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.

Solution 1

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.

Solution 2

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.

Comments

[keyser]

Does this have anything to do with visual studio? Is it their error? (I'm asking because of the tag)

[tim_a]

I am coding this in visual studio that's all. I get the error while coding but also during compiling (C2683).

[atoMerz]

Why are you casting Cell to obstacle in a Cell method?

[YoungJohn]

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.

[atoMerz]

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.

[David Rodríguez - dribeas]

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.

[tim_a]

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?

[Neil Kirk]

@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.

[mradul dubey]

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

[Neil Kirk]

@mraduldubey stackoverflow.com/questions/461203/…