cs-1030 dr. mark l. hornick 1 inheritance in c++

CS-1030Dr. Mark L. Hornick

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Inheritance in C++


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Inheritance Syntax class Pet {

protected: string name; public: int speak(); int eat(); }

class Dog : public Pet {// Java uses extendsprivate: bool hasFleas;public: int speak(); // override}

class Cat : public Pet {private: bool isDeclawed;public: int purr();

} Example Program…


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C++ Inheritance rules

Protected and public data and methods in the base class are inherited by the derived classWith some exceptions:

Constructors Destructors operator=

Are accessible in the derived class.


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Public/protected/private rulesclass Dog : <inheritance specifier> Pet {

private: bool hasFleas;public: int speak();}

<Inheritance specifier> is one of the following public

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

protected 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


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C++ Construction Order

1. Base class data members are constructed in order of declaration

2. Base class constructor is called

3. Derived class data members are constructed in order

4. 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

Example Program…


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Destruction order

Destructor order (reverse of ctor order)1. Derived class destructor is executed

2. Derived class data members are destroyed

3. The base class destructor is called

4. Base class data members are destroyed


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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…


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Functional binding

void main() {

Dog spot; spot.speak();

Pet* pet = &spot; pet->speak();

Pet aPet; apet.speak();}

Dog::speak()

Pet::speak()

Pet::speak()


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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


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Polymorphism

The solution to variant behavior Version is chosen at run time Delay the choice until we know the real type

Even when upcast!


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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


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virtual Example

class Pet {public:

virtual void speak(); …};

class Dog : public Pet {public:

void speak(); …}; virtual is optional here


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Calling virtual Functions

void main() {

Dog spot; spot.speak(); //virtual

Pet* pet = &spot; pet->speak();

Pet aPet; apet.speak();}

Dog::speak()

Dog::speak()

Pet::speak()


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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


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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)


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The C++ equivalent of Java abstract classes and interfaces

Done in C++ by declaring pure virtual methods

Language syntaxvirtual 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


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Upcasting

Inheritance embodies “is a” A derived object is a base object

Derived objects are base objects 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()


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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


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virtual Destructors

Allows upcast objects to properly destroy Forces use of the most derived destructor All the destructors are still called

Most derived base (in order)

ALWAYS make base class destructors virtual Good habit

Pure virtual destructors must still be defined


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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


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Polymorphic method Example

int Max(int a, int b)

{ return (a<b) ? b : a;}

double Max(double a, double b)


Complex Max(const Complex& a,

const Complex& b)


All the functions are the same!

cs-1030 dr. mark l. hornick 1 inheritance in c++

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