Re: Alternative to Abtract Class?

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 16 Apr 2009 18:59:27 -0400

Message-ID:

<daniel_t-22F625.18592716042009@earthlink.vsrv-sjc.supernews.net>

Immortal Nephi <Immortal_Nephi@hotmail.com> wrote:

I want to know. Is first version of abtract class the same as second
version of class with protected constructor() and destructor()?

No. For example:

// This works:
class C2 : public A2 {
   A2 anotherA2;
public:
   //...
};

// but this doesn't
class C : public A {
   A anotherA; // compiler error, can't instantiate an abstract class
public:
   //...
};

Also:

// This works:
void func(A& a) {
   a.print();
}

// but this doesn't
void func(A2& a) {
   a.print(); // class A2 has no member function 'print'
}

Here
is an example. You can't use class A so class B is used to derive
from class A. Also, you can't use class A2 with protected constructor
() so class B2 is used to derive from class A2.

#include <iostream>
using std::cout;
using std::endl;

class A
{
public:
        A() : m_x(0) { cout << "Constructor A" << endl; }
        ~A() { cout << "Destructor A" << endl; }

        void set(int x) { m_x = x; }
        int get() { return m_x; }

        virtual void print() = 0;

private:
        int m_x;
};

class B : public A
{
public:
        B() : A() { cout << "Constructor B" << endl; }
        ~B() { cout << "Constructor B" << endl; }

        void print() { cout << "m_x: " << get() << endl; }
};

class A2
{
protected:
        A2() : m_x(0) { cout << "Constructor A2" << endl; }
        ~A2() { cout << "Destructor A2" << endl; }

        int m_x;
};

class B2 : public A2
{
public:
        B2() : A2() { cout << "Constructor B2" << endl; }
        ~B2() { cout << "Destructor B2" << endl; }

        void set(int x) { A2::m_x = x; }
        int get() { return m_x; }

        void print() { cout << "m_x: " << get() << endl; }
};

int main5()
{
// Compilation Error because of abtract class
// A a;
// a.set( 5 );
// a.print();

        B b;
        b.set( 5 );
        b.print();

// Compilation Error because of protected constructor
// A2 a2;

        B2 b2;
        b2.set( 10 );
        b2.print();

        return 0;
}

Thanks....