Thread: Another beginner question

This one is about object oriented programming. In java I could just add new files and have them be classes so if I wanted a Player class in java I would make a file that said:

Code:

public class Player
{
     public Player(int a, int b)
     {some stuff}

     --- some more methods
}

And in my driver I could say, Player p = new Player(1,2); or something.

In C++ I'm not really sure what to do...here's what I have so far and its giving me an error about /clr options or something:

Code:

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

public class Player 
{
  public:
    double points,value;
	string name, pos;
    Player (string,string,double);
};

Player::Player (string n, string p, double pts) 
{
  name = n;
  pos = p;
  points = pts;
}

Ok I did this, Player a(name, pos, ppoints); and I got a undeclared identifier error. Whasts the difference between doing Player a(stuff) and doing Player* a = new Player(stuff)

What is the exact error.

As to the difference between

Player a(stuff) and doing Player* a = new Player(stuff)

Player a(stuff) means that you create a variable a, of the type Player.

Player *a = new Player(stuff) means that you care a variable a, that is a pointer to the type Player, which is then initialized to the memory location choosen by "new" for something of type Player.

In many cases, the difference isn't much at all - which is "better" really depends on how you want to use the variable - it is quite a complex matter to choose between the two, as they have all sorts of different benefits and drawbacks. The typical case where you want to have pointers is when you want to store a reference to this player elsewhere - whilst it's possible to do that even when you don't create them with new, it's the most natural choice in such a case.

--
Mats

Generally you want to use Player a(stuff) unless you have a good reason. Doing this allows the compiler to manage the lifetime of the object (it lasts until the end of the scope it is declared in).

Originally Posted by Daved

Generally you want to use Player a(stuff) unless you have a good reason. Doing this allows the compiler to manage the lifetime of the object (it lasts until the end of the scope it is declared in).

Yes, I completely agree with that.

One of the biggest problems with modern software is that it "leaks memory", which means that if you run it for a long time, it uses more memory than it did in the beginning, without a change in the ACTUAL data being kept by the application. The cause for this is that the programmer did a "new" (or "malloc") without a corresponding "delete" (or "free"). Avoiding using pointers to things, and thus needing to use "new" is a good idea if it's possible (within reason - with experience you'll learn to know when you need one and when you need the other).

--
Mats

Originally Posted by matsp

The typical case where you want to have pointers is when you want to store a reference to this player elsewhere - whilst it's possible to do that even when you don't create them with new, it's the most natural choice in such a case.

To expand upon this, here's how you would use a pointer and a reference without creating a variable with new:

Code:

class Player a("abc", "def", 12.0); //Constructs objects a
Player *ap = &a; //creates a pointer to type Player and puts the address of a in ap
ap->memberFunction();
Player &ar = a; //creates a reference and points it to a
ar.memberFunction();

Also, if you do use new, don't forget to use delete, or else you'll have a memory leak.

Code:

Player* b = new Player("abc", "def", 12.0); //constructs an object dynamically
delete b;

Yes, robwhit has it right - I was tempted to write a bit about that, as well as what DaveD commented on - but tried to keep it brief. This is obviously a forum where people comment when the initial post isn't complete :-)

--
Mats

the error just says error C2065: 'Player' : undeclared identifier. I don't have Player defined in this file though, its in Player.cpp like I would do in java

Ok so I have a Player.h, a Player.cpp, and a main file. I left out some unimportant code. I'm getting a Cannot open include file: 'Player.h': No such file or directory

Main:

Code:

#include <Player.h>
int main () 
{
Player a(name, pos, ppoints);
}

Class:

Code:

#include <Player.h>
Player::Player (string n, string p, double pts) 
{
  name = n;
  pos = p;
  points = pts;
}

Header:

Code:

class Player 
{
	double points,value;
	string name, pos;
	public:
		Player (string,string,double);
};

Ok I did "Player.h" instead of <Player.h> and now I have lots of errors:

Code:

class Player 
{
	double points,value;
	string name, pos;
	public:
		Player (string,string,double);
};

Syntax error, missing ';' before identifier 'name'

The compiler doesn't know the type "string", so you need to include the support for that.
#include <string>

--
Mats