Why can't i use global variable in a QThread based class?

Suppose the minimal case , which contains one class named Worker based on QThread , and another named Parser , which is a global class , resided in Worker c开发者_如何学Golass.

# parser.h
class Parser;
extern Parser *App_Parser;

class Parser {
    bool init() { App_Parser = new Parser(); }
}

# parser.cpp
Parser *App_Parser = 0;

# worker.cpp
class Worker: public QThread {
    Worker(int thread_id , QObject *parent) {
        Parser::init ();
        connect ( App_Parser , .... , this , SLOT(parseCompleted()) );
    }
    private slots:
           void parseCompleted () { 
                qDebug() << "Thread ID: " << thread_id << " completed"; 
           }
}

So here goes the problem , if i created 12 Worker in a higher class , i sometimes could see invalid thread_id , and it's always just one above it , in this case , always 13.

And if i don't use App_Parser , but just use new Parser() for an simple instance , it works.

So i'm thinking about why global variables shouldn't be used here.

---

Read this. It makes using threads in Qt a little bit cleaner: http://labs.qt.nokia.com/2010/06/17/youre-doing-it-wrong/

Whole problem is about where does QObject lives. When you create new Worker, you create QThread in main thread, so all it's events/signals are also parsed in main thread. You misunderstood whole qt "thread thing".