Class that use another class

Question

I have a problem with my classes.

I have two separate headers. Color.h and Painter.h:

1). Color.h

class Color{
       int number;
    public:
       void initialize();
       void change (Painter draw);
}

2). Painter.h

class Painter{
      Color a,b;
   public:
      void get();
      void draw();
}

My problem is, that i need to use Painter in class Color, and class Painter use Color. In Qt, i get a error that Painter is not a type. How can i fix this? What is solution for that problem?

You can make that happen by forward declaring one class in the other and use only pointers to the forward declared class. But if you have circular dependencies in such a simple case, consider changing the design of your software — Simon Warta, Dec 23 '14 at 23:21

score 3 · Accepted Answer · answered Dec 23 '14 at 23:22

3

In Painter.h you need to include Color.h, because you have objects of type Color. But in color.h you can add a forward declaration for the Painter class:

class Painter;
class Color{
   int number;
public:
   void initialize();
   void change (Painter draw); //a forward declaration is enough for this
}

And the method void change (Painter draw); you will define it in the color.cpp and there you include painter.h

answered Dec 23 '14 at 23:22

Zlatomir

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Don't you need to change `void change (Painter draw);` to `void change (Painter & draw);` for this to work? – drescherjm Dec 23 '14 at 23:29
1

@drescherjm I was thinking about that too, but it looks okay (http://stackoverflow.com/questions/553682/when-can-i-use-a-forward-declaration) – Simon Warta Dec 23 '14 at 23:30
1

@Zlatomir: that is incorrect explanation: `because you don't construct an object there`. You do not need it, but not because of that as you do not construct the base class in the header either, yet you need it. – László Papp Dec 24 '14 at 00:26

Class that use another class

1 Answers1