C++ final specifier

Cast all your worries on him,
because he cares for you.
1 Peter 5: 7 (Bible)

In Java, we can use final for a function to make sure that it cannot be overridden. We can also use final in Java to make sure that a class cannot be inherited. Similarly, the latest C++ standard C++ 11 added final.

Use of final specifier in C++ 11:
Sometimes you don’t want to allow derived class to override the base class’ virtual function. C++ 11 allows built-in facility to prevent overriding of virtual function using final specifier.
Consider the following example which shows use of final specifier. This program fails in compilation.

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#include <iostream>
using namespace std;
  
class Base
{
public:
    virtual void myfun() final
    {
        cout << "myfun() in Base";
    }
};
class Derived : public Base
{
    void myfun()
    {
        cout << "myfun() in Derived\n";
    }
};
  
int main()
{
    Derived d;
    Base &b = d;
    b.myfun();
    return 0;
}

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Output:

prog.cpp:14:10: error: virtual function ‘virtual void Derived::myfun()’
     void myfun()
          ^
prog.cpp:7:18: error: overriding final function ‘virtual void Base::myfun()’
     virtual void myfun() final 

2nd use of final specifier:
final specifier in C++ 11 can also be used to prevent inheritance of class / struct. If a class or struct is marked as final then it becomes non inheritable and it cannot be used as base class/struct.
The following program shows use of final specifier to make class non inheritable:

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#include <iostream>
class Base final
{
};
  
class Derived : public Base
{
};
  
int main()
{
    Derived d;
    return 0;
}

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Output:

error: cannot derive from ‘final’ base ‘Base’ in derived type ‘Derived’
 class Derived : public Base

final in C++ 11 vs in Java
Note that use of final specifier in C++ 11 is same as in Java but Java uses final before the class name while final specifier is used after the class name in C++ 11. Same way Java uses final keyword in the beginning of method definition (Before the return type of method) but C++ 11 uses final specifier after the function name.

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class Test
{
    final void fun()// use of final in Java
    { }
}
class Test
{
public:
    virtual void fun() final //use of final in C++ 11
    {}
};

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Unlike Java, final is not a keyword in C++ 11. final has meaning only when used in above contexts, otherwise it’s just an identifier.
One possible reason to not make final a keyword is to ensure backward compatibility. There may exist production codes which use final for other purposes. For example the following program compiles and runs without error.

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#include <iostream>
using namespace std;
  
int main()
{
    int final = 20;
    cout << final;
    return 0;
}

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Output:

20

In java, final can also be used with variables to make sure that a value can only be assigned once. this use of final is not there in C++ 11.

This article is contributed Meet Pravasi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above



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