What is the correct way to use Singleton pattern with QObject which is gonna be used with multithreading? Of course this way should exclude all Qt multithreading errors like
QObject: Cannot create children for a parent that is in a different thread.
I do not think that there is a generally accepted correct way to do this. The problem is that the Singleton pattern is avoided in modern software, because having a global reference to to a certain object makes unit testing really hard, sometimes impossible.
Do not misunderstand me, it is ok to have singletons where it seems logical. The problem is with the pattern, where you access them through a global variable or function like MySingletonClass::getInstance().
Try injecting it instead where it is needed through constructor parameter, or setter methods.
About multithreading: I do not think that Singletons are any special in this aspect. They are the same as any object accessed concurrently.
The run-time error message is specific to Qt and not C++. Of course you can use the object created on the other thread every each way you want. BUT you cannot pass it as a parent (that is Qt) to objects created on another thread so that the parent cannot destroy its children while itself getting destructed. It is also has to do with Qt messaging mechanism and timers. You may find QObject::moveToThread description interesting as well.
