How does QIODevice::readLine(qint64 maxSize = 0) works?

Question

The documentation of this method states:

QByteArray QIODevice::readLine(qint64 maxSize = 0)
This is an overloaded function.
Reads a line from the device, but no more than maxSize characters, and returns the result as a byte array.

However, I notice that even if we pass maxSize = 0, viz. don't pass anything, the readLine() sends several bytes in the line. Is this a well defined behaviour?

Besides, here is one decade old issue on the same topic: QIODevice::readLine(qint64 maxSize = 0) default argument performs poorly (should be documented). Is this still relevant?

Answer 1

It's not mentioned in the Qt API documentation, but passing in a value of 0 as the maxSize argument is treated as having a special meaning of "as many bytes as possible". Evidence of that intent can be seen at line 1466 of the qiodevice.cpp source file in Qt:

if (maxSize == 0) maxSize = MaxByteArraySize - 1;