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.
CPP
#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;
}
|
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:
CPP
#include <iostream>
class Base final
{
};
class Derived : public Base
{
};
int main()
{
Derived d;
return 0;
}
|
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.
CPP
class Test
{
final void fun()
{ }
}
class Test
{
public :
virtual void fun() final
{}
};
|
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.
CPP
#include <iostream>
using namespace std;
int main()
{
int final = 20;
cout << final;
return 0;
}
|
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.
Last Updated :
03 Nov, 2020
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