Thread: polymorphism

I've run into a small problem regarding polymorphism. I have a base class named TTT (tic tac toe), I then have 2 derived classes, OnlineGame, and OfflineGame. At the beginning of the program I make a pointer to TTT, then make it a new OnlineGame or OfflineGame depending on user input. My problem is that OnlineGame will need a function to accept the opponent's IP address. I made said function virtual, but now it's requiring me to declare it inside OfflineGame which is illogical to my design. Is there a better way to do this, or am I stuck defining a SetIPAddress for both classes?

This is my current code. It works, but I don't like it.

Code:

class TTT
{
public:
	TTT();
	void GetUserMove();
	bool GameOver();
	virtual void SetIPAddress(string &addr) = 0;
	virtual void GetOpponentMove();
};

class OnlineGame : public TTT
{
public:
	OnlineGame();
	void SetIPAddress(string &addr);
	void GetOpponentMove();
};

class OfflineGame : public TTT
{
public:
	OfflineGame();
	void SetIPAddress(string &addr);
	void GetOpponentMove();
};

Try to dynamic_cast your pointer to the online version. If the pointer is NULL, the cast did not work (it was not the online version), and proceed as normal. If the cast worked, then set the IP address.

Thanks for the replies. Spark, when I try it that way I get an error.

Code:

TTT *Game;
if (menu_choice() == 2) // Setup for online play
{
	Game = new OnlineGame;
	Game->SetIPAddress(get_ipaddr());
}

The error states SetIPAddress() is not a member of TTT, which it isn't as it's only declared in OnlineGame.

>> The error states SetIPAddress() is not a member of TTT, which it isn't as it's only declared in OnlineGame.

Hence the problem. You can't access the members from one class in another class. You can try casting the pointer however using dynamic_cast.

See this thread about dynamic_cast: http://cboard.cprogramming.com/showt...threadid=41401

Note: If you do use dynamic_cast on a reference, do so in a try{ } block.

Don't see why you need to have SetIPAddress in the base class...it seems that it would work if you just remove it from the base class and the OfflineGame class

Code:

#include <iostream>
#include <string>
using namespace std;

class TTT
{
public:
	TTT(){cout<<"something"; }
	void GetUserMove();
	bool GameOver();
	void GetOpponentMove();
};

class OnlineGame : public TTT
{
public:
 OnlineGame()  {cout<<"constructor";}
	void SetIPAddress(string &addr);
	void GetOpponentMove();
};

class OfflineGame : public TTT
{
public:
	OfflineGame();
	void GetOpponentMove();
};


void OnlineGame::SetIPAddress (string &addr)
{
cout<<"Whatever";
}
bool TTT::GameOver()
{
cout<<"Whatever" ;
}
int main(int argc, char *argv[]) {
    string str = "Hi"  ;
          OnlineGame one;
    one.SetIPAddress(str) ;
     one.GameOver();
    return 0;
}

Although I get errors when I don't have the some of the functions do anything

>> Don't see why you need to have SetIPAddress in the base class...it seems that it would work if you just remove it from the base class and the OfflineGame class

The point is to keep a pointer to an object of the base class, so you can use all/most of the external functionality without regard to which particular derived class you have.

Thanks again Zach, but I'm still having problems.
I changed my code to this:

Code:

TTT *Game;
TTT *p = new OnlineGame;
Game = dynamic_cast<OnlineGame*>(p);
if (Game != 0)
{
	cout << "Cast Succeeded!\n";
	Game->SetIPAddress(get_ipaddr());
}

And it's still giving me an error about SetIPAddress not being a member of TTT. It's also warning me about using dynamic_cast on a polymorphic type.

I think slaveofthenet wants to avoid creating a pointer to the derived type. I think he/she wants to just use a pointer to the base class, so that later on, he/she only has to have one set of code, rather than code for derived class OnlineGame and code for derived class OfflineGame. But then again, I barely understand deriving as it is.