Question

Does Rewrite clear file contents of existing file or does it delete it and create a new one?
I have a text file in my app.exe folder which I need to clear. Any example?

Answer 1

From the Delphi XE2 documentation, topic Rewrite – read the last quoted paragraph:

Creates a new file and opens it.

In Delphi code, Rewrite creates a new external file with the name assigned to F.

F is a variable of any file type associated with an external file using AssignFile. RecSize is an optional expression that can be specified only if F is an untyped file. If F is an untyped file, RecSize specifies the record size to be used in data transfers. If RecSize is omitted, a default record size of 128 bytes is assumed.

If an external file with the same name already exists, it is deleted and a new empty file is created in its place.

From the same documentation, link at the bottom of page for System.Rewrite, modified to use your app’s folder:

procedure TForm1.Button1Click(Sender: TObject);
var 
  F: TextFile;
  AppDir: string;
begin
  // Instead of ParamStr(0), you can use Application.ExeName
  // if you prefer
  AppDir := ExtractFilePath(ParamStr(0)); 
  AssignFile(F, AppDir + 'NEWFILE.$$$');
  Rewrite(F);  // default record size is 128 bytes
  Writeln(F, 'Just created file with this text in it...');
  CloseFile(F);
  MessageDlg('NEWFILE.$$$ has been created in the ' + AppDir + ' directory.',
    mtInformation, [mbOk], 0, mbOK);
end;

You should know, though, that Rewrite is outdated and doesn’t support Unicode. You should be using more modern methods to read and write files like TFileStream or TStringWriter (or even the simple solution of TStringList).

var
  SL: TStringList;
  AppDir: string;
begin
  AppDir := ExtractFilePath(ParamStr(0));
  SL := TStringList.Create;
  try
    SL.Add('Just created file with this text in it...');
    // Add more lines here if needed, and then only save once
    SL.SaveToFile(AppDir + 'NEWFILE.$$$');
    MessageDlg('NEWFILE.$$$ has been created in the ' + AppDir + ' directory.',
      mtInformation, [mbOk], 0, mbOK);
  finally
    SL.Free;
  end;
end;

Note that you cannot use TStrings; it’s an abstract class. You need to use one of it’s descendants instead (TStringList is the one used most often).