Creation and writing to .dat file using QFile on Windows some problem…

[freely]

Hello,
Does anybody know what is wrong with next:
I need to create file in some directory with rw permissions on disk then write to it data and open after. On Windows and Linux, MacOS also.
For that I use next code:

...
QString outputDir = "some_dir"; // with "/" separators
QString fileName = "some_file_name" + ".dat";
QFile file(outputDir+"/"+fileName);
if (!file.open(QIODevice::WriteOnly|QFile::WriteOnly))
     {
         QMessageBox::warning(0,"Could not create Project File",
                                    QObject::tr( "\n Could not create Project File on disk"));
 
          return false;
     }
else
    {
              QDataStream stream(&file);
              ...
              file.close();
    }
...

...
QString outputDir = "some_dir"; // with "/" separators
QString fileName = "some_file_name" + ".dat";
QFile file(outputDir+"/"+fileName);
if (!file.open(QIODevice::WriteOnly|QFile::WriteOnly))
     {
         QMessageBox::warning(0,"Could not create Project File",
                                    QObject::tr( "\n Could not create Project File on disk"));
 
          return false;
     }
else
    {
              QDataStream stream(&file);
              ...
              file.close();
    }
...

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On Linux ( Ubuntu ) it works ok ( file.open(…) returns true ) – file is created , written , closed ,reopened ,read. But when I run the same code on Windows file.open(…) returns false in any directory I choose. It is written in Qt docs that Qt automatically treats right directory path names ( separators ) for underlying OS ( translates “/” ( default ) to underlying OS ).
I’ve tried to use QDir::toNativeSeparators(outputDir+”/”+fileName) inside file.open(…) but still the file couldn’t be created on Windows – file.open(…) returns false.
***
Does anybody can help/point what is wrong with creating new file on windows in this case ? ( what is wrong with file.open(…) ) ?
How to solve so that I can open new file on Windows using QFile?

Thank you in advance.