If any cast were to be used there, the safest would be dynamic_cast, which requires RTTI (Run Time Type Information) to be enabled at compile time. This guarantees a valid cast if the object is indeed of the type being cast to. reinterpret_cast will work also, without needed RTTI, but is completely "unsafe".

That said, I'm 99% sure that this is a sign of bad design. As a general rule, if you are using inherritance anything that takes a base class should be able to take ANYTHING derived from the base class and function. There are of course the occassional exceptions to this, but this sounds like a case where a little reshuffling might save you a lot of headaches down the line [smile] One simple way I can think of restructuring things given how you've stated them is adding a GetNewData() virtual method to the base class, which the derived classes can use to gather the information it needs.

EDIT: A good line from MSDN on the topic of reinterpret_cast

Quote:The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Other uses are, at best, nonportable.

EDIT2: Completely forgot that while the cast may be legal, in the vast majority of cases it will not infact point to a valid object.

Quote:Original post by ZeroSum
I have a BaseGameState* which holds the currently running state, some derived states have extra functions to pass in data so I would like to cast the BaseGameState* to whatever gamestate is active to then use the functions.

If the behavior of your code depends on the dynamic type of an object, put that code in a virtual function.

Quote:I'm sure I've seen reinterpret_cast used for this but I'm not sure and google has left me slightly confused. Am I on the right track? While on the subject, is this a normal thing to do, or maybe a sign of bad design?

reinterpret_cast does not work for this, precisely because a pointer to a base class has no reason to be a pointer to a derived class. You can either use static_cast (at which point you have to assume that you are right about the dynamic type of the object) or use dynamic_cast. However, in the overwhelming majority of situations, casting to a derived class pointer is a sign of bad design that could be replaced with virtual functions.