Help with static member function

[wiiarethesound]

Hi everyone, this is my first post here, so please forgive any mistakes made on my part. I am having trouble implementing a static member function for a class called Car. Here is the code in my Car.h file:

#ifndef CAR_H
#define CAR_H

#include <string>
#include "Vehicle.h"

using namespace std;

static int carCount = 0;

class Car : public Vehicle
{
public:
	Car();
	Car(string, string, int, int, int);
	~Car();
	void PrintName();
	void PrintTorque();
	void PrintHorsepower();
	static int GetNumberOfCars()
	{
		return carCount;
	}
private:
	string make;
	string model;
	int VIN;
	int horsepower;
	int torque;
};

#endif 

And here is the implementation in Car.cpp:

#include "stdafx.h"
#include <string>
#include <iostream>
#include "Car.h"

using namespace std;

Car::Car()
{
	make = "unknown";
	model = "unknown";
	VIN = 0;
	torque = 0;
	horsepower = 0;
	carCount++;
}

Car::Car(string mk, string mdl, int vIDnum, int trq, int hp)
{
	make = mk;
	model = mdl;
	VIN = vIDnum;
	torque = trq;
	horsepower = hp;
	carCount++;
}

Car::~Car()
{
	carCount--;
}

void Car::PrintName()
{
	cout << "Make: " << make << endl;
	cout << "Model: " << model << endl;
}

void Car::PrintTorque()
{
	cout << "Torque: " << torque << endl;
}

void Car::PrintHorsepower()
{
	cout << "Horsepower: " << horsepower << endl;
}

Everything seems to compile properly, but when I run a simple driver program the carCount variable doesn't seem to increment and the GetNumberOfCars() function returns garbage.

Any help would be greatly appreciated! Thanks!

[jsmith]

You should move the static carCount variable inside the Car class. Then you'll need to instantiate the variable in Car.cpp, and then it will work.

As it is, carCount is a global variable, and never declare global variables in header files.

[wiiarethesound]

Hi jsmith, thanks for your quick reply. I have moved static int carCount from the header to underneath the private part of class Car. Is that correct?

Also, what do you mean by "instantiate the variable"? Do I need to use the new function somehow?

[pain sama]

you haave delcare both your static variables inside the class. to acess your static datamember outside in your class ,
(in car.cpp,may be line 7)

int car :: count;

[wiiarethesound]

Thanks to both jsmith and pain sama!! You helped me figure out my problem :)

For any others having the same problems here is the code:

// Car.h
#ifndef CAR_H
#define CAR_H

#include <string>
#include "Vehicle.h"

using namespace std;

class Car : public Vehicle
{
public:
	Car();
	Car(string, string, int, int, int);
	~Car();
	void PrintName();
	void PrintTorque();
	void PrintHorsepower();
	static int GetCarCount()
	{
		return carCount;
	}
private:
	string make;
	string model;
	int VIN;
	int horsepower;
	int torque;
	static int carCount;
};

#endif 

// Car.cpp
#include "stdafx.h"
#include <string>
#include <iostream>
#include "Car.h"

using namespace std;

int Car::carCount = 0;

Car::Car()
{
	make = "unknown";
	model = "unknown";
	VIN = 0;
	torque = 0;
	horsepower = 0;
	carCount++;
}

Car::Car(string mk, string mdl, int vIDnum, int trq, int hp)
{
	make = mk;
	model = mdl;
	VIN = vIDnum;
	torque = trq;
	horsepower = hp;
	carCount++;
}

Car::~Car()
{
	carCount--;
}

// All that other stuff..... 

Again, thanks to those that helped me! Much appreciated!!