Calling subclass methods from superclass object, virtual functions

Question

I'm designing a chess game (C++ beginner) however I'm stuck on my checking process for individual piece types, eg pawn moving only 1 space at a time, except the first time moving 2 spaces, and so on.

I have a piece class, and a subclass of that is pawn, king, etc, which contains the method:

check(string position,string destination);

and return a boolean value whether it is possible to move to the destination.

I have a pointer to each piece which I have defined by doing:

pawn pawnPieces[16];
piece *pointer[16];

for (i=0;i<16;i++)
{
    pointer[i]=&pawnPieces[i];
}

After my initial checking, I want to call the check function above from main, as a test:

cout << pointer[1]->check("B1","C1") << endl;

This gives me the error "no member named 'check' in piece" which makes sense, however I'm sure there would be a way to link the piece class to the pawn, king etc.

I think I need to do something with virtual functions from what I have read, but I am not sure how to approach this. If anyone could offer a few pointers that would be much appreciated.

This approach of trying to call the subclass function from a pointer to the class above it may be fundamentally flawed, perhaps I need to modify my design to achieve this goal? I still want to keep the check method of the pawn class in the same position, as I believe it encapsulates it well.

EDIT: I made a pure virtual function in the piece class:

virtual bool check(string positionIN,string destination)=0;

Now when I call the cout line above, I get a segmentation fault and I'm unsure why. I'm assuming it's because I'm using pointers?

EDIT2: Thank you for that! I have implemented this however I got a small error, is virtual meant to be attached to the pawn and king class? From my understanding I thought the virtual tag only goes on the base class.

EDIT3: I understand, I tagged the check function in classes pawn and king with the virtual tag and it compiled.

Now I am getting a segmentation fault through calling the object itself

pawnPieces[1].check("B1","C1")

and by calling the pointer to the object

pointer[1]->check("B1","C1")

from main, and I am not sure why.

EDIT4: All working now, I was calling it from main to test, however when I called it from within my program everything worked, thank you all!

Well, lookup interfaces and virtual , pure virtual respectively. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Oct 23, 2015 at 0:07
Virtual/pure methods are definitely the way to go. Declare check() in piece, and override it in pawn, king, etc. If you are still having problems, please show your actual code. — Remy Lebeau
– Remy Lebeau, Commented Oct 23, 2015 at 0:33
Edit 2: It only needs to be in the base class, but I'd prefer using it in all of them (as well as override) to make it more obvious. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jan 27, 2016 at 1:23

Remy Lebeau · Accepted Answer · 2015-10-23 00:35:44Z

2

What you are attempting to do is exactly what virtual methods are meant for.

class piece
{
public:
    virtual bool check(string position, string destination) = 0;
};

class pawn : public piece
{
public:
    virtual bool check(string position, string destination)
    {
        return ...;
    }
};

class king : public piece
{
public:
    virtual bool check(string position, string destination)
    {
        return ...;
    }
};

answered Oct 23, 2015 at 0:35

Remy Lebeau

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1 Comment

Homer Simpson Over a year ago

Thank you so much, I am starting to get somewhere!

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