Delphi: Is it necessary to convert string to WideString?

Question

I found a Windows API function that performs "natural comparison" of strings. It is defined as follows:

int StrCmpLogicalW(      
    LPCWSTR psz1,
    LPCWSTR psz2
);

To use it in delphi, I declared it this way:

interface
  function StrCmpLogicalW(psz1, psz2: PWideChar): integer; stdcall;

implementation
  function StrCmpLogicalW; external 'shlwapi.dll' name 'StrCmpLogicalW';

Because it compares unicode strings, I'm not sure how to call it when I want to compare ansi strings. It seems to be enough to cast strings to WideString and then to PWideChar, however, I have no idea whether this approach is correct:

function AnsiNaturalCompareText(const S1, S2: string): integer;
begin
  Result := StrCmpLogicalW(PWideChar(WideString(S1)), PWideChar(WideString(S2)));
end;

I know very little about character encoding so this is the reason of my question. Is this function OK or should I first convert both the compared strings somehow?

Thanks for your answers in advance.

Mariusz.

Answer 1

Yes, that's the right way to get a PWideChar from an AnsiString. Kind of ugly, but correct.

You might want to try wrapping it in a helper function, to improve readability. For example:

function WideCString(const value: AnsiString): PWideChar;
begin
  result := PWideChar(WideString(value));
end;

Answer 2

There might be an ANSI variant for your function to (I haven't checked). Most Wide API's are available as an ANSI version too, just change the W suffix to an A, and you're set. Windows does the back-and-forth conversion transparantly for you in that case.

PS: Here's an article describing the lack of StrCmpLogicalA : http://blogs.msdn.com/joshpoley/archive/2008/04/28/strcmplogicala.aspx

Answer 3

Keep in mind that casting a string to a WideString will convert it using default system codepage which may or may not be what you need. Typically, you'd want to use current user's locale.

From WCharToChar in System.pas:

Result := MultiByteToWideChar(DefaultSystemCodePage, 0, CharSource, SrcBytes,
  WCharDest, DestChars);

You can change DefaultSystemCodePage by calling SetMultiByteConversionCodePage.