Why does this work and how? Non-static Qt object

I'm a little confused and would like to clear this up.

//QDir()::rmdir is from Qt Creator auto complete.
//It does not w开发者_如何学JAVAork. 
//Says no such static function.I looked it up, turns out to be true.
//Fair enough...though I'm not sure why auto-complete suggested it.
bool success = QDir()::rmdir("Y:/dir1/dir2/dir3");   //Does not work.

//Now I could make a QDir object as such.
//I didn;t test this but I'm sure it would work fine.
//However it seems clumsy.
QDir d("Y:/");     //This seems like a waste.
d.rmdir("Y:/dir1/dir2/dir3");

//Lastly, the source of my confusion.  QDir().rmdir
//This works, but WHY?
//There is no empty constructor for QDir in Qt Documentation.
//http://doc.qt.nokia.com/4.7/qdir.html
//Yet this empty constructor version works.  Why?
bool success = QDir().rmdir("Y:/dir1/dir2/dir3");

My main concern is why does the last examaple [QDir().rmdir] work? I've noticed this on a number of Qt classes. Is this an anonymous object and if so what does this mean with regards to object clean up? Is this form safe to use?

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One of the QDir constructors is:

QDir ( const QString & path = QString() )

Your QDir().xxx code is calling this constructor, which then uses default to using a QString() as one argument.

This is safe and normal to do.

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QDir creates a temporary object. Pretty the same if you callsth. like:

QString s("123"); 
int answer = 40 + s.left(2).right(1).toInt();

Second line produces 2 temporary objects.