C++: undefined reference to Constructor - c++

Question

I just started working with C++ and am working on an exercise that deals with polymorphic pointers. I'm having trouble trying to solve an error message I believe I'm getting from my Rectangle.cpp as I call the class from my main.cpp.
The error message:
undefined reference to 'Rectangle::Rectangle(double, double)'
Main.cpp
#include <iostream>
#include "Rectangle.h"
using namespace std;
//////////////////////////////////////////////
// --- FUNCTIONS DECLARATION---
void introduceShape(Shape*);
double calculateShapeArea(Shape*);
double calculateShapeCircumferece(Shape*);
int main()
{
Rectangle rectangle1(5,2);
// Rectangle *rec1 = new Rectangle(5,2);
introduceShape(&rectangle1);
cout << "My area is: " << calculateShapeArea(&rectangle1) << ", my circumference is: " << calculateShapeCircumferece(&rectangle1) << endl << endl;
return 0;
}
//////////////////////////////////////////////
// --- FUNCTIONS ---
void introduceShape(Shape* shapeToIntroduce)
{
return shapeToIntroduce->introduce();
}
double calculateShapeArea(Shape* shapeToCalculate)
{
return shapeToCalculate->calculateArea();
}
double calculateShapeCircumferece(Shape* shapeToCalculate)
{
return shapeToCalculate->calculateCircumference();
}
Rectangle.h
#ifndef RECTANGLE_H_INCLUDED
#define RECTANGLE_H_INCLUDED
#include "Shape.h"
#include <iostream>
using namespace std;
class Rectangle: public Shape
{
double width;
double height;
public:
Rectangle(double , double );
void introduce();
double calculateArea();
double calculateCircumference();
};
#endif // RECTANGLE_H_INCLUDED
Rectangle.cpp
#include "Rectangle.h"
#include <iostream>
using namespace std;
Rectangle::Rectangle(double width, double height)
{
this->width = width;
this->height = height;
}
void Rectangle::introduce()
{
cout << "I AM A RECTANGLE !" << endl;
}
double Rectangle::calculateArea()
{
return width*height;
}
double Rectangle::calculateCircumference()
{
return 2*(width+height);
}
Shape.h
#ifndef SHAPE_H_INCLUDED
#define SHAPE_H_INCLUDED
class Shape
{
public:
virtual void introduce() = 0;
virtual double calculateArea() = 0;
virtual double calculateCircumference() = 0;
};
#endif // SHAPE_H_INCLUDED

Answer 1

The error is generated by the linker because it can not see where the definition of the constructor is located.
If you are using an IDE, you should add .cpp file to the project so that it can be compiled and the definition would be found by the linker. It not, then you have to compile it yourself -assuming you are using gcc:
g++ Rectangle.cpp
will combine cpp files into one executable and should not show you that error.
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