Conversion between absolute and relative paths in Delphi

Question

Are there standard functions to perform absolute <--> relative path conversion in Delphi?

For example:

• 'Base' path is 'C:\Projects\Project1\'
• Relative path is '..\Shared\somefile.pas'
• Absolute path is 'C:\Projects\Shared\somefile.pas'

I am looking for something like this:

function AbsToRel(const AbsPath, BasePath: string): string; // '..\Shared\somefile.pas' = //   AbsToRel('C:\Projects\Shared\somefile.pas', 'C:\Projects\Project1\')   function RelToAbs(const RelPath, BasePath: string): string; // 'C:\Projects\Shared\somefile.pas' = //   RelToAbs('..\Shared\somefile.pas', 'C:\Projects\Project1\')   

Answer 1

To convert to the absolute you have :

ExpandFileName

To have the relative path you have :

ExtractRelativePath

Answer 2

I would use PathRelativePathTo as the first function and PathCanonicalize as the second. In the latter case, as argument you pass the string sum of the base path and the relative path.

function PathRelativePathTo(pszPath: PChar; pszFrom: PChar; dwAttrFrom: DWORD;   pszTo: PChar; dwAtrTo: DWORD): LongBool; stdcall; external 'shlwapi.dll' name 'PathRelativePathToW';  function AbsToRel(const AbsPath, BasePath: string): string; var   Path: array[0..MAX_PATH-1] of char; begin   PathRelativePathTo(@Path[0], PChar(BasePath), FILE_ATTRIBUTE_DIRECTORY, PChar(AbsPath), 0);   result := Path; end;  function PathCanonicalize(lpszDst: PChar; lpszSrc: PChar): LongBool; stdcall;   external 'shlwapi.dll' name 'PathCanonicalizeW';  function RelToAbs(const RelPath, BasePath: string): string; var   Dst: array[0..MAX_PATH-1] of char; begin   PathCanonicalize(@Dst[0], PChar(IncludeTrailingBackslash(BasePath) + RelPath));   result := Dst; end;   procedure TForm4.FormCreate(Sender: TObject); begin   ShowMessage(AbsToRel('C:\Users\Andreas Rejbrand\Desktop\file.txt', 'C:\Users\Andreas Rejbrand\Pictures'));   ShowMessage(RelToAbs('..\Videos\movie.wma', 'C:\Users\Andreas Rejbrand\Desktop')); end; 

Of course, if you use a non-Unicode version of Delphi (that is, <= Delphi 2007), you need to use the Ansi functions (*A) instead of the Unicode functions (*W).