error LNK2001: unresolved external symbol -- I can't get a SIMPLE INHERITANCE to work in C++

Question

I'm struggling to use inheritance. I am studying computer science and have to work it out... Rihgt now I 'm trying to make my own class 'Copy' which contains a reference to a 'Drawable' object, For instance I have an object of the class 'circle' which inherits from 'Drawable'. I want to make a copy of the said object 'circle', which seems to work out ok. But when I want to call upon the 'draw' method things go sour.

Error Message:

1>------ Build started: Project: Project2, Configuration: Debug Win32 ------  
1>  main.cpp  
1>  Drawable.cpp  
1>  copy.cpp  
1>  Cirkel.cpp  
1>  Generating Code...  

1>Cirkel.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Drawable::draw(class sf::RenderWindow &,class sf::Vector2<float>)" (?draw@Drawable@@UAEXAAVRenderWindow@sf@@V?$Vector2@M@3@@Z)  

1>copy.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall Drawable::draw(class sf::RenderWindow &,class sf::Vector2<float>)" (?draw@Drawable@@UAEXAAVRenderWindow@sf@@V?$Vector2@M@3@@Z)  

**This is my main: **

#include <SFML\Graphics>
#include "circle.hpp"
#include "copy.hpp"


int main(){

    sf::RenderWindow window{ sf::VideoMode{ 640, 480 }, "SFML window" };

    circle blauwe_circle{ 40.0F, sf::Color::Blue };
    circle rode_circle{ 10.F, sf::Color::Red };

    copy cBlauw{ sf::Vector2f{ 40.f, 40.f }, blauwe_circle };
    copy cRoodA{ sf::Vector2f{ 50.f, 50.f }, rode_circle };

    sf::Vector2f nul{ 0.f, 0.f };



    while (window.isOpen()){

        sf::Event event;
        while (window.pollEvent(event)){
            if (event.type == sf::Event::Closed) window.close();
        }
        sf::sleep(sf::milliseconds(20));
    }
    return 0;
}`

drawable.hpp

class Drawable
{
    public:
        virtual void draw(sf::RenderWindow & window, sf::Vector2f vector);
};

Circle.hpp:

#include "Drawable.hpp"

class circle : public Drawable{

    public:

    circle(float size = 30, sf::Color color = sf::Color::Red);

    void draw(sf::RenderWindow & window, sf::Vector2f position) override;

    private:

    float size;
    sf::Color color;
    sf::CircleShape c;

};

cirlce.cpp

#include "circle.hpp"
#include "Drawable.hpp"

circle::circle(float fSize, sf::Color color) :
    size{ fSize },
    color{ color }
{
    c.setRadius(size);
    c.setPointCount(30U);
    c.setFillColor(color);
}

void circle::draw(sf::RenderWindow & window, sf::Vector2f position){
    c.setPosition(position);
    window.draw(c);
}

copy.hpp:

#include "Drawable.hpp"

class copy: public Drawable
{
    public:
        copy(sf::Vector2f position, Drawable & c);
        void draw(sf::RenderWindow & window, sf::Vector2f vector) override;
    private:
        sf::Vector2f position;
        Drawable & c;
};

copy.cpp

#include "copy.hpp"
#include "Drawable.hpp"

copy::copy(sf::Vector2f vPosition, Drawable & c) :
    position{ vPosition },
    c{ c }
{}

void copy::draw(sf::RenderWindow & window, sf::Vector2f v){
    c.draw(window, v);
}

Answer 1

If Drawable::draw is intended to be a pure virtual function without implementation, add = 0:

class Drawable
{
    public:
        virtual void draw(sf::RenderWindow & window, sf::Vector2f vector) = 0;
};