Re: Smart Pointers and virtual

kwikius <>
Mon, 18 Aug 2008 11:04:37 +0100
mosfet wrote:


I am trying to modify existing code to use smart pointers and I get some
issues with virtual methods :

class Folder : public Object
friend class PimItemCollection;
friend class ContactCollection;
friend class TaskCollection;
friend class AppointmentCollection;

// Constructor/destructor
Folder(OutlookSession* pOutlookSession) = 0;
virtual ~Folder();

virtual RefPtr<PimItemCollection> get_Items();

class ContactFolder : public Folder
// Constructor/destructor
ContactFolder(OutlookSession* pOutlookSession);

virtual ~ContactFolder();

virtual RefPtr<ContactCollection> get_Items();

1>c:\testpoom\System.WindowsMobile.PocketOutlook.h(640) : error C2555:
overriding virtual function return type differs and is not covariant
from 'System::WindowsMobile::PocketOutlook::Folder::get_Items'

I Don't understand the error because ContactCollection is inheriting
from PimItemCollection.

Unfortunately this is a problem with smart pointers in replacing raw

  When overrriding virtual functions one can modify the result type in
the override to a derived class of the return in the interface class,
known as covariant return. Unfortunately this doesnt translate if you
change to smart pointer as smart pointer to derived is not a derived
class of smart pointer to base (though it is convertible), but is a
separate class, so there is no covariant mechanism.

All you can do is change sig to base class, smart pointer return
expression in body should be automatically converted , but you then may
have to downcast again at other places in code, if you use derived class
direct, though myself I have found this rare in practise. Perhaps
covariant return is a case of being too clever anyway, breaks encapsulation

Andy Little

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