Question

I’m writing an SDL/input library for my game in Free Pascal, and I’m facing an issue.

I’ve got a variant record that, when I access an element of it, changes the other elements.

The record type is:

tInput = Record
    case Device: TInputDevice of
        ID_KeyOnce, ID_KeyCont:                     (Key:         TSDLKey);
        ID_MouseButton:                             (MouseButton: Byte);
        ID_MouseAxis, ID_JoyAxis,
          ID_JoyBall, ID_JoyHat:                    (Axis:        Byte);
        ID_JoyButton, ID_JoyButtonOnce, ID_JoyAxis,
          ID_JoyHat, ID_JoyBall:                    (Which:       Byte);
        ID_JoyButton, ID_JoyButtonOnce:             (Button:      Byte);

end;

The code that crashes it is:

with Input do begin
    Device := ID_JoyAxis;
    Which  := 0;
    Axis   := 1;
end;

When axis is set to one, all of the other variables in the record go to one two!

Is this a known bug? Or some functionality I’m not aware of? Or something I’ve screwed up?

Answer 1

This is called a union and intended behavior of this type of record declaration.

case Device : TInputDevice of

… is the “magic” here.

In a union the storage of members is “shared”.

Edit: taking the record you have in terms of byte offsets (… under the assumption that sizeof(TSDLKey) = 4):

------------------------------------------------
00 | Key | MouseButton | Axis | Which | Button |
---|     |-------------|------|-------|--------|
01 |     |             |      |       |        |
---|     |-------------|------|-------|--------|
02 |     |             |      |       |        |
---|     |-------------|------|-------|--------|
03 |     |             |      |       |        |
------------------------------------------------

By the rules I know, TInputDevice should be an enum type, otherwise you’d have to explicitly give Integer there:

type xyz = record
  case integer of
  0: (foo: Byte);
  1: (bar: Integer);
end;

NB: it is customary for variant types to have one member describe which of the union members should be picked and valid (nested unions).