Don't override the pure virtual function in derived class

Question

In the following example, the class Base is an abstract class. the Derived class inherits from a Base class. I don't override the pure virtual function in derived class. Then, I have been trying to create derived class object, but the compiler gives following error.

error: cannot declare variable 'd' to be of abstract type 'Derived'
     Derived d;

Code here:

#include<iostream>
using namespace std;

class Base
{
public:
    virtual void show() = 0;
};

class Derived : public Base
{

};

int main(void)
{
    Derived d;
    return 0;
}

Why Derived class also become abstract class?

Yes. The `show` function will be abstract until you define it. — Some programmer dude, Jul 19 '17 at 13:29
I don't feel this question demonstrates any research or thought about the problem. How can `Derived` _not_ be abstract? i.e. if it wasn't, what would happen when someone thinking they had a `Base` tried to call `show()` on it? Nothing good, if somehow the compiler allowed it. — underscore_d, Jul 19 '17 at 13:31
@Stefan Not my downvote but your question is "Is Derived class also become abstract class?" and the compiler error message is: "cannot declare variable 'd' to be of abstract type 'Derived'" seems to me like the compiler answered it for you. — Borgleader, Jul 19 '17 at 13:31
As a sidenote `int main(){ }` is enough. No need for the `void` nor the `return 0;`. — Ron, Jul 19 '17 at 13:36
@Ron I would argue that explicitly writing `return 0;` is good style; omitting it is only allowed because so many people couldn't be bothered to type it that the Committee buckled to make such code well-formed, AFAICT. But yes, that's not how to idiomatically write empty argument lists. — underscore_d, Jul 19 '17 at 13:37
I would imagine any half-decent C++ tutorial/book would cover this. — StoryTeller - Unslander Monica, Jul 19 '17 at 13:49

aoiee · Answer 1 · 2017-07-19T14:16:16.467

2

Yes, Derived is an abstract class. This is because there is a pure virtual method show in the Base class that has been inherited by Derived but never resolved. This means that in order for Derived to become a concrete class, you must first implement a show method either in the Base class itself, or in Derived.

edited Jul 19 '17 at 14:16

answered Jul 19 '17 at 13:30

aoiee

345
3
15

Just a minor quibble over terminology: `Derived` is not "in a way" an abstract class; it simply **is** an abstract class, because it has a pure virtual function that has not been overridden. – Pete Becker Jul 19 '17 at 14:09

score 2 · Answer 2 · answered Jul 19 '17 at 13:32

2

Yes. A pure virtual function must be implemented in derived classes, or else they remain abstract classes: C++ Virtual/Pure Virtual Explained

answered Jul 19 '17 at 13:32

Alex

780
4
12

Don't override the pure virtual function in derived class

2 Answers2