QFile::copy says “can't open source file for input”?

[.:saeed:.]

I'm going to copy one file using QFile::copy function but this function always returns false and errorString says :

"Cannot open D:/tmp/buf34.txt for input"
I tried to run this program with administrator privilege but nothing changed. My code is really simple :

QString source = url.toLocalFile();
  QString destination = _dir.absolutePath()
            + QString("/%1").arg(QFileInfo(source).fileName());
  qDebug()<<"Cp from :" << source << " to : "<< destination;
  QFile file(source);
  qDebug()<<file.copy(destination);
  qDebug()<<file.errorString();

QString source = url.toLocalFile();
  QString destination = _dir.absolutePath()
            + QString("/%1").arg(QFileInfo(source).fileName());
  qDebug()<<"Cp from :" << source << " to : "<< destination;
  QFile file(source);
  qDebug()<<file.copy(destination);
  qDebug()<<file.errorString();

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I have QListView occupied with a QFileSystemModel. I try to drag one file from this ListView to a QLabel. For the QLabel a destination path is set. In drop event I try to copy file.