I had an interesting issue with a MenuBar and a THISBACK.
I made three classes: MenuBarWindow, FnWindow and MyWindow.
- MenuBarWindow only has the MenuBar attribute and the method I want as its callback.
- FnWindow inherits from MenuBarWindow and has some buttons for the F1-F8 keys and the corresponding callback methods.
- MyWindow inherits from FnWindow and is the main window in my application.

Here is an example code:

class MenuBarWindow : public TopWindow
{
protected:
    MenuBar m_menuBar;
    
public:
    typedef MenuBarWindow CLASSNAME;
    MenuBarWindow(int _sizeX, int _sizeY);

protected:
    virtual void MenuBarMain(Bar& _bar) = 0;
};

MenuBarWindow::MenuBarWindow(int _sizeX, int _sizeY)
{
    SetRect(0, 0, Zx(_sizeX), Zy(_sizeY + m_menuBar.GetStdHeight(m_menuBar.GetFont())));
    AddFrame(m_menuBar);
    m_menuBar.Set(THISBACK(MenuBarMain));
}


class FnWindow : public MenuBarWIndow
{
protected:
    Button b_fn1;
    // ...
    Button b_fn8;

public:
    typedef FnWindow CLASSNAME;
    FnWindow(int _sizeX, int _sizeY);

private:
    virtual void OnClickFn1() = 0;
    // ...
    virtual void OnClickFn8() = 0;
};

FnWindow::FnWindow(int _sizeX, int _sizeY)
    : MenuBarWindow(_sizeX, _sizeY)
{
    // Adding buttons here...

    b_fn1 <<= THISBACK(OnClickFn1);
    // ...
    b_fn8 <<= THISBACK(OnClickFn8);
}

// MyWindow header...
MyWindow::MyWindow(int _sizeX, int _sizeY)
    : FnWindow(_sizeX, _sizeY)
{
    // do sth...
}

In the MyWindow files are declarations and definitions for the virtual methods.

If I run this the program stops and shows me this and exits the application when you continue:


If I remove

virtual void MenuBarMain(Bar& _bar) = 0;

and move

m_menuBar.Set(THISBACK(MenuBarMain));

from MenuBarWindow to MyWindow everything works fine.

At first I thought this is because the method is pure virtual and needs an implementation in the class from where I call it or use THISBACK but then I should get this strange behaviour after my changes as well because I do the same with the button callbacks but they work perfectly.

So does anyone has an idea why this is happening? Is this a generell C++ thing or is it a U++ thing?