A dependency loop - c++

Question

I've designed an object inherits from CDialog (called NBDialog, and some derived objects of controls, such as CEdit, CDateTimeCtrl, CComboBox etc.
The NBDialog is one project, and the controls are in other projects.
Naturally, All of the controls are put on the dialog and use dialog's methods, so I have to
#include NBDialog.h, and to add its .lib file for the linker.
I also want to handle all those controls from the dialog, so I wrote in NBDialog.h the following lines:
class NBCommonEditBox;
class NBDateTimeCtrl;
class NBCommonComboBox;
CMapWordToOb* NBGetEditBoxMap();
NBCommonEditBox* NBGetEditBoxById(unsigned long ID);
CMapWordToOb* NBGetDateTimeMap();
NBDateTimeCtrl* NBGetDateTimeById(unsigned long ID);
CMapWordToOb* NBGetComboBoxMap();
NBCommonComboBox* NBGetComboBoxById(unsigned long ID);
This way NBDialog.h doesn't know the context of the object, but it knows they are exist and stores them in the maps.
Now I want to extend the NBDialog project and add a method which will get the print information of all controls, so all objects which inhertied from NBDialog will be able to use this method. The print information is defined in the controls implementation.
EDIT: If I write this method in NBDialog.cpp, I can't compile it, because NBDialog doesn't know the context of the controls' classes:
CStringList* NBDialog::NBGetMainTexts()
{
CStringList* mainTexts = new CStringList();
POSITION pos;
WORD key;
NBCommonEditBox* currEdit = NULL;
for (pos = this->NBGetEditBoxMap()->GetStartPosition(); pos != NULL;)
{
this->NBGetEditBoxMap()->GetNextAssoc(pos, key, (CObject*&)currEdit);
currEdit->NBStringsToPrint(mainTexts);
}
return mainTexts;
}
Is there a way to write the desired method?

Answer 1

Easiest way is to define an interface for this and add that interface instead of the CObject. The interface can offer a method to get hold of the control itself. Don;t be afraid of multiple inheritance - yes it can have a slight performance penalty but it is not going to be an issue for you. In this case it will be similar to interface inheritance in Java since you would use a pure interface.
You could also implement this in a similar way that avoids multiple inheritance but it adds more complexity that you don't need.
// Interface goes in the NBDialog project
class INBControl {
public:
virtual ~INBControl() = 0;
virtual CWnd* getWnd() = 0;
virtual void getStringsToPrint(CStringList& strings) = 0;
};
inline INBControl::~INBControl() {}
class NBCommonComboBox : public CComboBox, public INBControl
{
public:
// ... stuff ...
virtual CWnd* getWnd() {
return this;
}
virtual void getStringsToPrint(CStringList& strings) {
strings.AddTail("foo"); // for example
}
};
// NBDialog
#include <map>
class NBDialog : public CDialog
{
public:
// .. stuff ..
private:
typedef std::map<int, INBControl*> ControlMap;
ControlMap control_map_;
};
void NBDialog::addNBControl(INBControl* control, int id)
{
CWnd* wnd = control->getWnd();
// Do stuff with the control such as add it
control_map_[id] = control;
}
// let the caller be responsible for [de]allocation of the string list
void NBDialog::NBGetMainTexts(CStringList& texts)
{
ControlMap::iterator i = control_map_.begin();
ControlMap::iterator e = control_map_.end();
for(; i != e; ++i) {
i->second->getStringsToPrint(texts);
}
}
Alternatively use a custom windows message and iterate all the controls, down-casting to CWnd and using SendMessage on its HWND. Each control will need to handle your custom windoes mesaage. You could pass a pointer to the string list in the LPARAM of the message. This apprach is flexible but somewhat brittle/unsafe and could crash if you end up using the same message ID for something else by accident.

Answer 2

Your implementation file (NBDialog.cpp) is free to #include the necessary headers to make this work (presumably things like NBCommonComboBox.h, etc.) Because the .cpp file isn't #include'd by anything you won't cause any circular include problems.