Question

I don’t know how to explain this properly, someone please edit my title and post as needed. I thought this could be solved with polymorphism, but I couldn’t get it to work. What I would like to have is as following.

I’m going to have different shapes and they all have different variables. For example, to describe a particular shape A I need:

Mandatory fields for every shape:

• starting and ending point (composed
  of 3 double variables: x, y and z)

• name (string)

A shape may or not need the following fields (maybe in the future I will need more)

• Length, width, height (all double)
• Angle
• Other variables which could be double or string

The thing is, this needs to flexible, as every shape is going to be described using XML, which I then need to load into the program. For example a cube would work like this:

<?xml version="1.0" encoding="utf-8" ?> 
<structure name="Cube">
    <variables>
        <point name="start" />
        <point name="end" />
        <length name="width" min="1" max="10000" unit="mm" />
        <length name="height" min="1" max="10000" unit="mm" />
    </variables>
    <points>
        <point name="topLeft">
            <x>start.x</x>
            <y>start.y - height/2</y>
            <z>start.z</z>
        </point>
        <point name="topRight">
            <x>end.x</x>
            <y>end.y - height/2</y>
            <z>end.z</z>
        </point>
        <point name="bottomLeft">
            <x>start.x</x>
            <y>start.y + height/2</y>
            <z>start.z</z>
        </point>
        <point name="bottomRight">
            <x>end.x</x>
            <y>end.y + height/2</y>
            <z>end.z</z>
        </point>
    </points>
    <connections>
        <connection type="line">
            <point name="topLeft" />
            <point name="topRight" />
            <point name="bottomRight" />
            <point name="bottomLeft" />
            <point name="topLeft" />
        </connection>
    </connections>
</structure>

So what I thought is, I have a base class called CVariable (with 2 abstract methods, parseString and toString), which I then inherit to make CLength (new attribute double length), CPoint (new attribute double x, y and z) and CName (new attribute string name). I can then have a class called CShape which has a list of CVariable, CPoint and CName (the connections, just a name identifying the point to be connected). The problem is, I believe, if I have a list of CVariable, I can’t access the new attributes. Am I right?

Sorry if it’s not clear, I will edit to add any information needed. Thanks a lot in advance.

Answer 1

The scheme you’re thinking in is usually called a “variant”. CVariable is not a good name for this, it’s just to unspecific. But you’re thinking in the right direction already. By deriving from a common base class you can pack the elements in a common container. The trick to access those derived types is using the runtime type information system: There is the typeof keyword telling you the actual type of the object you’re referring to: http://msdn.microsoft.com/de-de/library/58918ffs.aspx

A word of warning: Getting into a situation where one needs a variant is often a hint, that something in the design of the data structure may be flawed and rethinking the problem is a good idea.