How to prevent a method from being overridden in derived class?

Question

How can I enforce that a base class method is not being overridden by a derived class?

Don't make it virtual in the base class? Then it can't be overridden, only overloaded. There is no direct equivalent to Java's `final` methods, if that's the question. — Steve Jessop, Dec 16 '10 at 21:51
so without using a virtual key word we cannot make it, so how did they implemented in JAVA then — sriks, Dec 17 '10 at 13:15
in Java, marking a method `final` forbids subclasses from implementing that method, and the compiler and/or byte code verifier will enforce this. In C++, there's no way to forbid it, it's just that by definition of "override", only virtual functions can be "overridden". — Steve Jessop, Dec 17 '10 at 14:33
Note that there is a `final` specifier added with C++11: http://stackoverflow.com/a/16896559/1025391 — moooeeeep, Jun 03 '13 at 12:37

moooeeeep · Answer 1 · 2017-07-03T08:38:25.300

If you are able to use the final specifier from C++11 you can prevent derived classes from override that method. (Microsoft compilers appear to support the similar sealed with similar semantics.)

Here's an example:

#include <iostream>

struct base {
    // To derived class' developers: Thou shalt not override this method
    virtual void work() final {
        pre_work();
        do_work();
        post_work();
    }
    virtual void pre_work() {};
    virtual void do_work() = 0;
    virtual void post_work() {};
};

struct derived : public base {
    // this should trigger an error:
    void work() {
        std::cout << "doing derived work\n";
    }
    void do_work() {
        std::cout << "doing something really very important\n";
    }
};

int main() {
    derived d;
    d.work();
    base& b = d;
    b.work();
}

Here's what I get when I try to compile it:

$ g++ test.cc -std=c++11
test.cc:17:14: error: virtual function ‘virtual void derived::work()’
test.cc:5:22: error: overriding final function ‘virtual void base::work()’

Question: Why should I declare "work()" as "virtual" when I don't want that it is overridden in derived classes - why not just mark it as final? — Sonic78, Dec 11 '17 at 09:13
@Sonic78 Good question! In real world code, you might have another base class to `base`, that provides a virtual function that you want to prevent to be overridden in classes deriving from your intermediate base. — moooeeeep, Dec 12 '17 at 07:55

score 19 · Accepted Answer · edited Aug 16 '19 at 14:35

19

If you make the method non-virtual, derived classes cannot override the method. However, in C++03 a class cannot override a method from a base class, and also prevent further derived classes from overriding the same method. Once the method is virtual, it stays virtual.

edited Aug 16 '19 at 14:35

plasmacel

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answered Dec 16 '10 at 21:51

Greg Hewgill

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with C++11 you can use final specifier to avoid a method being overridden in derived classes – tharinduwijewardane Feb 10 '16 at 07:43

score 3 · Answer 3 · answered Dec 16 '10 at 21:52

Don't make it virtual.

This won't prevent deriving from your class and hiding the function (by providing another member function with the same name). However, if your class is not meant to be derived anyway (no virtual destructor, no virtual member functions), that shouldn't be an issue.

Stephane Rolland · Answer 4 · 2010-12-19T01:21:01.107

1

well if you want to keep it public, dont declare it virtual.

EDIT: Srikanth commented wondering about overriding a private member function in a derived class.

class A
{
public:
    virtual ~A(){};
    void test()
    {
        foo();
    };
private:
    virtual void foo()
    {
        std::cout << "A";   
    };
};


class B : public A
{
public:
    virtual void foo()
    {
        std::cout << "B";   
    };
};


void test()
{
    B b;
    A& a = b;

    a.test(); // this calls the derived B::foo()

    return 0;
}`

edited Dec 19 '10 at 01:21

answered Dec 16 '10 at 21:51

Stephane Rolland

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Derived classes can still override virtual private member functions. – Fred Larson Dec 16 '10 at 22:01
I have edited, thanx for the correction. – Stephane Rolland Dec 16 '10 at 22:21
but how can we override a private method without making the derived a friend or decreasing the access specifier – sriks Dec 18 '10 at 23:28
@Srikanth, I have edited the answer and added an example code. – Stephane Rolland Dec 19 '10 at 01:18

score 0 · Answer 5 · answered Dec 16 '10 at 21:53

0

well as far as i know you can't do that on c++, you can try to declare it as private. . find more info on this link http://en.allexperts.com/q/C-1040/prevent-overriding-functions-derived.htm

answered Dec 16 '10 at 21:53

Luponk

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Derived classes can still override virtual private member functions. – Fred Larson Dec 16 '10 at 22:02

score -1 · Answer 6 · answered Dec 16 '10 at 23:30

-1

Short answer: there is no need for that. Long answer you can do some twists, but is it worth it?

answered Dec 16 '10 at 23:30

Gene Bushuyev

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How to prevent a method from being overridden in derived class?

6 Answers6

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