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\\Projec

Answer 1

For what it's worth, my codebase uses SysUtils.ExtractRelativePath in one direction and the following home-grown wrapper coming back:

function ExpandFileNameRelBaseDir(const FileName, BaseDir: string): string;
var
  Buffer: array [0..MAX_PATH-1] of Char;
begin
  if PathIsRelative(PChar(FileName)) then begin
    Result := IncludeTrailingBackslash(BaseDir)+FileName;
  end else begin
    Result := FileName;
  end;
  if PathCanonicalize(@Buffer[0], PChar(Result)) then begin
    Result := Buffer;
  end;
end;

You'll need to use the ShLwApi unit for PathIsRelative and PathCanonicalize.

The call to PathIsRelative means that the routine is robust to absolute paths being specified.

So, SysUtils.ExtractRelativePath can be your AbsToRel only the parameters are reversed. And my ExpandFileNameRelBaseDir will serve as your RelToAbs.

Answer 2

Another version of RelToAbs (compatible with all Delphi XE versions).

uses
  ShLwApi;

    function RelPathToAbsPath(const ARelPath, ABasePath: string): string;
    var Buff:array[0..MAX_PATH] of Char;
    begin
      if PathCombine(Buff, PChar(IncludeTrailingPathDelimiter(ABasePath)), PChar(ARelPath)) = nil then
        Result := ''
      else Result:=Buff;
    end;

Answer 3

An alternate solution for RelToAbs is simply:

ExpandFileName(IncludeTrailingPathDelimiter(BasePath) + RelPath)

Answer 4

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).

Answer 5

Check if your solution will works with Relative Path To Full Path in case when you change current directory. This will works:

function PathRelativeToFull(APath : string) : string;
var
  xDir : string;
begin
  xDir := GetCurrentDir;
  try
    SetCurrentDir('C:\Projects\Project1\');
    Result := ExpandFileName(APath);
  finally
    SetCurrentDir(xDir);
  end{try..finally};
end;

function PathFullToRelative(APath : string; ABaseDir : string = '') : string;
begin
  if ABaseDir = '' then
    ABaseDir := 'C:\Projects\Project1\';
  Result := ExtractRelativePath(ABaseDir, APath);
end;

Answer 6

I just brewed this together:

uses
  ShLwApi;

function RelToAbs(const ARelPath, ABasePath: string): string;
begin
  SetLength(Result, MAX_PATH);
  if PathCombine(@Result[1], PChar(IncludeTrailingPathDelimiter(ABasePath)), PChar(ARelPath)) = nil then
    Result := ''
  else
    SetLength(Result, StrLen(@Result[1]));
end;

Thanks to Andreas and David for calling my attention to the Shell Path Handling Functions.