Lecture 12

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Lecture 12

• _{this Pointer}

• _{Scope Resolution Operator}

• _{Friend Function}

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this pointer

• _{Member functions of every object have access to a}

special constant pointer named this.

• _{It points to the object itself.}

• _{It is passed automatically as an implicit argument to}

the invoking member function.

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class ABC { int a; public:

void set()

{ a = 10; } };

class ABC { int a; public:

void set()

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Example

1. #include <iostream> 2. using namespace std; 3. class ABC

4. { private:char charray[10]; 5. public: void reveal()

6. { cout << "\nMy object's address is " << this; } 7. };

8. int main()

9. { ABC w1, w2;

10. w1.reveal();

11. w2.reveal();

12. cout << endl; 13. return 0;

14. }

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Contd…

• _{Static member functions do not have 'this' pointer}

as they can be called without any object using the class name.

• _{Not passed to friend functions as they are not}

member functions of a class.

• _{Used when a binary operator is overloaded using a}

member function.

• _{Used to return the object it points to.}

return 0;

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Example

1. #include<iostream> 2. #include<cstring>

3. using namespace std; 4. class person

5. { char name[20]; 6. int age;

7. public:

8. void setData(const char *s, int a) 9. { strcpy(name, s);

10. age = a; }

11. person& greater(person &x) 12. { if(x.age >= age) return x;

13. else return *this; }

14. void display()

15. { cout << name << " with age " << age; }

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Contd…

17. int main()

18. { person p1,p2,p3;

19. p1.setData("Abha", 21); 20. p2.setData("Akhil", 29); 21. p3.setData("Mitali", 31);

22. person p = p1.greater(p2);

23. cout << "Elder person is: ";

24. p.display();

25. p = p2.greater(p3);

26. cout << endl << "Elder person is: ";

27. p.display();

28. return 0;

29. }

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Elder person is: Akhil with age 29 Elder person is: Mitali with age 31

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Scope resolution operator (::)

• _{It links class name with member name and tells}

compiler what class the member belongs to.

• _{It allows access to a name in an enclosing scope}

that is "hidden" by a local declaration of the same name.

• _Example:

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int I; //Global

void f()

{ int I; //Local

I = 10; //Local

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Example

1. #include<iostream> 2. using namespace std; 3. int m = 10;

4. int main()

5. { int m = 20;

6. { int k = m;

7. int m = 30;

8. cout << "Inner block: k = " << k << ", m = "; 9. cout << m << ", ::m = " << ::m;

10. }

11. cout << endl << "Outer block: m = " << m << ", ::m = " << ::m;

12. return 0; }

Thapar University UTA007 - Computer Programming I 8

Inner block: k = 20, m = 30, ::m = 10 Outer block: m = 20, ::m = 10

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Friend Function

• _{A function that can access the private and protected}

members of a class to which it is a friend.

• _{It is not in the scope of a class to which it is a friend.} • _{It cannot be called using the object of a class to}

which it is a friend.

– _{Invoked like a normal function.}

• _{Has to be declared either in the public or the}

private section within a class (to which it is a friend)

preceded with a keyword friend.

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Contd…

• _{Defined elsewhere in the program like a normal C++}

function.

• _{Can be a simple function or a member function of}

some other class.

• _{Unlike member functions, it cannot access the}

member names directly and has to use an object name and dot membership operator with each member name.

• _{Usually, it has the objects as arguments.}

• _{Best suited in operator overloading.}

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Example

1. #include <iostream> 2. using namespace std; 3. class myclass

4. { int a, b; 5. public:

6. friend int sum(myclass x); 7. void set_ab(int i, int j); 8. };

9. void myclass::set_ab(int i, int j) 10.{ a = i; b = j; }

11.int sum(myclass x) 12.{ return x.a + x.b; }

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13. int main()

14. { myclass n;

15. n.set_ab(3, 4); 16. cout <<

sum(n);

17. return 0; 18. }

7

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Friend of more than one class

1. #include <iostream> 2. using namespace std; 3. const int IDLE = 0; 4. const int INUSE = 1;

5. class C2; // forward declaration 6. class C1 {

7. int status; 8. public:

9. void set_status(int state); 10. friend int idle(C1 a, C2 b); 11. };

12. class C2 {

13. int status; 14. public:

15. void set_status(int state); 16. friend int idle(C1 a, C2 b);

17. };Thapar University UTA007 - Computer Programming I 12

18. void C1::set_status(int state) 19. { status = state; }

22. int idle(C1 a, C2 b)

23. { if(a.status || b.status) 24. return 0;

25. else

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Contd…

27. int main() 28. { C1 x; 29. C2 y;

30. x.set_status(IDLE); 31. y.set_status(IDLE); 32. if(idle(x, y))

33. cout << "Screen can be used.\n"; 34. else

35. cout << "In use.\n"; 36. x.set_status(INUSE); 37. if(idle(x, y))

40. cout << "In use.\n"; 41. return 0;

42. }09/22/2020 UTA009 13

Screen can be used. In use.

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Class member as a friend function

1. #include <iostream> 2. using namespace std; 3. const int IDLE = 0; 4. const int INUSE = 1;

5. class C2; // forward declaration 6. class C1 {

7. int status; 8. public:

9. void set_status(int state); 10. int idle(C2 b);

11. };

12. class C2 {

13. int status; 14. public:

15. void set_status(int state); 16. friend int C1::idle(C2 b);

17. };Thapar University UTA007 - Computer Programming I 14

22. int C1::idle(C2 b)

23. { if(status || b.status) 24. return 0;

25. else

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Contd…

27. int main() 28. { C1 x; 29. C2 y;

30. x.set_status(IDLE); 31. y.set_status(IDLE); 32. if(x.idle(y))

35. cout << "In use.\n"; 36. x.set_status(INUSE); 37. if(x.idle(y))

40. cout << "In use.\n"; 41. return 0;

42. }09/22/2020 UTA009 15

Screen can be used. In use.

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Friend Class

• _{A class can also be made a friend of another class.} • _Example:

• All the member functions of a friend class A become friend of class B.

• _{Any member function of}_{class A}_{can access the private data of}_class

B.

• But, member functions of class B cannot access the private data of

class A.

class B { ...

friend class A; ... };

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Example

1. #include <iostream> 2. using namespace std; 3. class TwoValues {

4. int a, b;

5. public:

6. TwoValues(int i, int j) { a = i; b = j; }

7. friend class Min;

8. };

9. class Min { 10. public:

11. int min(TwoValues x);

12. };

13. int Min::min(TwoValues x)

14. { return x.a < x.b ? x.a : x.b; }

10

Output

15. int main()

16. { TwoValues ob(10,

20);

17. Min m;

18. cout << m.min(ob);

19. return 0;