Thread: interface, implementation, & application files

Hi all,
This program works well. The the object of data type of class used to count something.
However, the compiler generated the error when I broke this program into 3 parts:
- The interface file, Problem5.h, contains the class definition.
- The implementation file, Problem5.cpp, contains the class's member functions, #include "Problem5.h" was written.
- The application file used to test above class, #include "Problem5.h" was written.

* When I compiled them, it was fine. However, when I ran them, it seemed to get trouble with the linker.
The weird thing is, the program works perfectly when I broke the program into 2 parts, the file contains the definiton of class and its member functions' definition, called Problem5.h; and the file contains the main() function.
Everyone can help me out please. Here is the code:

Code:

#include <iostream>
using namespace std;

class CounterType
{
public:
  CounterType();//Default Constructor

  CounterType(int number); //Constructor
  

  void increase();
  

  void decrease();
  

  int get();
  
  void output(ostream& numberout);
  

private:
  int count;
  void check();

};

// <---- interface file
int main()
{
	CounterType myclass;
	CounterType fortest(100);
	int countvalue,i;
	for (i=0;i<10;i++)
	{
		myclass.increase();
		if ((i==3)||(i==7))
			myclass.decrease();
	}
	countvalue=fortest.get();
	cout<<"The first object that counts from 1-->10, except 3&7 :";
	myclass.output(cout);
	cout<<endl<<endl;
	cout<<"The second object that is initilized with 100: "<<countvalue<<endl<<endl;

	return 0;
}
//<---- application (drive) file

CounterType::CounterType() : count(0)
{
}

CounterType::CounterType(int number)
{
	count=number;
	check();
}

void CounterType::check()
{
	if (count<0)
	{
		cout<<"Count contains negative number. No negative number is allowed!\n\n";
		exit(1);
	}
}

void CounterType::increase()
{
  count++;
  check();
}

void CounterType::decrease()
{
  if (count > 0)
    count--;
	check();
}

int CounterType::get()
{
  return count;
}

void CounterType::output(ostream& numberout)
{
  numberout << count;
}
//<----- implementation file