How to create cross platform `QFileDialog` with default directory?

I have a code like 

```
QFileDialog::getSaveFileName(this,"Save File","/home",tr("Any File (*.*);;Text file (*.txt);;Compressed file (*.zip *.tar *.rar);;Image (*.png *.xpm *.jpg *.tiff)"))

```

This works fine with Linux. But in windows the users directory will be `"C:\Users"`. I don't know in Mac. I need this dialogue with default directory as user folder. How to do that?

I saw the `toNativeSeparators()` but this need more line of codes. Is there any easiest way to do that?