Casting pointer from base type to child type

Question

I'm building a simple game design for a project of mine. I have the following classes:

class Character
{
public: 
   virtual void Display();
   virtual void SetParameters( char* param, ... );
};

class NonPlayableCharacter : public Character
{

public:
   virtual void Display();
   virtual void SetParameters( char* paaram, ... );
   int GetNPCState();
}

And then I have a bunch of classes that derive either from Character or NonPlayableCharacter. I define it like so:

std::vector<Character*> _allChar;

My problem is that at any given time I would want to perform some operation on one of the element of the vector. So getting an element out of the vector I can't directly call the method GetNPCState() because the element in the vector are of Character* type. So doing this:

_allChar[0]->GetNPCState();

doesn't work. So I tried doing it with the famous dynamic_cast:

NonPlayableCharacter* test = dynamic_cast<NonPlayableCharacter*>(_allChar[0]);
test->GetNPCState();

The problem with this last attempt is that GetNPCState() crashes because the object is null, and I know for a fact (via debugging) that _allChar[0] isn't null.

Answer 1

There are several types of casts in C++ (4), of which 2 are of interests here:

• static_cast assumes that you know what you are doing
• dynamic_cast checks, at runtime, that you "guessed" right

Note: simplified, as dynamic_cast also allows cross-casts and casts involving virtual bases.

There are 3 versions of dynamic_cast, really, depending on the nature of the target:

• if the target is a reference (ie dynamic_cast<T&>(u)), then if the check fails dynamic_cast throws a std::bad_cast exception
• if the target is a pointer (ie dynamic_cast<T*>(p)), then if the checks fails dynamic_cast returns a null pointer
• finally, as a special case, if the target is void*, then dynamic_cast instead returns the address of the full object

In this case, you may:

• switch from dynamic_cast<NonPlayableCharacter*>(_allChar[0])->getNPCState() to dynamic_cast<NonPlayableCharacter&>(*_allChar[0]).getNPCState(), and let the exception propagates
• test the result of the cast (NonPlayableCharacter* test here) for non-nullity