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The components of a class such as its data members or functions are called members of a class i.e. class members. A class member is declared with an visibility labels, that specifies how it will be accessed outside its class. A C++ class member can take any of the visibility labels, such as - public, protected and private.

Public Visibility Label

A class member, be it an data variable or a function can be declared with the public keyword. A class member declared with a public visibility label is accessible and visible outside its class by using the object of its class. Let's see example-


A.cpp

#include<iostream>

using namespace std;

class A
{
public:
int a = 10;
void message(); 	//declaration of the function named message()
};


//Defition of the function message of class A
void A :: message()
{
cout<< "Hello from the class A";
}



int main()
{
A ob;		//Creating an object of class A

//Accessing the data member a, associated with the object of class A
cout<< "The value of a : " << ob.a << "\n";

//Calling the function message of class A, using its object
ob.message();
}

Output is :

The value of a : 10
Hello from the class A

Program Analysis

• In this code, we have created a class with two members - a data member named a and a function named message(), declared under public visibility mode.
• Next, the main() function first creates an object of class A and then use it with a dot operator to access the value of the data member named a of class A.
• Using the same object of class A with a dot operator, this main() function also calls the function named message() of class A.

Note :

Class A has declared the function message() and data member a with a public visibility mode and that's why it is visible and accessible outside class A(in the main() function), using its object.

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Private Visibility Label

A class member - data member or function declared with the private keyword will be hidden and inaccessible outside its class and it won't serve any purpose. Private member of a class is only accessible in the class in which it is declared or defined and not outside it. Let's see an example -

A.cpp

#include<iostream>

using namespace std;

class A
{
public:
int a = 10;
void message(); 	//declaration of the function named message()
};


//Defition of the function message of class A
void A :: message()
{
cout<< "Hello from the class A";
}



int main()
{
A ob;		//Creating an object of class A

//Accessing the data member a, associated with the object of class A
cout<< "The value of a : " << ob.a << "\n";

//Calling the function message of class A, using its object
ob.message();
}

B.java

class B
{
public static void main(String... ar)
{
A ob= new A();
System.out.println(ob.a);
}
}

Output is -

A.cpp: In function 'int main()':
A.cpp:26:34: error: 'int A::a' is private within this context
 cout<< "The value of a : " << ob.a << "\n";
                                  ^
A.cpp:8:9: note: declared private here
 int a = 10;
         ^~
A.cpp:29:12: error: 'void A::message()' is private within this context
 ob.message();
            ^
A.cpp:14:6: note: declared private here
 void A :: message()

Program Analysis

Compiler shows a compile error which clearly states that data member named a of class A and the function message has a private access, hence it can't be used outside class A(in which it is declared and defined.)

Protected Visibility Label

A C++ class member such as - data member or function declared with the protected keyword will be accessible and visible to -

• Inside its class.
• Inside the subclasses i.e. the class which inherits its parent class(more on it in protected visibility label in inheritance.
  Let's understand this by an example -
  
  A.cpp

  #include<iostream>
  
  using namespace std;
  
  class A
  {
  protected:
  int a = 10;
  void message(); 	//declaration of the function named message()
  };
  
  
  //Defition of the function message of class A
  void A :: message()
  {
  cout<< "Hello from the class A";
  }
  
  
  
  int main()
  {
  A ob;		//Creating an object of class A
  
  //Accessing the data member a, associated with the object of class A
  cout<< "The value of a : " << ob.a << "\n";
  
  //Calling the function message of class A, using its object
  ob.message();
  }
  

  
  
  

  After compiling :

  
  

  A.cpp: In function 'int main()':
  A.cpp:26:34: error: 'int A::a' is protected within this context
   cout<< "The value of a : " << ob.a << "\n";
                                    ^
  A.cpp:8:9: note: declared protected here
   int a = 10;
           ^~
  A.cpp:29:12: error: 'void A::message()' is protected within this context
   ob.message();
              ^
  A.cpp:14:6: note: declared protected here
   void A :: message()
        ^
  

  
  
  

  Program Analysis

  
  Compiler shows a compile error which clearly states that data member named a of class A and the function message has a protected access, hence it can't be used outside class A(within the main() function)
  
  
  
  
  

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