polymiorphism and access privilege ?

[sun1991]

I recently come to this thought, and did a test:

class B1
{
public:
virtual void Func()
{
std::cout << "Here is B1" << "\n";
}
};

class B2: public B1
{
private:
virtual void Func()
{
std::cout << "Here is B2" << "\n";
}
};

class B3: public B2
{
public:
virtual void Func()
{
std::cout << "Here is B3" << "\n";
}
};

int main()
{
B1* pB2 = new B2();
B1* pB3 = new B3();
pB2->Func();
pB3->Func();

delete pB2;
delete pB3;

return 0;
}

---
Func() in class B2 is under private tag, but seems it has no problem
to be called in main(), why? Is that means when polymiorphism, the
access privilege is just up to base class?

[Barry]

sun1991 wrote:

I recently come to this thought, and did a test:

class B1
{
public:
virtual void Func()
{
std::cout << "Here is B1" << "\n";
}
};

class B2: public B1
{
private:
virtual void Func()
{
std::cout << "Here is B2" << "\n";
}
};

class B3: public B2
{
public:
virtual void Func()
{
std::cout << "Here is B3" << "\n";
}
};

int main()
{
B1* pB2 = new B2();
B1* pB3 = new B3();
pB2->Func();
pB3->Func();

delete pB2;
delete pB3;

return 0;
}

---
Func() in class B2 is under private tag, but seems it has no problem
to be called in main(), why? Is that means when polymiorphism, the
access privilege is just up to base class?

Access control has nothing to do with polymorphism,
Your code compiles, because all the function you call (Func) is a member
of B1 , which is *public*; and pB2 and pB3 are both pointers to B1.
Which Func to called is to be decided during *run-time*.

C++ is more flexible on the derived class with its choosing access
control for overridding functions

In C# and Java,
You can't override a public function with protected/private access.
--
Thanks
Barry

[Victor Bazarov]

sun1991 wrote:

I recently come to this thought, and did a test:

class B1
{
public:
virtual void Func()
{
std::cout << "Here is B1" << "\n";
}
};

class B2: public B1
{
private:
virtual void Func()
{
std::cout << "Here is B2" << "\n";
}
};

class B3: public B2
{
public:
virtual void Func()
{
std::cout << "Here is B3" << "\n";
}
};

int main()
{
B1* pB2 = new B2();
B1* pB3 = new B3();
pB2->Func();
pB3->Func();

delete pB2;
delete pB3;

return 0;
}

---
Func() in class B2 is under private tag, but seems it has no problem
to be called in main(), why?

Because access specifiers and overriding are orthogonal.

Is that means when polymiorphism, the
access privilege is just up to base class?

It's up to the class through which you're calling it. In your case
'pB2' and 'pB3' are both pointers to 'B1', where the function is
declared public. If you make them pointers to 'B2', you won't be
able to call 'Func'.

V
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