separate class using an interface

[qt_gotcha]

A very basic question! I am porting a program from Borland builder. This is the setup: a ui interface with a button and a label. A separate clas that is a mathematical model called TWorld, doing a number of calculations. The ui is a class generated in the Eclipse IDE, the main function looks like:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    mod3 w;
    w.show();
    return a.exec();
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    mod3 w;
    w.show();
    return a.exec();
}

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where mod3 is a QWidget. Eclipse generates the necessary ui related files. The class TWorld is declared in the class mod3 as:

...
public:
TWorld *W;
...

...
public:
TWorld *W;
...

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If the button is pushed the mathmodel class is created and calculations are executed:

void mod3:: on_pushButton_clicked()
{
 W = new TWorld();	
 W->docalc();
}

void mod3:: on_pushButton_clicked()
{
 W = new TWorld();	
 W->docalc();
}

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at the end of the calculations the result should be displayed as the label text on the interface:

void TWorld::docalc()
{
    ... lots of calculations ...
    w.label->setNum(result);  //doesn't work
}

void TWorld::docalc()
{
    ... lots of calculations ...
    w.label->setNum(result);  //doesn't work
}

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this doesn't work of course because label is not part of W (TWorld) but of the interface mod3. mod3 is defined and created in the main function as "w" and only exists there it seems to me. If I make w a global variable, defined as extern in a header file with the definition in main.cpp, I get the error "QWidget: Must construct a QApplication before a QPaintDevice"
I don't understand how I can make the class TWorld "see" the interface. Probably something very simple I am overloocking!
thanks!