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