oops concepts

1. Access Modifiers in C++
Access modifiers are used to implement an important feature of Object Oriented Programming known
as Data Hiding.
Consider a real-life example: Consider Indian secret informatic system having 10 senior members, have some
top secret regarding national security. So we can think that 10 people as class data members or member
functions who can directly access secret information from each other but anyone can’t access this information
other than these 10 members i.e. outside people can’t access information directly without having any
privileges. This is what data hiding is.
Access modifiers or Access Specifiers in a class are used to set the accessibility of the class members. That is, it
sets some restrictions on the class members not to get directly accessed by the outside functions.Here are 3
types of access modifiers available in C++:
 Public
 Private
 Protected

2. Public
All the class members declared under public will be available to everyone. The data members and
member functions declared public can be accessed by other classes too. The public members of a
class can be accessed from anywhere in the program using the direct member access operator (.)
the object of that class.
Private
The class members declared as private can be accessed only by the functions inside the class. They
are not allowed to be accessed directly by any object or function outside the class. Only the
functions or the friend functions are allowed to access the private data members of a class.
Protected
Protected access modifier is similar to that of private access modifiers, the difference is that the
member declared as Protected are inaccessible outside the class but they can be accessed by any
subclass(derived class) of that class.

3. // C++ program to demonstrate public
// access modifier
#include<iostream>
using namespace std;
// class definition
class Circle
{
public:
double radius;
double compute_area()
{
return 3.14*radius*radius;
}
};
// main function
int main()
{
Circle obj;
// accessing public data member outside class
obj.radius = 5.5;
cout << "Radius is:" << obj.radius << "n";
cout << "Area is:" << obj.compute_area();
return 0;

4. // C++ program to demonstrate private
// access modifier
#include<iostream>
using namespace std;
class Circle
{
// private data member
private:
double radius;
// public member function
public:
double compute_area()
{ // member function can access private
// data member radius
return 3.14*radius*radius;
}
};
// main function
int main()
{
// creating object of the class
Circle obj;
// trying to access private data member
// directly outside the class
obj.radius = 1.5;
cout << "Area is:" << obj.compute_area();
return 0;
}

5. The output of above program will be a compile time error because we are not allowed to access
the private data members of a class directly outside the class.
we can access the private data members of a
class indirectly using the public member
functions of the class.
#include<iostream>
using namespace std;
class Circle
{
// private data member
private:
double radius;
// public member function
public:
void compute_area(double r)
{ // member function can access private
// data member radius
radius = r;
double area = 3.14*radius*radius;
cout << "Radius is:" << radius << endl;
cout << "Area is: " << area;
}
};
// main function
int main()
{
// creating object of the class
Circle obj;
// trying to access private data member
// directly outside the class
obj.compute_area(1.5);
return 0;
}

6. // C++ program to demonstrate
// protected access modifier
#include <iostrem>
using namespace std;
// base class
class Parent
{
// protected data members
protected:
int id_protected;
};
// sub class or derived class
class Child : public Parent
{
public:
void setId(int id)
{
// Child class is able to access the inherited
// protected data members of base class
id_protected = id;
}
void displayId()
{
cout << "id_protected is:" << id_protected << endl;
}
};
// main function
int main() {
Child obj1;
// member function of the derived class can
// access the protected data members of the base class
obj1.setId(81);
obj1.displayId();
return 0;
}

7. Access Control
A derived class can access all the non-private members of its base class. Thus base-class members
that should not be accessible to the member functions of derived classes should be declared
in the base class.
Access public protected private
Same class yes yes yes
Derived classes yes yes no
Outside classes yes no no
We can summarize the different access types according to - who can access them in the following way −
A derived class inherits all base class methods with the following exceptions
•Constructors, destructors and copy constructors of the base class.
•Overloaded operators of the base class.
•The friend functions of the base class.

8. Accessibility private variables protected variables public variables
Accessible from own
class?
yes yes yes
Accessible from derived
class?
no yes yes
Accessible from 2nd
derived class?
no yes yes
ccessibility private variables protected variables public variables
Accessible from own
class?
yes yes yes
Accessible from derived
class?
no yes
yes
(inherited as protected
variables)
Accessible from 2nd
derived class?
no yes yes

9. Accessibility private variables protected variables public variables
Accessible from own
class?
yes yes yes
Accessible from
derived class?
no
yes
(inherited as private
variables)
yes
(inherited as private
variables)
Accessible from 2nd
derived class?
no no no

10. Type of Inheritance
 When deriving a class from a base class, the base class may be inherited through public,
protected or private inheritance. The type of inheritance is specified by the access-
specifier as explained above.
We hardly use protected or private inheritance, but public inheritance is commonly used.
While using different type of inheritance, following rules are applied −
 Public Inheritance − When deriving a class from a public base class, public members of
the base class become public members of the derived class and protected members of
base class become protected members of the derived class. A base class's private
are never accessible directly from a derived class, but can be accessed through calls to
the public and protected members of the base class.
 Protected Inheritance − When deriving from a protected base
class, public and protected members of the base class become protected members of the
derived class.
 Private Inheritance − When deriving from a private base
class, public and protected members of the base class become private members of the
derived class.

11. Scope resolution operator in C++
C++, scope resolution operator is ::. It is used for following purposes.
1) To access a global variable when there is a local variable with same name:
// C++ program to show that we can access a global variable
// using scope resolution operator :: when there is a local
// variable with same name
#include<iostream>
using namespace std;
int x; // Global x
int main()
{
int x = 10; // Local x
cout << "Value of global x is " << ::x;
cout << "nValue of local x is " << x;
return 0;
} OUTPUT : global x is 0, local x is 10

12. 2) To define a function outside a class.
// C++ program to show that scope resolution
operator :: is used
// to define a function outside a class
#include<iostream>
using namespace std;
class A
{
public:
// Only declaration
void fun();
};
// Definition outside class using ::
void A::fun()
{
cout << "fun() called";
}
int main()
{
A a;
a.fun();
return 0;
}

13. 3) To access a class’s static variables.
// C++ program to show that :: can be used to access static
// members when there is a local variable with same name
#include<iostream>
using namespace std;
class Test
{
static int x;
public:
static int y;
// Local parameter 'a' hides class member
// 'a', but we can access it using ::
void func(int x)
{
// We can access class's static variable
// even if there is a local variable
cout << "Value of static x is " << Test::x;
cout << "nValue of local x is " << x;
}
};
// In C++, static members must be explicitly defined
// like this
int Test::x = 1;
int Test::y = 2;
int main()
{
Test obj;
int x = 3 ;
obj.func(x);
cout << "nTest::y = " << Test::y;
return 0;
}

14. 4) In case of multiple Inheritance:
// Use of scope resolution operator in multiple
inheritance.
#include<iostream>
using namespace std;
class A
{
protected:
int x;
public:
A() { x = 10; }
};
class B
{
protected:
int x;
public:
B() { x = 20; }
};
class C: public A, public B
{
public:
void fun()
{
cout << "A's x is " << A::x;
cout << "nB's x is " << B::x;
}
};
int main()
{
C c;
c.fun();
return 0;
}