Re: Polymorphism - accessing derived class data members

Victor Bazarov <>
Mon, 01 Feb 2010 15:03:15 -0500
Alexh wrote:

I have implemented various graphic elements (classes) which are
derived from a base class. The primary advantage of this is that any
operations on the derived classes can be done via virtual functions
and thus the code that performs the operation does not have to
identify the element (avoids a huge switch statement for every
operation). This is typical polymorphism.

The problem I'm having is modification of existing elements. The
properties of the elements are set up from a dialog (specific to each
type of element) which executes a new statement to create the element.
Since the pointer for element is the base type (and not the derived
class type), it cannot be used to access the derived class variables
which is necessary if the user wants to modify the element options for
an existing element. Here is some basic code -

class CElement : public CObject //base class

//vars here

virtual void Draw(...) const {};

CElement() {};

class CDog: public CElement //derived class

//vars here

virtual void CDog::Draw(...);

CDog(const CDogDlg& DogDlg); //constructor
CDog () {}


//implementation code generating Dog
CDogDlg DogDlg;

if (DogDlg.DoModal == IDOK){

CElement* pElement = new CDog(DogDlg);

//save pElement to a CTypedPtrList

Later, a user may need to modify an existing Dog. Currently, I do this
by opening DogDlg from a modify menu and on IDOK I delete the
exisiting Dog and simultaneously create a new one from the new dialog
data (delete/replace is transparent to user). The problem arises when
there is more than one Dog because then the dialog may not reflect the
correct data for the selected element (dialog reflects last useage).

It seems that you need to give the existing Dog to the DogDlg so the
dialog could extract the data from the object (instead of simply
generating a new one).

Ideally, I would like to have the dialog data reflect that actual data
associated with the Dog I am modifying.

Yes, that way it's certainly better. Generally, you might consider
creating a stand-alone "default Dog" when your dialog is invoked to
"create" a Dog, and pass the object to the same "edit the Dog" dialog.
And when editing an existing Dog, you need to create a copy of that Dog
before passing it to DogDlg. The decision to copy the resulting Dog
into the permanent storage (whether by creating a new one, or by copying
back into the existing one) should be made by the caller of the DogDlg -
if OK, store it, otherwise discard.

I asked this question on Codeproject and the only possible solution I
got was to use a dynamic_cast operator. I don't necessarily think
there is anything wrong with this but I'd like to see if there are any
other solutuions.

One possibility I thought of is to generate a dialog for each element
so there is a one to one association between element and dialog. This
does not in itself allow the dialog to get data from the derived class
but it obviates the need since it will always retain the last
settings. However, it does not retain the info after a restore from
serialization. I suppose the dialog control states could be serialized
if the dialog pointers are also stored in a CTypedPtrList. Here would
be some possible code -

CDogDlg* pDogDlg = new CDogDlg;

if (pDogDlg->DoModal() == IDOK){

CElement* pElement = new CDog(DogDlg);

//save pElement and pDogDlg to a CTypedPtrList


Some other methods that were suggested to me which won't work -

1. Change base class pointer to a derived class pointer. i.e. CDog*
pElement = new .... defeats purpose of polymorphism

You need to elaborate on that.

2. Define a virtual function that returns the dialog data - problem is
that there are other derived classes with different types of dialog
data (i.e. CCAt, CMouse, etc.). The virtual function prototype
definition has to be consistent for all derived classes.

Yes, that's probably not a good idea. You might end up with the base
class that has attributes of all derived classes, many mutually
exclusive, and you don't want to have to edit the base class when
creating a new derived class.

3. Instantiate the dialog class inside the derived class. This is a
catch 22 since the derived class constructor requires the dialog data.

I think that the caller of the DogDlg should do the dynamic_cast<Dog*>
and pass the pointer (if it's non-null) to DogDlg for editing. The
dialog will work with Dog only.

Alternatively, every object might know how it's to be edited, so you
basically ask the object (Dog, Cat, whatever) to invoke known to it
dialog and pass the proper object in. It might look like this:

     Dog* pDog; // somehow set
     if (bEditing) {
        CElement *pNew = pDog->Edit(); // will clone internally
        if (pNew)

        delele pNew;
     else if (bCreating) {
        CElement *pNew = Dog::createAndEdit(); // we know it's a Dog
        if (pNew) {
            // store the new element somewhere

That will require your CElement to have the 'Edit' virtual function
(overridden in every concrete descendant), and the 'copyAttributesFrom'
virtual function (this one can 'dynamic_cast' and/or do whatever it
likes). Every class that can create another instance should have the
'createAndEdit' static function...

Anyway, it seems that you need to do several more iterations in your
design effort. Good luck!

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