Visibility Modes in C++ with Examples

When a base class is derived by a derived class with the help of inheritance, the accessibility of base class by the derived class is controlled by visibility modes. The derived class doesn’t inherit access to private data members. However, it does inherit a full parent object, which contains any private members which that class declares. 

Compile Errors:

prog.cpp: In function 'int main()':
prog.cpp:14:6: error: 'int A::y' is protected
 int y;
 ^
prog.cpp:34:12: error: within this context
 cout << b.y << endl;
 ^
prog.cpp:17:6: error: 'int A::z' is private
 int z;
 ^
prog.cpp:37:12: error: within this context
 cout << b.z << endl;
 ^

• Since the member 'x' in A is public, its visibility will be open to all. It means that any class can access and use this x. That is the reason there is no error in 'b.x'. 
  
• The member 'y' in A is protected, its visibility will be only to the derived class. It means that any derived class can access and use this y.
• The member 'z' in A is private, its visibility will not be open to any other class. It means that any derived class cannot access and use this z.

There are three types of Visibility modes: 

1. Public Visibility mode: If we derive a subclass from a public base class. Then the public member of the base class will become public in the derived class and protected members of the base class will become protected in the derived class.
   

1. 
   Compile Errors:
   

prog.cpp: In function 'int main()':
prog.cpp:14:9: error: 'int A::y' is protected
 int y;
 ^
prog.cpp:37:15: error: within this context
 cout << b.y << endl;
 ^
prog.cpp:17:9: error: 'int A::z' is private
 int z;
 ^
prog.cpp:42:15: error: within this context
 cout << b.z << endl;
 ^

1. 
   
2. Protected Visibility mode: If we derive a subclass from a Protected base class. Then both public member and protected members of the base class will become protected in the derived class.
   

1. 
   Compile Errors:
   

prog.cpp: In function 'int main()':
prog.cpp:11:9: error: 'int A::x' is inaccessible
 int x;
 ^
prog.cpp:33:15: error: within this context
 cout << b.x << endl;
 ^
prog.cpp:14:9: error: 'int A::y' is protected
 int y;
 ^
prog.cpp:37:15: error: within this context
 cout << b.y << endl;
 ^
prog.cpp:17:9: error: 'int A::z' is private
 int z;
 ^
prog.cpp:42:15: error: within this context
 cout << b.z << endl;
 ^

1. 
   
2. Private Visibility mode: If we derive a subclass from a Private base class. Then both public member and protected members of the base class will become Private in the derived class. 
   

1. 
   Compile Errors:
   

prog.cpp: In function 'int main()':
prog.cpp:11:9: error: 'int A::x' is inaccessible
 int x;
 ^
prog.cpp:33:15: error: within this context
 cout << b.x << endl;
 ^
prog.cpp:14:9: error: 'int A::y' is protected
 int y;
 ^
prog.cpp:37:15: error: within this context
 cout << b.y << endl;
 ^
prog.cpp:17:9: error: 'int A::z' is private
 int z;
 ^
prog.cpp:42:15: error: within this context
 cout << b.z << endl;
 ^

After inheriting a base class with the help of a specific Visibility mode, the members will automatically change its visibility as mentioned above. But inorder to change the visibility after this inheritance, we need to do it manually.
Syntax:

<visibility_mode>:
 using base::<member>;

For example:

// inorder to change the visibility of x to public
<public>:
 using base::<x>;

Example: Consider a base class containing a public member 'a', protected members 'b' and 'c', and private members 'd' and 'e'. Below program explains how to change the visibility of 'b' from protected to public.

Compile Errors:

prog.cpp: In function 'int main()':
prog.cpp:22:9: error: 'int BaseClass::d' is private
 int d;
 ^
prog.cpp:47:26: error: within this context
 cout << derivedClass.d << endl;
 ^