Code:Im working on a a circle class program in which I have to find the diameter and area of the circle. I have divided the program into 3 files, 1 main.cpp 1 circle.cpp and 1 circle.h My code produces and error: 1>main_Circle.obj : error LNK2019: unresolved external symbol "public: __thiscall Circle::~Circle(void)" (??1Circle@@QAE@XZ) referenced in function _main 1>main_Circle.obj : error LNK2019: unresolved external symbol "public: __thiscall Circle::Circle(double)" (??0Circle@@QAE@N@Z) referenced in function _main 1>C:\Users\RKLO\documents\visual studio 2010\Projects\Rectangle class2\Debug\Rectangle class2.exe : fatal error LNK1120: 2 unresolved externals Here is my code //Cirlcle.h #pragma once //Circle class declaration class Circle { private: double radius; public: Circle(double=0); Circle (const Circle &); ~Circle(); void setRadius(double); double getRadius()const; double getDiameter()const; double getArea()const; }; //Circle.cpp //Implementation file for Circle class #include "Circle.h"//Needed for the rectangle class #include<iostream>//Needed for the cout #include<cstdlib>//Needed for the exit function using namespace std; //sets the value of the member radius void Circle::setRadius(double r) { if (r>=0) { radius=r; } else { cout << "Invalid radius\n"; exit(EXIT_FAILURE); } } //returns the value mamber of the variable radius double Circle::getRadius()const { return radius; } //returns the value member of the variable Diameter double Circle:: getDiameter()const { return 2*radius; } //get area returns the pruduct times pi*radius^2 double Circle:: getArea()const { return 3.1416*radius*radius; } //main_Circle.cpp #include <iostream> #include "Circle.h"//Neede for the rectangle class using namespace std; int main() { Circle box; double radiusCircle;//Local variable for radius cout<<"This program will calculate the area of a\n" ; cout << "circle. What is the radius? "; cin>>radiusCircle; box.setRadius(radiusCircle); cout << "Here is the circle's data: "; cout << "Radius: "<<box.getRadius()<<endl; cout << "Diameter "<<box.getDiameter()<<endl; cout << "The Area of the Circle is "<<box.getArea()<<endl; return 0; } Any help will be really appreciated I have uploaded the three files for your convenience.