twm’s blog: When const doesnt mean const

Consider this code:

procedure SplitAt(const _Input: string; _Position: integer; out _Head, _Tail: string);
begin
  _Head := Copy(1, _Position-1);
  _Tail := Copy(_Position);
end;

It’s meant to split a given string into two parts at the given position.

Nothing fancy, really, isn’t it?

Now, consider this call to the procedure above:

var
  s1: string;
  s2: string;
begin
  s1 := 'hello world';
  SplitAt(s1, 5, s1, s2);
  WriteLn('s1: "', s1, '"');
  WriteLn('s2: "', s2, '"');
end;

Which output do you expect?

The output I got certainly wasn’t what I expected:

s1: ""
s2: ""

I took me a while to understand what happened:

The compiler passes the parameters right to left, so the first parameter it passes is s2, then s1, then 5 and last s1 again as the input. Since the two rightmost parameters are *out* parameters it clears them before passing them, so s1 is already an empty string when it gets passed as input.

Not quite what I would have expected. It doesn’t help to remove the const modifier either.