Презентация Nheritance, polymorphism, and virtual functions онлайн
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Слайды и текст к этой презентации:
№1 слайд
Содержание слайда: Lesson 15
Inheritance, Polymorphism, and Virtual Functions
№2 слайд
Содержание слайда: What Is Inheritance?
Provides a way to create a new class from an existing class
The new class is a specialized version of the existing class
№3 слайд
Содержание слайда: Example: Insect Taxonomy
№4 слайд
Содержание слайда: The "is a" Relationship
Inheritance establishes an "is a" relationship between classes.
A poodle is a dog
A car is a vehicle
A flower is a plant
A football player is an athlete
№5 слайд
Содержание слайда: Inheritance – Terminology and Notation in C++
Base class (or parent) – inherited from
Derived class (or child) – inherits from the base class
Notation:
class Student // base class
{
. . .
};
class UnderGrad : public student
{ // derived class
. . .
};
№6 слайд
Содержание слайда: Back to the ‘is a’ Relationship
An object of a derived class 'is a(n)' object of the base class
Example:
an UnderGrad is a Student
a Mammal is an Animal
A derived object has all of the characteristics of the base class
№7 слайд
Содержание слайда: What Does a Child Have?
An object of the derived class has:
all members defined in child class
all members declared in parent class
An object of the derived class can use:
all public members defined in child class
all public members defined in parent class
№8 слайд
Содержание слайда: Protected Members and Class Access
protected member access specification: like private, but accessible by objects of derived class
Class access specification: determines how private, protected, and public members of base class are inherited by the derived class
№9 слайд
Содержание слайда: Class Access Specifiers
public – object of derived class can be treated as object of base class (not vice-versa)
protected – more restrictive than public, but allows derived classes to know details of parents
private – prevents objects of derived class from being treated as objects of base class.
№10 слайд
Содержание слайда: Inheritance vs. Access
№11 слайд
Содержание слайда: Inheritance vs. Access
№12 слайд
Содержание слайда: Inheritance vs. Access
№13 слайд
Содержание слайда: Inheritance vs. Access
№14 слайд
Содержание слайда: Constructors and Destructors in Base and Derived Classes
Derived classes can have their own constructors and destructors
When an object of a derived class is created, the base class’s constructor is executed first, followed by the derived class’s constructor
When an object of a derived class is destroyed, its destructor is called first, then that of the base class
№15 слайд
Содержание слайда: Constructors and Destructors in Base and Derived Classes
№16 слайд
Содержание слайда: Constructors and Destructors in Base and Derived Classes
№17 слайд
Содержание слайда: Constructors and Destructors in Base and Derived Classes
№18 слайд
Содержание слайда: Passing Arguments to
Base Class Constructor
Allows selection between multiple base class constructors
Specify arguments to base constructor on derived constructor heading:
Square::Square(int side) : Rectangle(side, side)
Can also be done with inline constructors
Must be done if base class has no default constructor
№19 слайд
Содержание слайда: Passing Arguments to
Base Class Constructor
№20 слайд
Содержание слайда: Redefining Base Class Functions
Redefining function: function in a derived class that has the same name and parameter list as a function in the base class
Typically used to replace a function in base class with different actions in derived class
№21 слайд
Содержание слайда: Redefining Base Class Functions
Not the same as overloading – with overloading, parameter lists must be different
Objects of base class use base class version of function; objects of derived class use derived class version of function
№22 слайд
Содержание слайда: Base Class
№23 слайд
Содержание слайда: Derived Class
№24 слайд
Содержание слайда: Driver Program
№25 слайд
Содержание слайда: Problem with Redefining
Consider this situation:
Class BaseClass defines functions x() and y(). x() calls y().
Class DerivedClass inherits from BaseClass and redefines function y().
An object D of class DerivedClass is created and function x() is called.
When x() is called, which y() is used, the one defined in BaseClass or the the redefined one in DerivedClass?
№26 слайд
Содержание слайда: Problem with Redefining
№27 слайд
Содержание слайда: Class Hierarchies
№28 слайд
Содержание слайда: Class Hierarchies
№29 слайд
Содержание слайда: Polymorphism and Virtual Member Functions
Virtual member function: function in base class that expects to be redefined in derived class
Function defined with key word virtual:
virtual void Y() {...}
Supports dynamic binding: functions bound at run time to function that they call
Without virtual member functions, C++ uses static (compile time) binding
№30 слайд
Содержание слайда: Polymorphism and Virtual Member Functions
№31 слайд
Содержание слайда:
№32 слайд
Содержание слайда:
№33 слайд
Содержание слайда: Static Binding
Program 15-10 displays 'C' instead of 'P' because the call to the getLetterGrade function is statically bound (at compile time) with the GradedActivity class's version of the function.
We can remedy this by making the function virtual.
№34 слайд
Содержание слайда: Virtual Functions
A virtual function is dynamically bound to calls at runtime.
At runtime, C++ determines the type of object making the call, and binds the function to the appropriate version of the function.
№35 слайд
Содержание слайда: Virtual Functions
To make a function virtual, place the virtual key word before the return type in the base class's declaration:
virtual char getLetterGrade() const;
The compiler will not bind the function to calls. Instead, the program will bind them at runtime.
№36 слайд
Содержание слайда: Updated Version of GradedActivity
№37 слайд
Содержание слайда: Polymorphism
№38 слайд
Содержание слайда:
№39 слайд
Содержание слайда:
№40 слайд
Содержание слайда: Polymorphism Requires References or Pointers
Polymorphic behavior is only possible when an object is referenced by a reference variable or a pointer, as demonstrated in the displayGrade function.
№41 слайд
Содержание слайда: Base Class Pointers
№42 слайд
Содержание слайда: Base Class Pointers
Base class pointers and references only know about members of the base class
So, you can’t use a base class pointer to call a derived class function
Redefined functions in derived class will be ignored unless base class declares the function virtual
№43 слайд
Содержание слайда: Redefining vs. Overriding
In C++, redefined functions are statically bound and overridden functions are dynamically bound.
So, a virtual function is overridden, and a non-virtual function is redefined.
№44 слайд
Содержание слайда: Virtual Destructors
It's a good idea to make destructors virtual if the class could ever become a base class.
Otherwise, the compiler will perform static binding on the destructor if the class ever is derived from.
See Program 15-14 for an example
№45 слайд
Содержание слайда: Abstract Base Classes and Pure Virtual Functions
Pure virtual function: a virtual member function that must be overridden in a derived class that has objects
Abstract base class contains at least one pure virtual function:
virtual void Y() = 0;
The = 0 indicates a pure virtual function
Must have no function definition in the base class
№46 слайд
Содержание слайда: Abstract Base Classes and Pure Virtual Functions
Abstract base class: class that can have no objects. Serves as a basis for derived classes that may/will have objects
A class becomes an abstract base class when one or more of its member functions is a pure virtual function
№47 слайд
Содержание слайда: Multiple Inheritance
№48 слайд
Содержание слайда: Multiple Inheritance
Problem: what if base classes have member variables/functions with the same name?
Solutions:
Derived class redefines the multiply-defined function
Derived class invokes member function in a particular base class using scope resolution operator ::
Compiler errors occur if derived class uses base class function without one of these solutions
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