QTCP | QTCP is a fucking TCP | TCP library

You are not using QTcpSocket in its proper way. In your Client class, you can handle all reading stuff and you won't even need those while loops. Use QIODevice::readyRead() signal and QIODevice::read(char *data, qint64 maxSize) method inside a slot which should read data and parse it.

The problem here is that it's not guaranteed that those data will be received with alignments happening on the sender side. For example, if you send >>>SOME_LENGTHY_DATA<<< from the sender, QTcpSocket may emit readyRead() signal two times and you can read >>>SOME_LENG and THY_DATA<<< in two different calls to your reading slot.

So, you should take care of parsing your data as data is coming in byte by byte. You are free to buffer the incoming data somewhere in your class or just use the QTcpSocket internal buffer. You should really take care of separating message chunks that are coming in.

Here is an implementation of Client. I use QPointF to store point instead of your Target class because I don't know what it is. You are free to create your own class and append it to Client::data based on parsed lat/lon. I did not test the code but it should work. I'm not sure if you ever need concurrency with this implementation but you won't need one IMHO.