Dynamic down cast to abstract class (C++)

Question

I am trying to learn some object orientated programming aspect I know from java in C++. However I am having some difficulties in using dynamic_cast where I would use instanceof in Java.

I have a base class Cell and a derived (abstract) class Obstacle. I have defined it like this: Obstacle : public Cell and Obstacle contains a pure virtual destructor. Now in the Cell class I want to implement a method bool Cell::isAccesible(). I've implemented this as follows:

bool Cell::isAccessible() { Obstacle *obs = dynamic_cast<Obstacle*>(this); if (obs != NULL) return false; return true; }

However I get the following error back:

"the operand of a runtime dynamic_cast must have a polymorphic class type".

What's wrong with the way I want to implement this? Any guidance is appreciated.

Does this have anything to do with visual studio? Is it their error? (I'm asking because of the tag) — keyser
– keyser, Commented Apr 15, 2014 at 16:35
This seems like a convoluted way to solve this problem. If isAccessible were a virtual method then Obstacle could override it and simply return false and this would no longer be an issue. — YoungJohn
– YoungJohn, Commented Apr 15, 2014 at 16:39
I don't know what you're trying to do, but the error is because your Cell class is not polymorphic i.e. it does not have any virtual methods. If you add a virtual method to Cell your code should compile. — atoMerz
– atoMerz, Commented Apr 15, 2014 at 16:43
Using instanceof in Java or dynamic_cast to perform type switches in C++ is something you should avoid in the first place. You are creating cyclic dependencies as the base needs to know about the derived and vice versa. — David Rodríguez - dribeas
– David Rodríguez - dribeas, Commented Apr 15, 2014 at 17:06

Neil Kirk · Accepted Answer · 2014-04-15 16:45:25Z

20

Cell class must have at least one virtual function to use dynamic_cast. Also, if Cell is your base class, it should have a virtual destructor.

You should make isAccessible a virtual function and override it in Obstacle to return false.

answered Apr 15, 2014 at 16:45

Neil Kirk

22k9 gold badges57 silver badges96 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

mradul dubey Over a year ago

Can you please elaborate on why a virtual destructor is required ?

Neil Kirk Over a year ago

@mraduldubey stackoverflow.com/questions/461203/…

atoMerz · Accepted Answer · 2014-04-15 16:47:02Z

What you're doing is wrong. Generally you shouldn't need to cast to a sub type of a class in its base class. If you need it, it is likely a design error. In your case the code should look like this.

virtual bool Cell:: isAccessible() { return true; } bool Obstacle::isAccessible() { return false; }

P.S. The cause of your error is that Cell class does not have a virtual method and thus it does not show polymorphic behaviour.

Thanks. However apart from the fact I was trying to solve the problem the wrong way (by casting) why should Cell always have at least one virtual method in order to be able to use a dynamic_cast?
@tim_a dynamic_cast and virtual functions need some information to be embedded in the object, but this increases its size slightly, so C++ lets you choose whether to add the information.

Collectives™ on Stack Overflow

Dynamic down cast to abstract class (C++)

2 Answers 2

2 Comments

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Linked

Related