Qt Creator - Main widow and second window

[ChrisW67]

but sir, is there a way to fix my error?
My professor wants me to do it that way

you say that my error is that I don't have a class dialog.
but don't I have one at secwindow.h line 23??

Actually, yes you do have a declaration of Dialog at that location. I misread the error message. While you do have a Dialog class, you do not have a Dialog class constructor that accepts an argument. Your code:

Dialog *dialog = new Dialog(this); //ERROR: no matching constructor for initialization of 'Dialog'

Dialog *dialog = new Dialog(this); //ERROR: no matching constructor for initialization of 'Dialog'

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is passing an argument and this does not match any constructor you have declared:

class Dialog : public QDialog
{
public:
    Dialog();  //  <<<< no arguments accepted
...

class Dialog : public QDialog
{
public:
    Dialog();  //  <<<< no arguments accepted
...

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Your Dialog constructor should have a similar declaration and implementation to the one you have for SecWindow.