how to use a global vars in QT?

[wter27]

Someone said you could write a static class and call it.
But I just wondered if I could use a global varieties to initialize my program.
Here is the code:

extern int var_name ;
void MainWindow::function_name_1()
{
int var_name;
var_name=1;
};
void MainWindow::function_name_2()
{
some_other_function(var_name);
};

extern int var_name ;
void MainWindow::function_name_1()
{
int var_name;
var_name=1;
};
void MainWindow::function_name_2()
{
some_other_function(var_name);
};

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The compiler says may be used uninitialized in this function.
So how to fix it?Anyone could give me some suggestions to use the global vars?