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.

Answer 1

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;
      }
  }