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Simulating final class in C++

  • Difficulty Level : Hard
  • Last Updated : 01 Apr, 2021

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).

Since in C++ 11 there is support for final specifier ,the third example shows its implementation.

c




/* 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. 

C




/* 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

This article is compiled by Gopal Gorthi and reviewed by GeeksforGeeks team. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Want to learn from the best curated videos and practice problems, check out the C++ Foundation Course for Basic to Advanced C++ and C++ STL Course for foundation plus STL.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.



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