Re: Question about polymorphism (or so I believe)

"Victor Bazarov" <>
Fri, 8 Jun 2007 17:16:13 -0400
Aaron wrote:

On Jun 8, 2:12 pm, "Victor Bazarov" <> wrote:

If you're looking to create a "polymorphic" container (or a
container of objects that you could use polymorphically), then you
need to store _pointers_ to the base class in that vector. If you
just store the base class objects, you will slice all the relevant
parts when trying to place the derived objects into that vector.

So this is where the whole void pointer thing comes in?

*Void* pointer? Definitely not. It has to be a pointer to the base

But if the derived class has made modifications that change the amount
of memory it consumes, and I store pointers to the base class instead
of to the derived one...hrm, obviously I'm missing something. I will
implement it using pointers to the base class and wait for the library
to get that book in so I can refresh the whole pointer thing in my

Make sure that you don't mix up addresses of dynamic objects with those
of static or automatic objects in that collection.

You have another option - a virtual function in the base 'Game' class
that should return the requested 'Move' object (by reference or pointer)
to be processed. That would put the burden of storing and maintaining
the collection of 'Move' objects (or their derived variations) on the
derived [from 'Game'] class. Example

    class Move {
        virtual char const* title() const = 0;

    #include <iostream>
    #include <cstring>

    class Game {
        virtual const Move& getMove(size_t ind) const = 0;
        virtual size_t moveCount() const = 0;
        void dump(std::ostream& os) const {
            for (size_t i = 0; i < moveCount(); ++i)
                os << getMove(i).title() << std::endl;

    class MyChessMove : public Move {
        char const* title_;
        char const* title() const { return title_; }
        MyChessMove(const char* t) : title_(t) {}
        friend class MyChessGame;

    #include <vector>

    class MyChessGame : public Game {
        std::vector<MyChessMove> history;
        MyChessGame() { history.push_back("e2-e4");
            history.push_back("I give up");
        const Move& getMove(size_t ind) const { return history[ind]; }
        size_t moveCount() const { return history.size(); }

    class MyBlackjackMove : public Move {
        size_t value;
        const char* title() const {
            char c = "_A23456789xJQK"[value % 14];
            char const *suit[] =
                { "Spades", "Clubs", "Diamonds", "Hearts" };
            static char what_of_what[32];
            what_of_what[0] = c; what_of_what[1] = 0;
            strcat(what_of_what, " of ");
            strcat(what_of_what, suit[value / 100]);
            return what_of_what;
        MyBlackjackMove(size_t card, size_t suit)
            : value((suit % 4)*100 + card % 14) {}

        friend class MyBlackjackGame;

    #include <list>

    class MyBlackjackGame : public Game {
        std::list<MyBlackjackMove> hand;
        MyBlackjackGame() {
            hand.push_back(MyBlackjackMove(1, 10));
            hand.push_back(MyBlackjackMove(2, 12));
        const Move& getMove(size_t ind) const {
            std::list<MyBlackjackMove>::const_iterator it = hand.begin();
            return std::advance(it, ind), *it;
        size_t moveCount() const { return hand.size(); }

    int main() {
        MyChessGame chess;
        MyBlackjackGame blackjack;



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