qint64 QIODevice::read(char *, qint64) and friends require the user to make sure that the buffer is large enough or valid, hence they are rather unsafe. Their overloads returning QByteArray cause a new buffer (within QByteArray) to be allocated.

If QIODevice had the following overloads it would be possible to reuse the capacity of an already available QByteArray instance safely:

qint64 QIODevice::read(QByteArrray *);
qint64 QIODevice::readAll(QByteArrray *);
qint64 QIODevice::readLine(QByteArrray *);
qint64 QIODevice::peek(QByteArrray *);

These overloads would make all sorts of data processing applications perform better.