Your ClientRunnable class declares the member variable "Parent" as type QWidget *. QWidget has no slot named "ClientStarted", so the compiler is telling you that. If you want this to work, you need to qobject_cast<ServerMTh *>( parent ) in the ClientRunnable constructor and store it explicitly as ServerMTh * Parent.

In line 79 of ServerMTh.cpp above, you create a ClientRunnable instance with no parent. This will cause a crash as soon as run() executes since Parent will be uninitialized.