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Under The Hood #2 - Long Strings See Also:  Delphi Papers and Dr.Bob's Examines Columns

As a long-term Turbo Pascal user, I've known Strings to be limited to 255 characters only, with the 0th 'character' being the actual length-Byte (range 0..255). This lead to dirty-but-fast code such as the following to trim the spaces on the right side of the String:

  procedure RightTrim16(var Str: String);
  var
    Len: Byte absolute Str; { Len is overlaid with the 0th length byte of Str }
  begin
    while Str[Len] = ' ' do Dec(Len)
  end {RightTrim16};

Length
Now that we know we cannot use the 0th Byte of a string as length indicator, how can we remove the trailing spaces from a long String? Well, we can still use the Length function to get the length of a String. We only need to realise that it's a 32-bit integer now. To remove trailing spaces from a (long) String, we now need to walk back from the end of the long String and count the number of spaces that are at the end. After having counted them, we can just Delete them from the String (you don't want to Delete the spaces one at a time, since this could involve copying the long String for every Delete operation, something you'd like to avoid as much as possible for performance's sake).

  procedure RightTrim32(var Str: String);
  var
    SPos,SLen: Integer;
  begin
    Spos := Length(Str);
    if Str[SPos] = ' ' then { are there trailing spaces to begin with? }
    begin
      SLen := 0;
      while (SPos > 0) and (Str[SPos] = ' ') do
      begin
        Inc(SLen);
        Dec(SPos)
      end;
      Delete(Str,SPos+1,SLen);
    { SetLength(Str,SPos); }
    end
  end {RightTrim32};

  {$IFNDEF WIN32} { to make sure it doesn't get added into Delphi 2 code }
  procedure SetLength(var Str: String; Len: Integer);
  begin
    Str[0] := Chr(Len);
  end {SetLength};
  {$ENDIF}

  procedure RightTrim(var Str: String);
  var
    SPos: Integer;
  begin
    Spos := Length(Str);
    if Str[SPos] = ' ' then { are there trailing spaces to begin with? }
    begin
      while (SPos > 0) and (Str[SPos] = ' ') do Dec(SPos);
      SetLength(Str,SPos);
    end
  end {RightTrim};

  function TrimRight(const S: string): string;
  { TrimRight trims trailing spaces and control characters from the given string.
  }
  var
    I: Integer;
  begin
    I := Length(S);
    while (I > 0) and (S[I] <= ' ') do Dec(I);
    Result := Copy(S, 1, I);
  end;

Don't depend on the {$H} compiler directive (which works on a unit basis), but use the ShortString and AnsiString predefined types instead. The compiler may use a String as a Long String while you think it's a Short String or vice versa, but when you use ShortString or AnsiString types this will never happen.

  unit Hood2;
  interface

  {$IFNDEF WIN32}
    procedure RightTrim16(var Str: String);
    procedure SetLength(var Str: String; Len: Integer);
  {$ELSE}
    procedure RightTrim32(var Str: String);
  {$ENDIF}

    procedure RightTrim(var Str: String);

  implementation

  {$IFNDEF WIN32} { to make sure it doesn't get added into Delphi 2 code }
    procedure RightTrim16(var Str: String);
    var
      Len: Byte absolute Str; { Len is overlaid with the 0th length byte of Str}
    begin
      while Str[Len] = ' ' do Dec(Len)
    end {RightTrim16};

    procedure SetLength(var Str: String; Len: Integer);
    begin
      Str[0] := Chr(Len);
    end {SetLength};
  {$ELSE}
    procedure RightTrim32(var Str: String);
    var
      SPos,SLen: Integer;
    begin
      Spos := Length(Str);
      if Str[SPos] = ' ' then { are there trailing spaces to begin with? }
      begin
        SLen := 0;
        while (SPos > 0) and (Str[SPos] = ' ') do
        begin
          Inc(SLen);
          Dec(SPos)
        end;
        Delete(Str,SPos,SLen);
      { SetLength(Str,SPos); }
      end
    end {RightTrim32};
  {$ENDIF}

    procedure RightTrim(var Str: String);
    var
      SPos: Integer;
    begin
      Spos := Length(Str);
      if Str[SPos] = ' ' then { are there trailing spaces to begin with? }
      begin
        while (SPos > 0) and (Str[SPos] = ' ') do Dec(SPos);
        SetLength(Str,SPos);
      end
    end {RightTrim};
  end.