The Delphi Bug List

Consider the following code:

function Test(AssignResult: Boolean): string;
begin
  if AssignResult then
    Result := 'a';
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  S: string;
begin
  S := Test(True);
  S := Test(False);
  Caption := S;
end;

What will the value of S be at the time it's assigned to Caption? Interestingly, it will be 'a'. The first call to the Test function will set Result to 'a'. The second call to the Test function will not assign anything to Result. The reason why it returns 'a' anyway is Delphi internally passes the same string variable in both calls without clearing it in between.

I consider the lack of a warning a compiler bug because Delphi does issue a warning on similar-looking functions that return Integers instead; for example:

function Test(AssignResult: Boolean): Integer;
begin
  if AssignResult then
    Result := 1;
end;

The above code sample should be changed to:

function Test(AssignResult: Boolean): string;
begin
  if AssignResult then
    Result := 'a'
  else
    Result := '';
end;

This problem also occures when the function result is a variant. (Tested under Delphi 5 SP1 Enterprise)

After spending some time on this issue I think I can be pretty sure that this happens for all non fixed-size result types like strings (any type), sets, records and arrays (dynamic and static). I think Borland should think about this issue as it becomes critical as the size of the project gets bigger.