Re: Virtual function behaviour

From:

Fei Liu <feiliu@aepnetworks.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 04 Apr 2007 09:51:53 -0400

Message-ID:

<ev0ahn$ftl$1@aioe.org>

dragoncoder wrote:

Hello experts,

I was just playing around wrote this code.

sundev1:/home/ptiwary/rnd $ cat a1.cpp
#include <iostream>

using namespace std;

class Base
{
    public:
        virtual void foo() { cout << "In Base::foo()" << endl; bar();}
    private:
        void bar() { cout << "In Base::bar()" << endl; }
};

class Der1: public Base
{
    public:
        virtual void foo() { cout << "In Der1::foo()" << endl; bar();}
    private:
        void bar() { cout << "In Der1::bar()" << endl; }
};

class Der2: public Base
{
    public:
        virtual void foo() { cout << "In Der2::foo()" << endl; bar();}
    private:
        void bar() { cout << "In Der2::bar()" << endl; }
};

int main()
{
    Base* b1 = new Base();
    Base* b2 = new Der1();
    Base* b3 = new Der2();

    b1->foo();
    b2->foo();
    b3->foo();

    return 0;
}

sundev1:/home/ptiwary/rnd $ g++ a1.cpp
sundev1:/home/ptiwary/rnd $ ./a.out
In Base::foo()
In Base::bar()
In Der1::foo()
In Der1::bar()
In Der2::foo()
In Der2::bar()

I have 2 questions regarding this.

1. Is the behaviour correct? Because someone told me I need to make
bar() also virtual to get the effect.

The behavior is correct. This is the idea of polymorphism, the run time
automatically picks up the correct function to use.

2. What is the deal with private virtual functions? Even if I make
bar() virtual, as its private, it won't be accessible from the derived
classes Der1 and Der2 so it does not make any sense having private
virtual functions. Am I right?

No, virtual tells the compiler/linker that instead of static linking,
let the run time decide which function to use. Check the following example:

#include <iostream>

using namespace std;

class Base
{
     public:
         void foo() { cout << "In Base::foo()" << endl; bar();}
     private:
         virtual void bar() { cout << "In Base::bar()" << endl; }
};

class Der1: public Base
{
     public:
         void foo() { cout << "In Der1::foo()" << endl; bar();}
     private:
         virtual void bar() { cout << "In Der1::bar()" << endl; }
};

class Der2: public Base
{
     public:
         void foo() { cout << "In Der2::foo()" << endl; bar();}
     private:
         virtual void bar() { cout << "In Der2::bar()" << endl; }
};

int main()
{
     Base* b1 = new Base();
     Base* b2 = new Der1();
     Base* b3 = new Der2();

     b1->foo();
     b2->foo();
     b3->foo();

     return 0;
}

Thanks in advance.