Array or list for drawing multiple shapes

  • Hi,

    I am building a Qt painting program for my master's thesis and have a technical hurdle.

    The user can select a couple of drawing modes (freehand, rectangle, ellipse, line, and so forth) and create shapes.
    The shapes are stored in a QList<Shape> and the PaintEvent iterates through the QList to draw all of them (just like in the undo example).

    However, I want to give each Shape it's own inherited class like Line, Rectangle, Ellipse, so I can give them specific functions like getRect, getLine, etc. (in particular I need a special function to detect when the mouse is hovering over the line to select it, but that's another issue).

    The QList<Shape> I am currently using will not let me add anything other than Shape objects, so I can't add these types of objects to the list.

    The way the PaintEvent is coded is it uses a switch to get the type of the current indexed list item, and then based on that it determines what to draw like (pseudo-code):
    case Rectangle:
    case Ellipse:


    But it will simply give me an error that for example getLine is not a member of Shape and so forth... even though there is a Line object at that index.

    I'm probably doing this all the wrong way though, so my question basically is: how can I give each shape it's own class but preferably keep all these objects in a single list/container to draw from, while still being able to call their individual functions?

  • Lifetime Qt Champion

    you must use pointers to the object to be allowed to use them


    then you also insert subclasses of Shape. Like Line etc.

    It's not possible to use object directly (non *) as object slicing would happen.

    I assume you understand virtual function and that Shape could have a Paint function and the subclasses also have a paint function and you can just call Qlist[0]->Paint() and the right paint will be called with no need for switch case pr type. If you use pointers. :)
    see here

  • Lifetime Qt Champion


    Out of curiosity, what is your thesis subject and why do you need to implement a painting program for it ? It's to know if there's could be a better/simpler way to get what you need.

  • This post is deleted!

  • @SGaist My subject is design of graphical user interfaces. I already picked out the design of a painting program so I can't change any details about that.

    It looks like I need to do some reading about polymorphism. That is a concept I clearly don't understand well enough yet. Thanks for the pointer, mrjj :)

  • Lifetime Qt Champion

    Rather than going up-front with building your own GUI for a painting program, I'd recommend taking a look at what is currently existing like Krita or KolourPaint. Painting is a vast subject that can cover many aspects. Just take a look at Qt's examples on the subject.

    You should also take the time to look at QtQuick for the GUI design part.

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