employee has the code to get name and SSN.
• Employee has the code to displayGross()
• The same code is used by SalariedEmployee and
  HourlyEmployee.

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Protected Members

• Protected members are the same as private members
  in the base and derived class.
• Member functions defined in a derived class can
  reference or call protected members directly
  within the derived class.
• For example SalariedEmployeecalcPay() can
  reference gross defined in Employee which is
  protected.

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Private Members

• Private members in the base class cannot be
  referenced or called directly by members of the
  derived class.
• Private members are still inherited, but must use
  accessors.

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Functions HourlyEmployee

• HourlyEmployeeHourlyEmployee() Employee()
• cout ltlt Enter pay rate
• cin gtgt rate
• cout ltlt Enter hours worked
• cin gtgt hours
• void HourlyEmployee calcPay()
• gross rate hours

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Redefinition

• Do not list member functions from base unless you
  want to change the definition.
• When a member function is redefined in a derived
  class you must list its prototype in the class
  definition, even though the prototype is the same
  as in the base class.
• See calcPay() in each derived class.

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Redefinition vs Overloading

• A function is redefined when the new function in
  the derived class has the same name, and the
  exact number and types of parameters.
• Overloading occurs when it is the same name, but
  different types or number of parameters.

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Function Signatures

• Signature includes function name and the sequence
  of types in the parameter list.Employee(string,
  string)
• When redefined, the function in the base and
  derived class have the same signature, but the
  derived overrides the base.
• When overloaded, they have different signatures.

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Using Derived Classes

• In the following code, what happens
• when each object is declared?
• after each call to calcPay()?
• after each call to displayGross()?

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• void main()
• Employee emp
• emp.calcPay()
• emp.displayGross()
• Employee emp2 (Joe Cool, 123-45-6789)
• SalariedEmployee empSalary
• empSalary.calcPay()
• empSalary.displayGross()
• HourlyEmployee empHourly
• empHourly.calcPay()
• empHourly.displayGross()

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Objects as Function Arguments

• An object of a derived class is also an object of
  the base class.
• base Employee
• derived SalariedEmployee,
  HourlyEmployee
• Object types of SalariedEmployee and
  HourlyEmployee can be passed through a parameter
  type of Employee.

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Objects as Arguments - Example

• Just like an ofstream can be passed through an
  ostream (see P11 SalesPersonClass).void
  outputSalesInfo(ostream target)//ostream can
  handle cout or fileOut (ofstream)void
  displayName(Employee emp)displayName(empSalary
  )displayName(empHourly)
• However, only the members defined in the base are
  available in the function (name, ssn, gross).

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Summary

• Inheritance
• Base and Derived Classes
• Constructor Overloading
• Base Initialization List
• Constructors in Derived Class
• Function Redefinition vs Overloading
• Function Signatures