QScopedPointer usage with Forward Declaration

[ca]

Hello everyone, this will be my very first thread. I am currently learning qt from tutorials and books. At some point, I implemented this class with Private
Implementation covered with QScopedPointer.

***1) Here is my MasterController class with a forward declaration of class "Implementation" with QScoped Pointer guards it.

class CMLIB_EXPORT MasterController : public QObject
{
    Q_OBJECT
 
public:
    explicit MasterController(QObject* parent = nullptr);
    ~MasterController();
 
    cm::controllers::NavigationController* ui_navigationController();
 
signals:
private:
 
    class Implemantation;
    QScopedPointer<Implemantation> implementation;
};

class CMLIB_EXPORT MasterController : public QObject
{
    Q_OBJECT

public:
    explicit MasterController(QObject* parent = nullptr);
    ~MasterController();

    cm::controllers::NavigationController* ui_navigationController();

signals:
private:
    
    class Implemantation;
    QScopedPointer<Implemantation> implementation;
};

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*** 2) Here is also my cpp file for MasterController class. Here we see previously declared Implemation class as well as constructor, deconstructor of Master Class.

class MasterController::Implemantation
{
public:
    Implemantation(MasterController* _masterController) : masterController(_masterController)
    {
        navigationController = new NavigationController(masterController);
    }
 
    ~Implemantation()
    {
    }
 
    MasterController* masterController;
    NavigationController* navigationController;
    QString welcomeMessage = "This is MasterController to Major Tom";
};
 
 
MasterController::MasterController(QObject* parent) : QObject(parent)
{
    //implementation is a scoped pointer
    implementation.reset(new Implemantation(this));
}
 
MasterController::~MasterController()
{
}
 
NavigationController* MasterController::ui_navigationController()
{
    return implementation->navigationController;
}

class MasterController::Implemantation
{
public:
    Implemantation(MasterController* _masterController) : masterController(_masterController)
    {
        navigationController = new NavigationController(masterController);
    }

    ~Implemantation()
    {
    }

    MasterController* masterController;
    NavigationController* navigationController;
    QString welcomeMessage = "This is MasterController to Major Tom";
};


MasterController::MasterController(QObject* parent) : QObject(parent)
{
    //implementation is a scoped pointer
    implementation.reset(new Implemantation(this));
}

MasterController::~MasterController()
{
}

NavigationController* MasterController::ui_navigationController()
{
    return implementation->navigationController;
}

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*** 3) Here is my Question: My private implemented Class (Implementation) has 3 members :
a- MasterController class that it lives on
b- Someother Navigation Class
c- A String

If my MasterController class comes to the end of scope , everythis is fine, all objects are deleted. From curiosity, i deleted Destructor from MasterController and i got following compile errors:

C:\Qt\5.14.2\msvc2017_64\include\QtCore\qscopedpoi nter.h:57: error: C2027: use of undefined type 'cm::controllers::MasterController::Implemantation '
c:\qtprojects\cm-learningqt\customermanager\cm-lib\source\controllers\mastercontroller.h:34: see declaration of 'cm::controllers::MasterController::Implemantation '
C:\Qt\5.14.2\msvc2017_64\include\QtCore\qscopedpoi nter.h:53: while compiling class template member function 'void QScopedPointerDeleter<T>::cleanup(T *)'
with
[
T=cm::controllers::MasterController::Implemantatio n
]

I would be pleased if someone explains what the problem is and why destructor is important here.
Thanks in advance