Problem using Interfce

[Stephen Jones]

I have a Widget interface which I was hoping would allow me to
subsume a set of classes {Button, Cursor, etc} as being Widgets
so I could keep an array of buttons, cursors, etc by initializing
them as Widgets.

private Widget[] widgets;
...
widgets[widx++]=new Button(fmt, unitw, unith, count);
...
widgets[widx++]=new Cursor(unitw, unith, count);


But when I try to step through the array I cannot access Button
or Cursor variables because "Error: no property 'vertStart' for
type 'Widget.Widget'":

foreach(Widget w; widgets){
     glDrawArrays(GL_TRIANGLES, w.vertStart, w.vertCount);
}


I am used to languages where the w under consideration in any
iteration would be known to have been initialized as a Button or
Cursor, etc, and the value of vertStart would be found without
error. I cannot cast without wrapping everything in if
statements. I could use access functions but I would prefer not
to incur function overhead. Is there a solution?