OOP object-oriented programming

Lecture 23

Date Class

class Date{
int day, month, year;
static Date defaultDate;

public:
void SetDay(int aDay);
int GetDay() const;
void AddDay(int x);
static void SetDefaultDate(
int aDay,int aMonth, int aYear);

private:
bool IsLeapYear();
};

int main(){
Date aDate;
aDate.IsLeapYear(); //Error
return 0;
}

Creating SpecialDate Class

class SpecialDate: public Date{

public:
void AddSpecialYear(int i){
if(day == 29 && month == 2 && !IsLeapyear(year+i)){ //ERROR!}
}
};

Modify Access Specifier:

We can modify the access specifier “IsLeapYear” from private to public

Modified Date Class

class Date{
public:
bool IsLeapYear();
};

Modified AddSpecialYear

void SpecialDate :: AddSpecialYear(int i){
	if(day == 29 && month == 2 && !IsLeapyear(year+i)){ 
	}
}

Protected members

Protected members can not be accessed outside the class
Protected members of the base class become protected members of the derived class in Public inheritance

Modified Date Class

class Date{
protected: 
bool IsLeapYear();
};



int main(){
Date aDate;
aDate.IsLeapYear(); //Error
return 0;
}

Modified AddSpecialYear

void SpecialDate :: AddSpecialYear(int i){
	if(day == 29 && month == 2 && 	!IsLeapyear(year+i)){ 
	}
}

Disadvantages: Breaks encapsulation The protected member is part of base class’s implementation as well as derived class’s implementation

“IS A” Relationship

Public inheritance models the “IS A” relationship
A derived object IS A kind of base object
The base class pointer can point toward an object of the derived class

class Person {
char * name;
public: 
const char * GetName();

};

class Student: public Person{
int rollNo;
public:
int GetRollNo();
};

int main()
{
Student sobj;
cout << sobj.GetName();
cout << sobj.GetRollNo();
return 0;
}

int main(){
Person * pPtr = 0;
Student s;
pPtr = &s;
cout << pPtr->GetName();

return 0;
}
int main(){
Person * pPtr = 0;
Student s;
pPtr = &s;
//Error
cout << pPtr->GetRollNo();
return 0;
}

“IS A” Relationship: We can use a reference of the derived object where the reference of base object is required

int main(){
Person p; 
Student s; 	
Person & refp = s;
cout << refp.GetName();
cout << refp.GetRollNo(); //Error
return 0;
}
void Play(const Person& p){
cout << p.GetName()
<< “ is playing”;
}

void Study(const Student& s){
cout << s.GetRollNo()
<< “ is Studying”;
}

int main(){
Person p; 
Student s; 
Play(p); 
Play(s);
return 0;
}

Static Type: The type that is used to declare a reference or pointer is called its static type

Member Access : The access to members is determined by static type

The static type of pPtr is Person
The static type of s is Student

Following call is erroneous
 pPtr->GetRollNo();

Hierarchy of Inheritance

We represent the classes involved in inheritance relation in tree like hierarchy

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