Can an Object hold and object list of Itself?

[AlanScottAgain 1]

Hi

I was wondering what the best way to approach this is?

Scenario:

I'm creating a simple drawing program based on Skia (Which is awesome)

I want to be able to create compound shapes.

 

My air code:

TDrawingObject= class

Brush

color

path

location

end;

 

Then

 

TDrawingObjectList = class (TObjectList<TDrawingObject>)

Holds and access the TDrawingObjects

How could I create something like:

 

TDrawingObject= class

Brush

color

path

location

DrawingObjectList

end;

 

So that I can recursively run through the one object that is made up of multiples of itself?

Thanks

Alan

 

[FPiette 382]

Quote

So that I can recursively run through the one object that is made up of multiples of itself?

As long as you avoid or detect recursive infinite loops, then it is OK.

 

[AlanScottAgain 1]

Thanks

 

How do I get past the issue that TDrawingObjectList  is not declared until after  TDrawingObject?  So  DrawingObject can't hold a list of DrawingObjectList 

TDrawingObjectList  can hold more TDrawingObjectList(s) 

 

I feel Like I'm missing something basic!  I can used an untyped list but that defeats the object.

I guess the question is how do I declare/ structure a recursive list. 

[Vandrovnik 214]

Just put this to the top:

 

type tDrawingObjectList = class;

Edited June 14, 2022 by Vandrovnik

[AlanScottAgain 1]

10 minutes ago, Vandrovnik said:

Just put this to the top:

 

type tDrawingObjectList = class; 

But then its an untyped list?

I guess that may be the way to go

[Vandrovnik 214]

20 minutes ago, AlanScottAgain said:

But then its an untyped list?

I guess that may be the way to go

No, it is just a forward declaration.

[FPiette 382]

Quote

But then its an untyped list?

No, it is a forward declaration. The class itself is declared after TDrawingObject.

That declaration is enough for the compiler to know it has to reserve space for the class field (Actually a pointer).

The code accessing the property DrawingObjectList is in the implementation section and after both classes have been completely defined.

 

[AlanScottAgain 1]

Thank you

I did not know about forward declarations 👍