Graphics editor - square/circle

[Tallmios]

I've created a graphics editor using Pascal through Turbo Delphi. 
Drawing a rectangle/ellipse works like this:
https://i.gyazo.com/aea5ea17c3fadcaf106c9f53acae49ff.gif  (the cursor is desynchronised in the gif, but works correctly in the program). 
How would I go about drawing a square/circle with the right mouse button, similarly to how Microsoft Paint allows you right angles/circles/squares when you're holding shift?

[Ñuño_Martínez]

When drawing, just check if Shift is pressed, if so then check which one is smaller (width or height) and use it as both values.  Something like this:

Code: Pascal  [Select][+][-]

1.   BEGIN
2.     Width := MouseX - OriginX; Height := MouseY - OriginY;
3.     IF ShiftPressed THEN
4.     BEGIN
5.       IF Width < Height THEN
6.         DrawEllipse (OrigX, OrigY, Width, Width)
7.       ELSE
8.         DrawEllipse (OrigX, OrigY, Height, Height)
9.     END
10.     ELSE
11.       DrawEllipse (OrigX, OrigY, Width, Height)
12.   END;
13.  

If you're asking how to know if Shift is pressed, it is different in each framework/library you're using (i.e. SDL, FCL, Allegro, ...).

[Tallmios]

Thank you. I'll try your code out. In my program it's going to be a function of RMB and I've already sorted that out.