OK, I cannot find an actual
“this is how to resolve this ”
apparently C++ syntax error
by …. defining the class this way…

What is confusing is
it complains about wrong declaration to be replaced with correct ****definition ****

So
what is it – declaration or definition ?

Here is the actual code snippet, class declaration of class MainWindow_Bluewtoothctl_Dialog
giving the error (.h file ) when class REG_EXP (class remember variable) is added to it :

class MainWindow_Bluewtoothctl_Dialog : public QDialog
{
    Q_OBJECT
public:
    // May30 class test
    class REG_EXP
    {
        int a;
        QString text;
    };
    // May24
    QString ClassResult;           

Please
provide actual code and maybe attach an explanation why or
better yet a reference to some manual.

It did not like this

class REG_EXP:

same error