Исходники.RU - Информационный сервер для программистов: исходники со всего света.

4.1 dynamic_cast bug?


Robert -- rcobb@deerssv1.MED.OSD.MIL
Friday, March 29, 1996

[Moderator's note: Robert found the answer, which is included below.]

[Platform: MSVC 4.1, NT 3.51 sp 3, Pentium 60]

Line 3 of the following code snippet causes:

error C2682: cannot use dynamic_cast to convert from 'const class CObject *'
to 'const class CStringList *'

......
    const CObject *anObject;
    const CStringList *aList;
    aList = dynamic_cast(anObject);
........

-  This code compiles fine under 4.0. 
-  The error is only generated when casting const objects.

QUESTION:  Why is this an error?  I know Microsoft fixed the hole that
allowed dynamic_cast to cast off const, did they break this legal cast in
the process?

To answer my own inquiry:  A support call to Microsoft revealed that this is
a bug in 4.1 to be fixed in 4.2.  Suggestions for work-arounds:
1.  Go back to 4.0 (no thanks)
2.  Use C style casts (absolutely not)
3.  Cast away the const and comment heavily to remove the unneeded casts
when a fix is available.

Example of suggestion #3:
......
    const CStringList *aList;
    const CObject *aObject = aList;
    CObject *aNonConstObject;
    aNonConstObject = const_cast(aObject);
    aList = dynamic_cast(aNonConstObject);
...........

- Robert Cobb


Mats Mеnhav -- manhav@connectum.skurup.se
Sunday, March 31, 1996

-- [ From: Mats Manhav * EMC.Ver #2.5.02 ] --

The #3 solution suggested heavy commenting.
I suggest using _MSC_VER like this

#define FIXFORVERSION 1000
#if (_MSC_VER != FIXFORVERSION)
#error "New version. Maybe the bugfix can be removed.
#endif

Whenever the compiler is updated you will get the error. Check if the bug
has been fixed and 
either remove the #if and the bugfix or update the FIXFORVERSION with the
new compiler version number.

-------- REPLY, Original message follows --------

> Date: Friday, 29-Mar-96 10:51 AM
> 
> From: Cobb, Robert             \ Internet:    (rcobb@deerssv1.med.osd.mil)
> To:   MFCList                  \ Internet:    (mfc-l@netcom.com)
> 
> Subject: 4.1 dynamic_cast bug?
> 
> [Moderator's note: Robert found the answer, which is included below.]
> 
> [Platform: MSVC 4.1, NT 3.51 sp 3, Pentium 60]
> 
> Line 3 of the following code snippet causes:
> 
> error C2682: cannot use dynamic_cast to convert from 'const class CObject
*'
> to 'const class CStringList *'
> 
> ......
>     const CObject *anObject;
>     const CStringList *aList;
>     aList = dynamic_cast(anObject);
> ........
> 
> -  This code compiles fine under 4.0. 
> -  The error is only generated when casting const objects.
> 
> QUESTION:  Why is this an error?  I know Microsoft fixed the hole that
> allowed dynamic_cast to cast off const, did they break this legal cast in
> the process?
> 
> To answer my own inquiry:  A support call to Microsoft revealed that this
is
> a bug in 4.1 to be fixed in 4.2.  Suggestions for work-arounds:
> 1.  Go back to 4.0 (no thanks)
> 2.  Use C style casts (absolutely not)
> 3.  Cast away the const and comment heavily to remove the unneeded casts
> when a fix is available.
> 
> Example of suggestion #3:
> ......
>     const CStringList *aList;
>     const CObject *aObject = aList;
>     CObject *aNonConstObject;
>     aNonConstObject = const_cast(aObject);
>     aList = dynamic_cast(aNonConstObject);
> ...........
> 
> - Robert Cobb
> 

-------- REPLY, End of original message --------


--
==========================================================================
Mats Mеnhav (Mats Manhav for 7-bit people)
email:manhav@connectum.skurup.se   WWW: http://connectum.skurup.se/~manhav
FAX:  (int) 46 (0) 414 243 05      Phone: (int) 46 (0) 414 243 05         
==========================================================================


Jeff Dickey -- jdickey@sevensigma.com
Sunday, March 31, 1996

Mats,

Good idea, but instead of doing

   #error "New version. Maybe the bugfix can be removed.

I suggest:

   #pragma message( "New version. Maybe the bugfix can be removed." )

This way, you see the message without tripping the compiler up on a bogus
"error".

Just my $0.01978962398 (damn Pentium bug!)

Jeff


At 23:37 3/31/96 -0500, Mats Manhav wrote:
>-- [ From: Mats Manhav * EMC.Ver #2.5.02 ] --
>
>The #3 solution suggested heavy commenting.
>I suggest using _MSC_VER like this
>
>#define FIXFORVERSION 1000
>#if (_MSC_VER !=3D FIXFORVERSION)
>#error "New version. Maybe the bugfix can be removed.
>#endif
>
>Whenever the compiler is updated you will get the error. Check if the bug
>has been fixed and=20
>either remove the #if and the bugfix or update the FIXFORVERSION with the
>new compiler version number.
>
>-------- REPLY, Original message follows --------
>
>> Date: Friday, 29-Mar-96 10:51 AM
>>=20
>> From: Cobb, Robert             \ Internet:   =
 (rcobb@deerssv1.med.osd.mil)
>> To:   MFCList                  \ Internet:    (mfc-l@netcom.com)
>>=20
>> Subject: 4.1 dynamic_cast bug?
>>=20
>> [Moderator's note: Robert found the answer, which is included below.]
>>=20
>> [Platform: MSVC 4.1, NT 3.51 sp 3, Pentium 60]
>>=20
>> Line 3 of the following code snippet causes:
>>=20
>> error C2682: cannot use dynamic_cast to convert from 'const class CObject
>*'
>> to 'const class CStringList *'
>>=20
>> ......
>>     const CObject *anObject;
>>     const CStringList *aList;
>>     aList =3D dynamic_cast(anObject);
>> ........
>>=20
>> -  This code compiles fine under 4.0.=20
>> -  The error is only generated when casting const objects.
>>=20
>> QUESTION:  Why is this an error?  I know Microsoft fixed the hole that
>> allowed dynamic_cast to cast off const, did they break this legal cast in
>> the process?
>>=20
>> To answer my own inquiry:  A support call to Microsoft revealed that this
>is
>> a bug in 4.1 to be fixed in 4.2.  Suggestions for work-arounds:
>> 1.  Go back to 4.0 (no thanks)
>> 2.  Use C style casts (absolutely not)
>> 3.  Cast away the const and comment heavily to remove the unneeded casts
>> when a fix is available.
>>=20
>> Example of suggestion #3:
>> ......
>>     const CStringList *aList;
>>     const CObject *aObject =3D aList;
>>     CObject *aNonConstObject;
>>     aNonConstObject =3D const_cast(aObject);
>>     aList =3D dynamic_cast(aNonConstObject);
>> ...........
>>=20
>> - Robert Cobb
>>=20
>
>-------- REPLY, End of original message --------
>
>
>--
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>Mats M=E5nhav (Mats Manhav for 7-bit people)
>email:manhav@connectum.skurup.se   WWW: http://connectum.skurup.se/~manhav
>FAX:  (int) 46 (0) 414 243 05      Phone: (int) 46 (0) 414 243 05        =
=20
>=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
>
----
Jeff Dickey
Seven Sigma Software and Services
PGP-secured mail encouraged!
'http://www.sevensigma.com/keylist.html' or finger 'jdickey@sevensigma.com'
       for PGP public key

Key fingerprint =3D  99 CA 69 E5 2C 95 E8 4E  7E B2 59 5E FB A5 C2 FD

| Вернуться в корень Архива |