i'm looking for a C++ replacement of the Python PubSub Library in which i don't have to connect a signal with a slot or so, but instead can register for a special Kind of messages, without knowing the object which can send it.
Perhaps you misunderstand what signals and slots are. With signals and slots you don't have to know who sends signals. Your "client" class just declares slots, and an outside manager can connect signals to them.
I recommend you to check out Qt. It's an amazing cross-platform library with much more than just GUI support. It has a convenient and efficient implementation of signals and slots which you can use.
These days it's also licensed with LGPL (in addition to GPL and commercial), so you can use it for practically any purpose.
Re your clarification comment, why not raise an exception for the error? The parent can notify the GUI, or alternatively the GUI can register for a signal the parent emits. This way the parent also doesn't have to know about the GUI.
Python's PubSub/observer Pattern for C++?