C++ fill array with objects derived from array type

Question

In C++ I have an array of pointers to Player objects and want to fill it with Fickle objects where Fickle is a class that is derived from Player. This is because I want a general Player array that I can fill with different objects from different classes that all are derived from the Player class.

How can I do this?

I create an array of pointers to Player objects

Player ** playerArray;

Then initialize the array to a certain size

playerArray = new Player *[numPlayersIn];

But then the following does not work for some reason:

playerArray[i] = new Fickle(0);

How can I fill the playerArray with Fickle objects (Fickel is a class derived from Player) ?

Thanks.

UPDATE:

I get the error message (in Eclipse IDE):

expected ';' before 'Fickle'

I think it might be something to do with the definition of Fickle.

The Fickle.hpp file contains:

#pragma once
#include "player.hpp";

class Fickle: public Player {
public:
    // constructor
    Fickle(int initChoice){
        choice = initChoice;
    }
}

Is this OK or is there a problem with this?

The Player class header file has:

class Player {
private:

public:
    int getChoice();
int choice; // whether 0 or 1
virtual void receive(int otherChoice); // virtual means it can be overridden in subclases
};

The receive method will be overridden in Fickle and other classes derived from the Player class

UPDATE 2:

OK I think the error is actually due to a different part of the code.

Player defines a method receive:

virtual void receive(int otherChoice);

That should be overridden by the subclass Fickle but the definition in Fickle:

void Fickle::receive(int otherChoice) {}

gives the error:

no 'void Fickle::receive(int)' member function declared in class 'Fickle'

But I don't know why this is because receive is defined in the Player class?

What error do you get? What is definition of Fickle (first line with the base classes)? — Yakov Galka
– Yakov Galka, Commented Oct 1, 2010 at 20:58
Any reason you're not using std::vector? std::vector<Player*> is simple enough. Even better (since you need to manage those pointers) would be boost::ptr_vector<Player> or std::vector<shared_ptr<Player>>. — GManNickG
– GManNickG, Commented Oct 1, 2010 at 20:58
What do you mean, "does not work"? As far as the pointer types are concerned it does work, so there must be some error elsewhere in your code. Does Fickle have the required constructor? Is 0 <= i < numPlayersIn? — Steve Jessop
– Steve Jessop, Commented Oct 1, 2010 at 21:00
Does Fickle derive from single Player base (i.e. no multiple inheritance)? — Yakov Galka
– Yakov Galka, Commented Oct 1, 2010 at 21:04
@ybungalobill: Irrelevant. This idea should work even if there is multiple inheritance. — aschepler
– aschepler, Commented Oct 1, 2010 at 21:16

Jerry Coffin · Accepted Answer · 2010-10-01 21:27:18Z

5

While you probably should be using a vector instead, there's no real reason a dynamically allocated array can't work. Here's a bit of working demo code:

#include <iostream>

class Player {
public:
    virtual void show() { std::cout << "Player\n"; }
};

class Fickle : public Player {
public:
    virtual void show() { std::cout << "Fickle\n"; }
};

int main() {
    Player **p = new Player *[2];
    p[0] = new Player;
    p[1] = new Fickle;

    for (int i=0; i<2; i++)
        p[i]->show();

    for (int i=0; i<2; i++)
        delete p[i];
    delete [] p;
    return 0;
}

edited Oct 1, 2010 at 21:27

answered Oct 1, 2010 at 21:13

Jerry Coffin

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2 Comments

tree-hacker Over a year ago

Why is Player declared as a struct here? Does it make a difference if Player is a class instead of a struct?

Jerry Coffin Over a year ago

@tree-hacker: it just saves putting in a public:. That's the only real difference it makes. To save confusion, I've made both classes with public:.

Yakov Galka · Accepted Answer · 2010-10-01 21:41:04Z

2

It looks like you forgot a semicolon at the end of Fickle class:

class Fickle: public Player {
    // ...
}; // <---- this semicolon

Maybe somewhere else.

UPDATE 2: When you override a virtual function in the derived class you must declare it there too.

edited Oct 1, 2010 at 21:41

answered Oct 1, 2010 at 21:25

Yakov Galka

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3 Comments

Mark Ransom Over a year ago

For such a common mistake it's surprisingly hard to track down.

Yakov Galka Over a year ago

@Mark Ransom: Blame C++ preprocessor. It's especially hard when it comes from the end of some include. When I was a newbie I wasted a hour to track this error once.

tree-hacker Over a year ago

OK I didn't know you had to declare overridden functions in the derived class as well.

MSN · Accepted Answer · 2010-10-01 21:29:18Z

1

If it's not compiling correctly, then you probably didn't #include the header that defines Fickle in the same source file you have that code. Usually an error like that means the compiler doesn't know what Fickle is.

answered Oct 1, 2010 at 21:29

MSN

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Comments

Mark B · Accepted Answer · 2010-10-01 21:17:27Z

0

Does Fickle have a constructor that accepts an int or pointer type? Is it privately derived from Player? Otherwise it looks like this should work.

answered Oct 1, 2010 at 21:17

Mark B

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1 Comment

tree-hacker Over a year ago

Yes Fickle does have an int constructor. It is publicly derived from Player I think (see code in question) but am not too sure about derived classes.

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