c++ inheritance virtual functions

Question

I am trying to learn inheritance in c++. I wrote some code to learn virtual functions.

#include <iostream>

using namespace std;

class A {
  int a;

  public:
    A() {}
    virtual int get_count() const = 0;
    int get_A() { return a; }
};

class B : public  A{
  public:
    int b;

    B() {}

    B(A& base)
      : b(base.get_count()) {}

    virtual int get_count() const { return 10; }
};

void func(A& base) {
  B derived(base);
  cout << derived.b;
}

int main() {
  A base;
  B derived;

  func(derived);
}

When I try to compile I get this error:

test_inheritance_vir.cpp: In function ‘int main()’:
test_inheritance_vir.cpp:32: error: cannot declare variable ‘base’ to be of abstract type ‘A’
test_inheritance_vir.cpp:5: note:   because the following virtual functions are pure within ‘A’:
test_inheritance_vir.cpp:10: note:  virtual int A::get_count() const

Can you please tell me what I am doing wrong?

ScarletAmaranth · Accepted Answer · 2014-01-07 22:32:38Z

4

You are trying to instantiate an object of type A with A base;. It's not possible as A contains a pure virtual function. (get_count()) Suppose I tried calling base.get_count().

answered Jan 7, 2014 at 22:32

ScarletAmaranth

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Comments

Paweł Stawarz · Accepted Answer · 2014-01-07 22:33:54Z

1

The method virtual int get_count() const = 0; is pure virtual. You can't create an object of a class that is abstract (or in other words - has a pure virtual member). If you want to create an object of A, remove the = 0 and define the function (with an empty body if you need):

virtual int get_count() const{};

should work.

answered Jan 7, 2014 at 22:33

Paweł Stawarz

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4 Comments

SOReader Over a year ago

This is not the problem. He is trying to create A instance as main's first variable and this is logical error. Should be A* or A& and and instance of a derived class.

Paweł Stawarz Over a year ago

What if he's creating the instance because he wants? :) And if he wants - he can do it.

SOReader Over a year ago

if so it shouldn't be purely virtual. It's a design issue

Paweł Stawarz Over a year ago

Yes, it shouldn't be purely virtual. That's what I've said in my answer. So - something's wrong? :)

Zac Howland · Accepted Answer · 2014-01-07 22:35:44Z

1

The way you have implemented A (below) causes it to be an abstract base class.

class A 
{
  int a;

  public:
    A() {}
    virtual int get_count() const = 0; // this is a pure virtual function
    int get_A() { return a; }
};

It can only be used as a pointer to a derived class that implements the pure virtual functions:

int main() 
{
    B derived;
    A* pA = new B; // this is okay

    delete pA;
    return 0;
}

answered Jan 7, 2014 at 22:35

Zac Howland

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Comments

Joel · Accepted Answer · 2014-01-07 23:34:58Z

0

The answers above are the technical reasons why it won't work, but there's a more insidious problem in that the design doesn't make sense. You're creating a B to add some functionality to your A. If you also create a C that extends A, do you really want to turn that into a B?

The classic example of inheritance is animals. Zebra and Giraffe are both Animals, so a class hierarchy would look like this

class Animal 
{
    stuff
}

class Zebra : public Animal
{
    more stuff
}

class Giraffe : public Animal
{
    different stuff
}

It doesn't make much sense to turn a Zebra into a Giraffe, though, even though both are animals.

edited Jan 7, 2014 at 23:34

answered Jan 7, 2014 at 22:41

Joel

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