• Ever wondered how can you design a class in C++ which can’t be inherited. Java and C# programming languages have this feature built-in. You can use final keyword in java, sealed in C# to make a class non-extendable.
• Below is a mechanism using which we can achieve the same behavior in C++. It makes use of private constructor, virtual inheritance and friend class.
• In the following code, we make the Final class non-inheritable. When a class Derived tries to inherit from it, we get compilation error.
• An extra class MakeFinal (whose default constructor is private) is used for our purpose. Constructor of Final can call private constructor of MakeFinal as Final is a friend of MakeFinal .
• Note that MakeFinal is also a virtual base class. The reason for this is to call the constructor of MakeFinal through the constructor of Derived, not Final (The constructor of a virtual base class is not called by the class that inherits from it, instead the constructor is called by the constructor of the concrete class).

/* A program with compilation error to demonstrate that Final class cannot
be inherited */
#include<iostream>
using namespace std;

class Final; // The class to be made final

class MakeFinal // used to make the Final class final
{
private:
MakeFinal() { cout << "MakFinal constructor" << endl; }
friend class Final;
};

class Final : virtual MakeFinal
{
public:
Final() { cout << "Final constructor" << endl; }
};

class Derived : public Final // Compiler error
{
public:
Derived() { cout << "Derived constructor" << endl; }
};

int main(int argc, char *argv[])
{
Derived d;
return 0;
}
Output: Compiler Error

In constructor 'Derived::Derived()':
error: 'MakeFinal::MakeFinal()' is private

• In the above example, Derived‘s constructor directly invokes MakeFinal’s constructor, and the constructor of MakeFinal is private, therefore we get the compilation error.
• You can create the object of Final class as it is friend class of MakeFinal and has access to its constructor. For example, the following program works fine.

/* A program without any compilation error to demonstrate that instances of
the Final class can be created */
#include<iostream>
using namespace std;

class Final;

class MakeFinal
{
private:
MakeFinal() { cout << "MakeFinal constructor" << endl; }
friend class Final;
};

class Final : virtual MakeFinal
{
public:
Final() { cout << "Final constructor" << endl; }
};

int main(int argc, char *argv[])
{
Final f;
return 0;
}
Output: Compiles and runs fine

MakeFinal constructor
Final constructor
C++ 11 Update :-
In C++ 11 we can make the base class non-inheritable by using final specifier. For eg, the following code gives a compile error as the base class is declared as final.

C++

#include <iostream>
using namespace std;

class Base final {
// body
};
class Derived : public Base // compile error because base class is final
{
// body
};

int main() { return 0; }
Output:

prog.cpp:8:7: error: cannot derive from ‘final’ base ‘base’ in derived type ‘derive’
class derive: public base // compile error because base class is final