VC4.2 & C2664 problem
Dave_Chestnutt@keyfile.com
Tuesday, August 06, 1996
Environment: VC 4.2, NT 3.51 SP4
We're using CTypedPtr<> templates. With VC 4.1, our code compiled and
ran ok. With 4.2, however, in a few places it fails with this
compile error:
C:\MSDEV\mfc\include\afxtempl.h(1561) : error C2664:
'struct __POSITION *CObList::AddHead(class CObject *)' :
cannot convert parameter 1 from 'class KConnectorModel *'
to 'class CObject *' (new behavior; please see help)
Some (hopefully) relevant lines from our .h file:
--Our class is based on CObject..
class KConnectorModel : public CObject {...};
--and elsewhere, inside a class definition:
CTypedPtrList m_listInputConnectors;
I believe that the line above is the offending one, causing
CTypedPtrList to barf.
I've read the release notes on C2664 (and "Technote: Improved
Conformance to ANSI C++"), but don't see how they apply to this case.
Anyone have any ideas or hints?
Thanks!
-Dave
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========================================================================
Stephen Wolstenholme -- steve@tropheus.demon.co.uk
Friday, August 09, 1996
[Mini-digest: 5 responses]
On Tue, 06 Aug 96 16:51:12 EDT, you wrote:
>Environment: VC 4.2, NT 3.51 SP4
>
>We're using CTypedPtr<> templates. With VC 4.1, our code compiled and
>ran ok. With 4.2, however, in a few places it fails with this
>compile error:
>
> C:\MSDEV\mfc\include\afxtempl.h(1561) : error C2664:
> 'struct __POSITION *CObList::AddHead(class CObject *)' :
> cannot convert parameter 1 from 'class KConnectorModel *'
> to 'class CObject *' (new behavior; please see help)
>
I hit this problem when I first recompiled with 4.2 and discovered
that it is no longer possible to use forward declarations for
CTyped... classes. My code has "evolved" from the scribble tutorial
that also used foward declarations. The scribble example has now been
changed but the associated documentation has not. MS must be assuming
that all the 4.2 users no longer need a tutorial.
-----From: Lance Lovette
Dave,
>> C:\MSDEV\mfc\include\afxtempl.h(1561) : error C2664: 'struct
>> __POSITION *CObList::AddHead(class CObject *)' : cannot convert
>> parameter 1 from 'class KConnectorModel *' to 'class CObject *'
>> (new behavior; please see help)
I had a similar sounding problem with CTypedPtrArray (although I don't
remember the exact error message so it may have been another problem).
My problem was that I was defining the template before defining the
objects the template was storing. For example,
CTypedPtrList m_listInputConnectors;
class KConnectorModel
{
};
This worked in 4.1 but because of some template changes, it did not
work in 4.2. I solved the problem by moving the class definition
before the CTypedPtrList.
Lance
lovette@iftech.com
+-------------------------------------------------------------------+
Interface Technologies, Inc.
For a collection of free tutorials covering a variety of programming
and computer-related topics such as Visual C++, MFC, and Windows NT
check out the ITI On-line Training Center at http://www.iftech.com.
-----From: "Michael Thayer"
Usually this just means that you have do do an explicit cast:
The compiler used to do this for you, and now you have to stick in a
(COBject*)
cast. Frankly this is one area where the standrds groups has made C++ even
weirder than it's C heritage requires.
Michael
-----From: Deepak Saxena
Text item:
I was having the same problem just a few days ago. My problem was that I had
used declared the class a CObject derived class but had used
DECLARE_DYNAMIC(CMyClass, CMyOtherClass). Fixing my DECLARE_DYNAMIC() macro
fixed this problem.
Deepak
-----From: Mike Blaszczak
At 04:51 PM 8/6/96 EDT, Dave_Chestnutt wrote:
>We're using CTypedPtr<> templates. With VC 4.1, our code compiled and
>ran ok. With 4.2, however, in a few places it fails with this
>compile error:
In MFC 4.2, I tightened up the type checking on several
typesafe template class member functions because they were either
not protected or poorly protected.
> C:\MSDEV\mfc\include\afxtempl.h(1561) : error C2664:
> 'struct __POSITION *CObList::AddHead(class CObject *)' :
> cannot convert parameter 1 from 'class KConnectorModel *'
> to 'class CObject *' (new behavior; please see help)
This message is pretty unfortunate. The error message is from
the compiler, and the compiler doesn't (can't) know anything about
the context in which you're using it. The "new behaviour" applies
_ONLY_ only OnLY ohhhhnulee to the compiler's interpretation
and implemnetation of the ANSI not-yet-standard. The message
implies that the compiler knows that you're playing with some
other part of the package (eg, the runtimes or the STL or MFC)
that changed and that the error you're getting is because of
some "new behaviour" of something other than the compiler.
>Some (hopefully) relevant lines from our .h file:
>--Our class is based on CObject..
> class KConnectorModel : public CObject {...};
Since KConnectorModel derives from CObject, you should always
be able to convert from a KConnectorModel* to a CObject*.
The only situation where you wouldn't is if you had an incomplete
(that is, forward) declaration of KConnectorModel and then tried
to declare the template.
If the compiler sees
class KConnectorModel : public CObject {...};
CTypedPtrList m_listInputConnectors;
you're just fine: the compiler knows that KConnectorModel "is-a"
CObject. But if the compiler doesn't see that information:
class KConnectorModel; // forward decl
CTypedPtrList m_listInputConnectors;
you'll get an error message like you describe.
.B ekiM
http://www.nwlink.com/~mikeblas <--- trip report central
1995 Honda VFR750F (Serial number 00050!)
1987 Yamaha FZ700 (damaged) AMA, HRC, VFROC
These words are my own: I do not speak for Microsoft.
Mike Blaszczak -- mikeblas@nwlink.com
Monday, August 12, 1996
At 10:37 AM 8/9/96 GMT, Stephen Wolstenholme wrote:
>The scribble example has now been
>changed but the associated documentation has not. MS must be assuming
>that all the 4.2 users no longer need a tutorial.
Or, maybe it was just an honest oversight. Maybe, just this once, there
_isn't_ a conspiracy.
Of course, I'm assuming you don't have anything to back up this flippant,
abrasive little quip. That's probably wrong; who would use words like
"must" without any real evidence?
.B ekiM
http://www.nwlink.com/~mikeblas/
These words are my own. I do not speak on behalf of Microsoft.
hsusc@nt.com
Friday, August 09, 1996
>We're using CTypedPtr<> templates. With VC 4.1, our code compiled and
>ran ok. With 4.2, however, in a few places it fails with this
>compile error:
>
> C:\MSDEV\mfc\include\afxtempl.h(1561) : error C2664:
> 'struct __POSITION *CObList::AddHead(class CObject *)' :
> cannot convert parameter 1 from 'class KConnectorModel *'
> to 'class CObject *' (new behavior; please see help)
>
>Some (hopefully) relevant lines from our .h file:
>
>--Our class is based on CObject..
>
> class KConnectorModel : public CObject {...};
>
>--and elsewhere, inside a class definition:
>
> CTypedPtrList m_listInputConnectors;
I ran into the same problem during upgrade from VC 4.1 to 4.2. If in your case, m_listInputConnectors is
a stored as a member variable, then I believe we have the same problem. So you should have the
following:
// MyClass header file
// forward declarations
class KConnectorModel; <======= this needs to be modified to a #include "KConnectorModel.h"
class MyClass {
.
.
.
CTypedPtrList m_listInputConnectors;
// template is instantiated at this point. So the compiler needs the #include in order to trace
// KConnectorModel's class hierarchy all the way now to CObject. For details see ANSI C++ draft,
// 14.3.2, Template Point of instantiation
}
The workaround is to #include "KConnectorModel.h" instead of using a class forward declaration. And
I believe the reason that the error code C2664 points you to the technote about VC 4.2's "Improved
Conformance to ANSI C++" is because with the new version of the compiler, templates of this type are
instantiated in the header file. And that also explains why the error is telling you it cannot convert
KConnectorModel to CObject. In your use, CTypedPtrList has CObList as the Base class, however,
CObList requires KConnectorModel to be of CObject type. But with a forward declaration, the compiler
is not able to find the header file of the KConnectorModel to trace the class hierarchy; thus the error.Hope this shed some lights.
- Stan
Nortel - Mission Park
Software Engineer
hsusc@nt.com
Mike Blaszczak -- mikeblas@nwlink.com
Wednesday, August 14, 1996
At 04:43 PM 8/9/96 +0100, hsusc@nt.com wrote:
>The workaround is to #include "KConnectorModel.h" instead of
>using a class forward declaration.
This isn't a workaround--it's a fix. There's no problem in the product
in this area; the problem is in your code, which is making use of a class
without having a complete definition for it.
>And I believe the reason that the error code C2664 points you
>to the technote about VC 4.2's "Improved Conformance to ANSI C++" is
because >with the new version of the compiler,
As I explained in a previous note with this subject line, this has nothing
to do with a compiler change. The templated collection wrapper classes,
like CTypedPtrList, were enhanced in MFC 4.2 to more carefully typecheck
their parameters.
>But with a forward declaration, the compiler
>is not able to find the header file of the KConnectorModel
>to trace the class hierarchy; thus the error.
Right. This an error, though, no matter which version of the compiler
you're using.
.B ekiM
http://www.nwlink.com/~mikeblas/
These words are my own. I do not speak on behalf of Microsoft.
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