Passing Variables between classes in modules

[okibi]

I was wondering how I would go about passing a variable between two classes that are within different modules in a program. Here is an example of what I mean:

Let this be modMain:

//begin modMain

module modMain;

//imports go here

import modPopUp;

class modMain : MainWindow //this is for gtkD
{
  //this() would build the MainWindow
  
  modPopOp popWindow = new modPopUp();
}

void main(char[][] args)
{
  GtkD.init(args);
  modMain mMain = new modMain();
  GtkD.main();
}

//end modMain

Now let this be modPopUp:

//begin modPopUp

module modPopUp:

//imports go here

class modPopUp : MainWindow
{
  //this() would build the MainWindow
  
  char[] myStr;
  //function runs to generate myStr
}

//end modPopUp

What I'm asking is how do I pass myStr from modPopUp back to modMain?

Thanks!