C++ Inheritances - Files - c++

Question

Im trying to seperate a class and its superclass into two different header and cpp files. In the main Method I want to include both of them.
Currently my main programm example.cpp looks like this:
#include <iostream>
#include "header.h"
using namespace std;
int main(void)
{
Rectangle Rect;
Rect.setWidth(5);
Rect.setHeight(7);
// Print the area of the object.
cout << "Total area: " << Rect.getArea() << endl;
return 0;
}
My Rectangle.cpp like this:
#include <iostream>
#include "Shape.h"
#include "Rectangle.h"
using namespace std;
int getArea()
{
return (Shape::width * Shape::height);
}
And my Rectangle.h:
class Rectangle: public Shape
{
public:
int getArea();
};
The Shape.cpp:
#include <iostream>
#include "Shape.h"
using namespace std;
void Shape::setWidth(int w)
{
width = w;
}
void Shape::setHeight(int h)
{
height = h;
}
And the Shape.h:
class Shape
{
public:
void setWidth(int w);
void setHeight(int h);
int width;
int height;
};
The header.h just includes the two headers of the classes:
#include "Shape.h"
#include "Rectangle.h"
If I compile it, the compiler says:
Lukass-MacBook-Pro:Oberklassenbeispiel Lukas$ g++ -c Rectangle.cpp
Rectangle.cpp:9:17: error: invalid use of non-static data member 'width'
return (Shape::width * Shape::height);
~~~~~~~^~~~~
Rectangle.cpp:9:32: error: invalid use of non-static data member 'height'
return (Shape::width * Shape::height);
~~~~~~~^~~~~~
2 errors generated.
It seems, that the Rectanlge.cpp cant see the attributes of the superclass. How do I fix this?

Answer 1

When you #include, you provide the file name, not the classes specified in the file. It is quite common to have multiple closely coupled classes in a single file. Your example isn't really the best (it is single inheritance, not multiple as the title suggests), but working with that...
You could have a file, shapes.h with the following contents :
class Shape
{
public:
void setWidth(int w)
{
width = w;
}
void setHeight(int h)
{
height = h;
}
protected:
int width;
int height;
};
// Derived class
class Rectangle: public Shape
{
public:
int getArea()
{
return (width * height);
}
};
Then in your main function (and the file containing it).
#include <iostream>
#include "shapes.h"
int main(void)
{
Rectangle Rect;
Rect.setWidth(5);
Rect.setHeight(7);
// Print the area of the object.
cout << "Total area: " << Rect.getArea() << endl;
return 0;
}