Problem in C++ class with static variables and functions

Can someone tell me what is problem in the following class, g++ is giving errors on ubuntu:

class FibonacciGenerator
{
    private:
        static int num1, num2, counting;

    public:

        static void Reset()
        {
            num1 = 0; num2 = 1;
            counting = 1;
        }

        static int GetCount()
        {
            return counting;
        }

        static int GetNext()
        {
            int val = 0;
            if(counting == 1) val = num1;
            else if(counting == 2) val = num2;
            else 
            {
                val = num1 + num2;
                num1 = num2;
                num2 = val;
            }
            counting ++;

            return v开发者_如何学Cal;
        }
};

The statement static int num1, num2, counting; in a class definition does not define those variables, it only declares them. If they are used they must also be defined.

A complete example of this is as follows:

//Begin FibonacciGenerator.hpp
#ifndef FIBONACCI_GENERATOR_HPP
#define FIBONACCI_GENERATOR_HPP
class FibonacciGenerator
{
private:
    static int num1, num2, counting;

public:
/* as above */
};
#endif //FIBONACCI_GENERATOR_HPP
//End FibonacciGenerator.hpp

//Begin FibonacciGenerator.cpp
#include "FibonacciGenerator.h"
int FibonacciGenerator::num1;
int FibonacciGenerator::num2;
int FibonacciGenerator::counting;
//End FibonacciGenerator.cpp

If FibonacciGenerator is declared in a namespace, then these static member definitions must also be in that namespace.

Using static members like this is probably a very bad idea. It would be better to make them instance variables, so that you could have multiple independent FibonacciGenerators in separate parts of the code.

You have only declared the static members. You should also define them in a cpp file like this

int FibonacciGenerator::num1 /*= 0*/;
int FibonacciGenerator::num2 /*= 1*/;
int FibonacciGenerator::counting /*= 0*/;

when you write

static T v; //if static were removed, this would also be a definition

inside a class, it is only a declaration, but every program must contain exactly one definition of every variable that is used (as per the One-Definition-Rule). hth