Question

Quick one; am I right in thinking that passing a string to a method ‘as a CONST’ involves more overhead than passing a string as a ‘VAR’? The compiler will get Delphi to make a copy of the string and then pass the copy, if the string parameter is declared as a CONST, right?

The reason for the question is a bit tedious; we have a legacy Delphi 5 utility whose days are truly numbered (the replacement is under development). It does a large amount of string processing, frequently passing 1-2Kb strings between various functions and procedures. Throughout the code, the ‘correct’ observation of using CONST or VAR to pass parameters (depending on the job in hand) has been adhered to. We’re just looking for a few ‘quick wins’ that might shave a few microseconds off the execution time, to tide us over until the new version is ready. We thought of changing the memory manager from the default Delphi 5 one to FastMM, and we also wondered if it was worth altering the way the strings are passed around – because the code is working fine with the strings passed as const, we don’t see a problem if we changed those declarations to var – the code within that method isn’t going to change the string.

But would it really make any difference in real terms? (The program really just does a large amount of processing on these 1kb+ish strings; several hundred strings a minute at peak times). In the re-write these strings are being held in objects/class variables, so they’re not really being copied/passed around in the same way at all, but in the legacy code it’s very much ‘old school’ pascal.

Naturally we’ll profile an overall run of the program to see what difference we’ve made but there’s no point in actually trying this if we’re categorically wrong about how the string-passing works in the first instance!

Answer 1

No, there shouldn’t be any performance difference between using const or var in your case. In both cases a pointer to the string is passed as the parameter. If the parameter is const the compiler simply disallows any modifications to it. Note that this does not preclude modifications to the string if you get tricky:

procedure TForm1.Button1Click(Sender: TObject);
var
  s: string;
begin
  s := 'foo bar baz';
  UniqueString(s);
  SetConstCaption(s);
  Caption := s;
end;

procedure TForm1.SetConstCaption(const AValue: string);
var
  P: PChar;
begin
  P := PChar(AValue);
  P[3] := '?';
  Caption := AValue;
end;

This will actually change the local string variable in the calling method, proof that only a pointer to it is passed.

But definitely use FastMM4, it should have a much bigger performance impact.