Re: Smart Pointers and virtual
mosfet wrote:
Hi,
I am trying to modify existing code to use smart pointers and I get some
issues with virtual methods :
class Folder : public Object
{
public:
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();
protected:
...
};
class ContactFolder : public Folder
{
public:
// Constructor/destructor
ContactFolder(OutlookSession* pOutlookSession);
ContactFolder();
virtual ~ContactFolder();
virtual RefPtr<ContactCollection> get_Items();
};
1>c:\testpoom\System.WindowsMobile.PocketOutlook.h(640) : error C2555:
'System::WindowsMobile::PocketOutlook::ContactFolder::get_Items':
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
pointers.
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
regards
Andy Little