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in Technique[技术] by (71.8m points)

inheritance - Public virtual function derived private in C++

I was trying to figure out what happens when a derived class declares a virtual function as private. The following is the program that I wrote

#include <iostream>
using namespace std;
class A
{
    public:
        virtual void func() {
        cout<<"A::func called"<<endl;
    }
    private:
};
class B:public A
{
    public:
    B()
    {
        cout<<"B constructor called"<<endl;
    }
    private:
    void func() {
        cout<<"B::func called"<<endl;
    }
};
int main()
{
    A *a = new B();
    a->func();
    return 0;
}

Surprisingly (for me) the output was:

B constructor called
B::func called

Isn't this violating the private access set for that function. Is this the expected behavior? Is this is a standard workaround or loophole? Are access levels bypassed when resolving function calls through the VTABLE?

Any insight in to this behavior would be greatly helpful.

Further it was mentioned that a privately overriding a virtual member would prevent further classes from inheriting it. Even this is having problems. Modifying the above program to include:

class C: public B
{
    public:
    void func() {
        cout<<"C::func called"<<endl;
    }
};

and the main test program to:

int main()
{
    A *a = new C();
    a->func();
    return 0;
}

output is:

C::func called
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0 votes
by (71.8m points)

This is well-defined behavior. If a were a B* this wouldn't compile. The reason is that member access is resolved statically by the compiler, not dynamically at run-time. Many C++ books suggest that you avoid coding like this because it confuses less experienced coders.


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