A protected method can become a private method when inherited in c++?

Question

I've been playing around with inheritance and I've tried this code:

#include <iostream> 
#include <string>

class Foo
{
public:
    virtual void func() = 0;

protected:
    virtual void doSum() const = 0;
};

class Bar : public Foo
{
public:
    void func() { 
        doSum();
    }

protected:
    void doSum() const
    {
        std::cout << "hi, i'm doing something" << std::endl;
    }
};

int main() 
{ 
    Foo* ptr = new Bar();
    ptr->func();

   return 0; 
}

So I've also tried replacing the protected keyword in the class Bar with private like this :

private:
    void doSum() const
    {
        std::cout << "hi, i'm doing something" << std::endl;
    }

and the code happened to work just the same...

So my question is, is there any difference if I declare a protected method private when implementing a derived class? If so, what are they? Am I even allowed to do this?

R Sahu · Accepted Answer · 2019-04-08T16:14:20.340

2

So my question is, is there any difference if I declare a protected method private when implementing a derived class?

Yes.

If so, what are they?

That will prevent the next level of derived class from being able to call the derived class's implementation.

class Foo
{
   protected:
      virtual void doSum() const = 0;
};

class Bar : public Foo
{
   private:
      void doSum() const
      {
         std::cout << "hi, i'm doing something" << std::endl;
      }
};

class Baz : public Bar
{
   public:
      void doSum() const
      {
         //===========================
         Bar::doSum(); // NOT ALLOWED
         //===========================
      }
};

Am I even allowed to do this?

Yes.

edited Apr 08 '19 at 16:14

answered Apr 08 '19 at 16:04

R Sahu

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Sure. but the `fooPtr->doSum()` would still end up calling the `Baz` implementation. – cplusplusrat Apr 08 '19 at 16:07
Thanks for the detailed answer! So, if i'm not intending to derive Bar, should I rather let it be protected or make it private? Or there's literally no relevance? – grybouilli Apr 08 '19 at 16:07
@Grybouilli, if you are absolutely certain, then yes, then it is not relevant. – R Sahu Apr 08 '19 at 16:08
1

@Grybouilli "So, if i'm not intending to derive Bar, should I rather let it be protected or make it private?" - If you do not want it to be possible to derive from `Bar`, you should make the class `final`. You can also make individual functions `final` to prevent them from being overridden while still letting others derive from `Bar`. – Jesper Juhl Apr 08 '19 at 16:12

score 1 · Answer 2 · edited Apr 08 '19 at 16:10

So my question is, is there any difference if I declare a protected method private when implementing a derived class?

No. There is no difference. Unfortunately, C++ standard does not impose any requirement on the derived class to place the overriding virtual function within any particular accessibility scope. This means, the base class could declare a virtual method protected, and the derived class could implement the method in public/protected/private scope and the code will still be legal and will work.

A protected method can become a private method when inherited in c++?

2 Answers2