CODEWITHSUNDEEP
c++inheritancevirtual
mariner
mariner

Reputation: 930

c++ inheritance virtual functions

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?

Upvotes: 1

Views: 138

Answers (4)

Joel
Joel

Reputation: 5674

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.

Upvotes: 0

Zac Howland
Zac Howland

Reputation: 15870

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;
}

Upvotes: 1

Paweł Stawarz
Paweł Stawarz

Reputation: 4012

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.

Upvotes: 1

ScarletAmaranth
ScarletAmaranth

Reputation: 5101

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().

Upvotes: 4

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