Issue declaring extern class object

Question

Let me start by saying I've extensively searched for answers on google and more specifically here.

The thing is I actually (at least I think I did) found people with similar problems, though the answer given to them gave me another problem.

I'm using Visual Studio 2010 Express and working with SFML libary (though i do not think this last part is relevant)

So here it goes:

I have a source file called player.cpp which holds class Player and I have a header file (included in all source files) called cc.h(command and control) that holds all the necessary includes and external variables/functions. The essential code can be summed up in the following:

Player.cpp:

#include "cc.h"
class Player
{
private:

//some variables here

public:

//more variables and some functions

}john;//example instance

cc.h:

#pragma once

//some #includes
//some externs

extern Player john;

Now in cc.h the word Player is underlined as a mistake saying it is an undefined identifier , but only sometimes, other times visual studio doesn't mark it as a mistake, instead it recognizes it as a class but doesn't recognize john as an object/instance (i hope it's called this way) of that same class. Furthermore, at compiling the first error it shows is "error C2146: syntax error : missing ';' before identifier 'john'" at the line of the extern declaration of john, in cc.h, which apparently (to me) does not make any sense.

Answer 1

The global declaration in cc.h would not help you, I guess - because you declare it to access it from else where (other than Player.cpp), but for this you need the method signatures - a soon as you want to access john from elsewhere and thus include Player.cpp, you get duplicates symbols.

Please consider creating a Player.h file where only the class and method signatures are declared - like this:

#ifndef PLAYER_H_
#define PLAYER_H_

class Player
{
     void doSomething();
};
#endif

and add this to cc.h:

#include <Player.h>
extern Player john;

and in your Player.cpp

#include <Player.h>

Player john;

void Player::doSomething()
{
    //...
}

This makes sure that the Player signatures are known and a valid instance is declared globally.

Answer 2

You need to put the definition of your Player class in the header file, before you declare the extern variable. Otherwise the compiler has no idea what Player is.

I suggest something like this:

player.h

#ifndef PLAYER_H_
#define PLAYER_H_

class Player {
    ...
};

#endif

player.cpp

#include "player.h"

Player john;

cc.h

#ifndef CC_H_
#define CC_H_

#include "player.h"

extern Player john;

#endif

Answer 3

1. You need to define the Player class, in your header file
2. Use extern to use variable that has an external linkage, and is already defined in some other file.

For example: you have file a.cpp, and inside this file has a global variable Player p. If you want to use the same exact instance p of Player in file c.cpp, then inside file c.cpp you write extern Player p.

I hope i made myself clear.

Answer 4

"extern Player john;" is considered to be undefined identifier as the compiler is unable to understand what Player is, as you have not included the file Player.cpp where the class Player is declared to cc.h . It is always recommended to declare the class and its methods in header files say for example in Player.h and then define these methods in the source file i.e Player.cpp. And to include Player.h in your cc.h so that compiler understands where " Player john;" is declared.