C++ Classes – Inheritance (Pt.2)

C. Papachristos

Robotic Workers Lab

University of Nevada, Reno

CS-202

Course , Projects , Labs:

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4^th Project Deadline was this Wednesday 2/26.

• NO Project accepted past the 24-hrs delayed extension (@ 20% grade penalty).
• Send what you have in time!

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• Always check out WebCampus CS-202 Samples for some help !

Course Week

CS-202 C. Papachristos

Today’s Topics

CS-202 C. Papachristos

Method Overriding

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• Overriding vs Overloading

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Inheritance Rules

• Base–Derived Constructors
• Derived–Base Destructors
• Assignment Operator

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Polymorphism Prelude

• Base class pointers to address Derived class Objects

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Advanced Examples

• Derived–to–Base forwarding via static_cast-ing
• protected Interface: Constructors, Destructor, Assignment Operator

Inheritance

CS-202 C. Papachristos

Inheritance Relationship

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Inheritance Syntax:

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class BaseClass {

public:

//operations

private:

//data

};

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class DerivedClass : public BaseClass {

public:

//operations

private:

//data

};

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Base Class

Derived

Class(es)

Indicates that this DerivedClass Inherits data and operations from this BaseClass

Inheritance

CS-202 C. Papachristos

Handling Access from within a Derived Class

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Derived/Child Class has access to Base/Parent Class’s:

• protected Member Variables/Functions.
• public Member Variables/Functions (as everything else also does).

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No access to Base/Parent Class’s private Member Variables/Functions:

• Not even through Derived/Child Class’ own Member Function.

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Remember:

private Member Variables are only directly accessible (“by name”) in Member Functions of their own Class (they one they are defined in).

Inheritance

CS-202 C. Papachristos

Inherited Member(s) of a Derived Class

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Vehicle

Parent Class

Car

Child Class

Child Class’ own:

• Member Fxns
• Member Vars

public Fxns & Vars

protected Fxns & Vars

private Vars

private Fxns

Copy-ctor

Assignment Operator

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ctor

dtor

public Fxns & Vars

protected Fxns & Vars

private Vars

Not directly accessible!

Can be accessed and invoked

But are not directly Inherited!

Method Overriding

CS-202 C. Papachristos

Overriding

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Remember: Interface of a Derived/Child Class:

• Extends: Contains declarations for its own new Member Functions.
• Overrides: Contains declarations for Inherited Member Functions to be changed.

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Implementation of a Derived/Child Class will:

• Define new Member Functions.
• Redefine Inherited Functions when you Declare them!

class Vehicle {

public:

int getMileage(){ return m_mileage; }

private:

int m_mileage;

} ;

class Car : public Vehicle {

public:

int getMileage();

} ;

Now that you re-Declared it, you have to Define it!

Method Overriding

CS-202 C. Papachristos

Overriding vs Overloading

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Overriding in a Derived/Child class means “Redefining what it does” :

• The same parameters list.
• Essentially “crossing-out & re-writing” what the one-and-same function does!
• Overridden functions share the same signature (because they are one function)!

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Overloading a Function means “Reusing its name” :

• Using a different parameter(s) list.
• Essentially defining a “new version of” a function (that takes different parameters).
• Overloaded functions must have different signatures !

Method Overriding

CS-202 C. Papachristos

Overriding vs Overloading

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Overriding in a Derived/Child class means “Redefining what it does” :

• Overridden functions share the same signature (because they are one function)!

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Overloading a Function means “Reusing its name” :

• Overloaded functions must have different signatures!

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Function “Signature” (or the information that participates in Overload Resolution):

• The unqualified name of the function.
• The specific sequence of types (names are irrelevant) in parameters list

(including order, number, types).

• Signature does NOT include: return type (later however we will encounter an issue here), const keyword for non-Reference type parameters (e.g. const int vs int)
• Signature DOES include: Class method cv-qualifiers (e.g. const keyword at the end)

Method Overriding

CS-202 C. Papachristos

Overriding vs Overloading

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Method Overriding (uses exact same signature):

• Derived Class Method can modify, add to, or replace Base Class methods.
• Derived Method will be called for Derived Objects.
• Base Method will be called for Base Objects.

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class Animal {

public:

void eat(){

cout << "I eat stuff" << endl;

}

};

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class Lion : public Animal {

void eat(){

cout << "I eat meat" << endl;

}

};

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int main(){

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Animal animal;

animal.eat(); // I eat stuff

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Lion lion;

lion.eat(); // I eat meat

}

Method Overriding

CS-202 C. Papachristos

Overriding vs Overloading

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Method Overloading (uses exact different signature):

• A different function (which however carries the same name!)
• Derived/Child Class has access to both functions.

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class Animal {

public:

void eat(){

cout << "I eat stuff" << endl;

}

};

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class Lion : public Animal {

public:

void eat(const char * food)

cout << "I ate a " << food << endl;

}

};

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int main(){

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Lion lion;

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lion.eat(); // I eat stuff

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lion.eat("Steak"); // I ate a Steak

}

Inheritance Rules

CS-202 C. Papachristos

Inheritance Rules - Further

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All “normal” functions in Base/Parent class are Inherited in Derived/Child Class.

Inheritance exceptions to the rules so far are:

• Constructor(s) - ctor
• Destructor(s) - dtor

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Inheritance Rules

CS-202 C. Papachristos

Constructor & Destructor in Derived Class(es)

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The Base/Parent Class ctors are not Inherited in Derived(Child) Classes.

• They can however be invoked within Derived(Child) Class’ ctor.

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Nothing more is required:

• A Base(Parent) Class ctor must instantiate all Base Class Member Variables.
• These are Inherited by Derived(Child) Class!

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The “First thing” that any Derived(Child) Class ctor does is to try and invoke the Base(Parent) Class Default ctor !

• Unless we otherwise explicitly specify to call another Base(Parent) Class ctor .�(We will see how in a little while…)

Inheritance Rules

CS-202 C. Papachristos

Constructor & Destructor in Derived Class(es)

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Constructor(s)

• Base/Parent Class ctor is called before Derived/Child Class ctor.

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DerivedClass dClass;

BaseClass();

DerivedClass();

Inheritance Rules

CS-202 C. Papachristos

Constructor & Destructor in Derived Class(es)

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Constructor(s)

• Base/Parent Class ctor is called before any Derived/Child Class ctor.

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Destructor

• Derived/Child Class dtor is called before the Base/Parent Class dtor.

DerivedClass dClass;

BaseClass();

DerivedClass();

~DerivedClass();

~BaseClass();

{

DerivedClass dClass;

} //end of scope

Inheritance Rules

CS-202 C. Papachristos

Inheritance Rules - Further

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All “normal” functions in Base/Parent class are Inherited in Derived/Child Class.

Inheritance exceptions to the rules so far are:

• Constructor(s) - ctor
• Destructor(s) - dtor
• Copy-ctor

If none is specified for Derived Class, compiler will still generate an “automatically� synthesized” one (which will also invoke the Base Class Copy-ctor.

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Inheritance Rules

CS-202 C. Papachristos

Constructor & Destructor in Derived Class(es)

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Copy-ctor

• Base/Parent Class ctor is called before any Derived/Child Class ctor.

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DerivedClass dClass;

BaseClass();

DerivedClass();

DerivedClass dClass_Cpy( dClass );

BaseClass();

DerivedClass(const DerivedClass & other);

Inheritance Rules

CS-202 C. Papachristos

Inheritance Rules - Further

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All “normal” functions in Base/Parent class are Inherited in Derived/Child Class.

Inheritance exceptions to the rules so far are:

• Constructor(s) - ctor
• Destructor(s) - dtor
• Copy-ctor

If none is specified for Derived Class, compiler will still generate an “automatically� synthesized” one.

• Assignment Operator ( = )

If none is specified for Derived Class, compiler will still generate an “automatically� synthesized” one (which will also invoke the Base Class assignment operator=) .

If one is implemented for the Derived Class that one will be called alone .

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Inheritance Rules

CS-202 C. Papachristos

Assignment operator= in Derived Class(es)

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Assignment Operator ( = )

• If none is specified for Derived Class, compiler will still generate an “automatically� synthesized” one (which will also invoke the Base Class assignment operator=) .
• If one is implemented for the Derived Class that one will be called alone .

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We can also directly call the Base Class Assignment from the Derived Class one !

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class Animal{

Animal & operator=(const Animal & rhs_animal ){

cout << "Animal assignment" << endl;

}

};

class Lion : public Animal{

Lion & operator=(const Lion & rhs_lion ){

Animal::operator=( rhs_lion );

cout << "Lion assignment" << endl;

}

};

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Inheritance Rules

CS-202 C. Papachristos

Constructor & Destructor in Derived Class(es)

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Sequence of Base-Derived ctor & Derived-Base dtor calls

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Example:

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class Animal {

public:

Animal(){cout << "Base constructor"<<endl;}

~Animal(){cout << "Base destructor"<<endl;}

};

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class Lion : public Animal {

public:

Lion(){cout << "Derived constructor"<<endl;}

~Lion(){cout << "Derived destructor"<<endl;}

};

Output:

Base constructor

Derived constructor

Derived destructor

Base destructor

int main(){

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Lion lion;

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return 0;

}

Inheritance Rules

CS-202 C. Papachristos

Parametrized Constructor in Derived Class(es)

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Calling the Base ctor from a Derived ctor explicitly

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Example:

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class Animal {

public:

Animal(const char * name) {

strcpy(m_name, name); cout<<m_name<<endl;

}

protected:

char m_name[MAXNAME];

};

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class Lion : public Animal {

public:

Lion(const char * name) : Animal(name){

/* Rest of Lion Parametrized ctor statements*/

}

};

int main(){

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Lion lion("King");

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return 0;

}

Output:

King

Note: Initializer-list is the only way that allows calling a Base ctor from a Derived ctor.

Inheritance Rules

CS-202 C. Papachristos

Parametrized Constructor in Derived Class(es)

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Calling the Base ctor from a Derived ctor explicitly

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Example:

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class Animal {

public:

Animal(const char * name) {

strcpy(m_name, name); cout<<m_name<<endl;

}

protected:

char m_name[MAXNAME];

};

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class Lion : public Animal {

public:

Lion(const char * name) : Animal(name){

strcat(m_name," Lion"); cout<<m_name<<endl;

}

};

int main(){

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Lion lion("King");

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return 0;

}

Output:

King

King Lion

Note: Calls Parametrized Base ctor by passing down argument from the Derived ctor.

Polymorphism (prelude)

CS-202 C. Papachristos

Inheritance & Polymorphism

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Polymorphism means “the ability to take many forms”.

• Allowing a single (Remember: Overriding vs Overloading) behavior to take on many type-dependent forms.
• Hence, grants the ability to manipulate Objects in a type-independent way.

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Pointers of Base Class-type:

• They are used to address “Common Ancestor”-type Objects from a Class Hierarchy.

Polymorphism (prelude)

CS-202 C. Papachristos

Inheritance & Polymorphism

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Base Class-type Pointers :

• A Pointer of a Base Class type can point to an Object of a Derived Class type.

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SUV suv1;

Sedan sedan1, sedan2;

Jeep jeep1;

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Car * suv1_Pt = &suv1;

Car * sedan1_Pt = &sedan1, * sedan2_Pt = &sedan2;

Car * jeep1_Pt = &jeep1;

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This is valid: A Derived Class (SUV, Sedan, Van, Jeep) “is a type of” Base Class (Car).

• Note: A 1-way relationship:

A Derived Class Pointer cannot point to a Base Class Object.

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Advanced Examples

CS-202 C. Papachristos

Derived to Base Class forwarding

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Parent Class “hides” data even from Children (but some Interface exists for its Type!) :

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class Animal {

friend ostream & operator<<(ostream & os, const Animal & animal){

os << "I have " << animal.m_legs << " legs" << endl;

return os;

}

private:

int m_legs;

};

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class Lion : public Animal {

friend ostream & operator<<(ostream & os, const Lion & lion){

os << "I am a " << lion.m_color << " lion and\n"

<< "I have " << lion.m_legs << " legs" << endl;

return os;

}

private:

char m_color[256];

};

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Advanced Examples

CS-202 C. Papachristos

Derived to Base Class forwarding

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Parent Class “hides” data even from Children (but some Interface exists for its Type!) :

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class Animal {

friend ostream & operator<<(ostream & os, const Animal & animal){

os << "I have " << animal.m_legs << " legs" << endl;

return os;

}

private:

int m_legs;

};

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class Lion : public Animal {

friend ostream & operator<<(ostream & os, const Lion & lion){

os << "I am a " << lion.m_color << " lion and\n"

<< "I have " << lion.m_legs << " legs" << endl;

return os;

}

private:

char m_color[256];

};

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Not possible !

The function is a friend of Lion,�not a friend of Animal !

Advanced Examples

CS-202 C. Papachristos

Derived to Base Class forwarding

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Parent Class “hides” data even from Children (but some Interface exists for its Type!) :

​

class Animal {

friend ostream & operator<<(ostream & os, const Animal & animal){

os << "I have " << animal.m_legs << " legs" << endl;

return os;

}

private:

int m_legs;

};

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class Lion : public Animal {

friend ostream & operator<<(ostream & os, const Lion & lion){

os << "I am a " << lion.m_color << " lion and\n"

<< static_cast< const Animal & >( lion );

return os;

}

private:

char m_color[256];

};

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Casting to a Reference of Base class�type invokes the corresponding�function overload for Animal !

Time for Questions !

CS-202

CS-202 C. Papachristos