Polymorphism and Dynamic Casting

Question

So I'm working on a text-based RPG, and I've run into an issue. I am currently working on equipping weapons from the character's inventory. I am trying to make it so that my program can tell if the item they want to equip is of class Weapon or not. Here is the clip of relevant code:

 Item tempChosenWeapon = myInventory.chooseItem();
cout << tempChosenWeapon.getName() << endl;
Item *chosenWeapon = &tempChosenWeapon;
cout << chosenWeapon->getName() << endl;//THE CODE WORKS UP TO HERE

Weapon *maybeWeapon = dynamic_cast<Weapon*>(chosenWeapon);
cout << maybeWeapon->getName() << endl;

Now, Weapon is a child class of Item, which is why I am using dynamic cast -- in an attempt to change chosenWeapon, which is of type Item, to type Weapon in order to compare the two classes. (I am using these cout<<s in or to test whether or not calling a function from these objects works).

My program compiles, and everything runs fine until we come to maybeWeapon->getName(), in which the program crashes. I've researched quite a bit, but I just don't understand what I am doing wrong. Any answer or alternative suggestion is much appreciated! Thanks!

Answer 1

The problem

The problem is that you try to make a dynamic cast to a Weapon but in reality the object pointed to is a true copy constructed Item and not a subclass. This is results in a nullptr and UB when you dereference it !

Why ?

Let's suppose that you have only Weapon objects in your inventory. The first instruction in your snippet is the root of your evil:

    Item tempChosenWeapon = myInventory.chooseItem();

This is statement is a copy construction of an Item object. If the source object was a Weapon, it will be sliced.

Later you take a pointer to this object:

    Item *chosenWeapon = &tempChosenWeapon;

But this Item* doesn't point to a Weapon object as you think. It points to a real crude Item object ! So when you do the dynamic cast here:

    Weapon *maybeWeapon = dynamic_cast<Weapon*>(chosenWeapon);

the code will find out that choosenWeapon is not a Weapon*, and the result of dynamic_cast will be a nullptr. Until now it's not necessarily a catastrophe. But when you then derefence this pointer you get UB:

    maybeWeapon->getName()     // OUCH !!!!!! 

Solution

Checking if the dynamic_cast was successful (i.e. result not nullptr) is a protection against the crash, but will not solve your root problem.

It is even possible that the problem is even deeper than expected: what type does the myInventory.chooseItem() return in reality ? Is it a plain Item ? Then you might have the slicing problem already in the inventory !

If you want to use polymorphism:

• you have to work with pointers (preferably smart pointers) or with references, in order not to loose the original type of an object, like it happened here.
• If you need to copy polymorphic objects, you can't just use an assignment with an Item: you'd need to invoke a polymorphic clone() function and ensure that the target of this cloning has a compatible type.

To start with a solution, it's something like this:

Item* chosenWeapon = myInventory.chooseItem();  // refactor choosItem() to return a pointer.
cout << chosenWeapon->getName() << endl; 
Weapon *maybeWeapon = dynamic_cast<Weapon*>(chosenWeapon);
if (maybeWeapon) 
    cout << maybeWeapon->getName() << endl;
else cout << "Oops the chosen item was not a weapon" <<endl; 

If this still not work, then your inventory container would be flawed. In this case, look at this question before opening a separate question with the code of your container

Answer 2

dynamic_cast will return nullptr if the pointer cast cannot be performed (for reference casts it will throw an exception), so your code should read something like:

Weapon *maybeWeapon = dynamic_cast<Weapon*>(chosenWeapon);
if ( maybeWeapon  ) {
   cout << maybeWeapon->getName() << endl;
else {
   // it's not a weapon
}

If you don't perform that test, and try to dereference the pointer containing nullptr, you are off in Undefined Behaviour Land.