I have a vector of Base and Derived Class Objects, but I can't access data members inherited by derived objects that are stored in the vector

Question

Bear with me, I'm relatively new at C++.

Here is my project. I want to essentially create a simple game of chess.

I've got a base class gamePiece, which I'm thinking will eventually become an abstract class ( but right now it isn't)

The base class gamePiece has a bunch of data members in it: colorOfPiece, rankOfPiece, fileOfPiece, etc. It also has a function, void displayPieceInfo() which simply displays all of these values on the console via cout.

I am planning on having a number of derived classes from this one. Right now I have a "rook" subclass.

I want to add various types of pieces in a single vector, so I can later traverse it with an iterator.

Here is the problem I am running into.

vector<gamePiece> vectorOfAllGamePieces;
vector<gamePiece>::iterator itGamePieces;

I push_back a Rook into the vector as the first element. All the constructors look like they are running fine, initializing the variables. Yet when I try to run the display function on the first element, the strings are empty/unitialized.

itGamePieces=vectorOfAllGamePieces.begin();
itGamePieces->displayPieceInfo();

If I were to push in a generic gamePiece into the vector instead, everything would display properly.

Am I allowed to even have a vector like this with mixed types of objects -- for example, an object of a derived class, an object of the parent class, an object of a second class derived from the same parent

And if so, why do you suppose the values of the data members aren't showing up properly, even after I have set them in the constructor?

The joys of slicing. You don't, in fact, have a vector of both base and derived. That vector only contains bases. — Mat
– Mat, Commented Feb 17, 2013 at 19:33

Luchian Grigore · Accepted Answer · 2013-02-17 19:33:46Z

3

Not really, because of slicing. When you add a derived object to a vector<gamePiece>, it gets sliced and a gamePiece object is actually added to the vector.

You'll need to use a vector of (smart) pointers.

answered Feb 17, 2013 at 19:33

Luchian Grigore

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Deamonpog · Accepted Answer · 2013-02-17 19:46:53Z

The problem is like this, Rook is a GamePiece. But You Cannot Store a Rook in a place for GamePieces. You can only cast a Pointer to Rook to a Pointer to GamePiece. So your vector must be saving NOT GamePieces but GamePiece * (Pointers to GamePieces).

std::vector<GamePiece *> vectorOfAllGamePieces;

But this could give you trouble in memory management. Another way is to store Handles without using pointers. ( a handle is something like a pointer to a pointer ). Or of course you can use smart pointers.

A quick hack will be ( this is a "hack" which is "evil" in the eyes of some people ),

std::vector<Rook> allRooks;
std::vector<Pawns> allPawns;
.....
std::vector<Queen> allQueens;
King theKing;

std::vector<GamePiece *> allPieces;

The allPieces will contain pointers to objects in the other vectors. I think this is kind of OK when you know exactly all the types of pieces ( i guess this is chess ).

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