linker error, using static member

Hello guru's,

>Studying C++: I am trying to understand static members in classes. I am getting a linker error when I try to compile the code below.
I am pretty sure the linker error is related to the memory definition or declaration of the static int.
Can someone explain me how to solve my problem, how to run this simple program without errors?

Thanks,
Mike

#include <iostream.h>

class Teller {
private:
int value;
static int amount;
public:
Teller( int W ) { value = W; amount++; }
// constructor, static variabele

static void init_amount() { amount = 0 ; }
int give_value() {return value; }

static int give_amount() { return amount; }
};

void main()
{
Teller::init_amount(); // Set 'amount' to 0

Teller T1(10); // Object T1
Teller T2(20); // Object T2

cout << "\nValue = " << T1.Teller::give_value() << " Amount = " << Teller::give_amount();

cout << "\nValue = " << T2.Teller::give_value()
<< " Amount = " << Teller::give_amount();
cout << "\nAmount through scope resolution operator = "
<< Teller::give_amount();
getchar();
}

There is no memory reserved for amount

Code:

---

//now there is
int Teller::amount = 0;

---

This is done in Init_Amount , Amount = 0

Or is this the wrong location to do this?

A static variable is defined in the class, but must be declare outside the class. So outside your class you should declare it, for example like this:

int Teller::amount = 0;

This is because a class definition does not allocate space for the variable, the declaration outside the class does.

Shiro is right in his idea, although I think he has the declaration and definition backwards.

A declaration allocates nothing, it only says (declares) to the compiler that something will exist.

A definition allocates space. It tells the compiler exactly how the variable, etc. should be created (defined).

Quote:

---

Originally posted by Shiro
A static variable is defined in the class, but must be declare outside the class. So outside your class you should declare it, for example like this:

int Teller::amount = 0;

This is because a class definition does not allocate space for the variable, the declaration outside the class does.

---

Mustn´t you put extern before the declaration?

extern int Teller::amount = 0;

>> Mustn´t you put extern before the declaration?

'extern' indicates that what you have provided is simply a declaration, and not a definition (which has to be elsewhere). In this case, you have a definition, so if you were to use 'extern', then that keyword would be ignored by the compiler.

You use 'extern' when you have a symbol which you need to use in multiple files to provide declarations (so that symbol is defined) without redefining it (which is illegal).