What is the implementation of QDir::count()

Question

All over the internet it seems the accepted way of getting a directory's child count is to loop through its entries manually and count them. This seems far too tedious and generally there are other frameworks and API's that offer a convenience function to get child counts, one of them being QDir::count().

Is there a POSIX API that returns the number of entries in a directory efficiently? If so, I would imagine that this is the implementation of QDir::count(). If not, then perhaps it's done the way so many people are doing it, looping through the entries and counting them if they're appropriate.

If you care about efficiency, you should take a look at `QDirIterator` itself. — Pavel Strakhov, Jun 17 '14 at 22:12

score 4 · Accepted Answer · answered Jun 17 '14 at 20:50

Internally, QDir iterates over the directory reading the entries into a list. QDir::count() returns the number of entries in that list (count() will populate the list if that hasn't happened yet).

See qtbase/src/corelib/io/qdir.cpp for the implementation of

QDir::count():

uint QDir::count() const
{
    const QDirPrivate* d = d_ptr.constData();
    d->initFileLists(*this);
    return d->files.count();
}

QDirPrivate::initFileLists():

inline void QDirPrivate::initFileLists(const QDir &dir) const
{
    if (!fileListsInitialized) {
        QFileInfoList l;
        QDirIterator it(dir);
        while (it.hasNext()) {
            it.next();
            l.append(it.fileInfo());
        }
        sortFileList(sort, l, &files, &fileInfos);
        fileListsInitialized = true;
    }
}

What is the implementation of QDir::count()

1 Answers1