Multiple inheritance with MFC and especially CCommandTarget
TA -- siemens@inet.uni-c.dk
Wednesday, January 29, 1997
Environment: MSVC 4.0, Windows 95
I have a question on the inheritance issues with MFC that I hope someone
could help me out with:
Can the message map problem with multiple inheritance involving
CCommandTarget be solved using virtual inheritance?
Clarification of question by example:
Suppose you have 2 classes, Class A and Class B, both derived at some point
from CCommandTarget. Deriving a class, Class C, from A and B using simple
inheritance will result in ambiguities for CCommandTarget when selecting
the proper handler for a message, but if C is inherited from A and B using
virtual inhertance, will the ambiguities be solved, then?
I've read som KB articles found by the search string "multiple NEAR
inheritance AND MFC" that stated that I might get problems with the
compiler due to some compiler options like /vms /vmb and /vmg, but I was
not able to determine if had anything directly to do with this question
(I'm sorry, but I don't remember the ID's of the articles).
Best regards,
Mike Jakobsen
Siemens@inet.uni-c.dk
Siemens A/S
Borupvang 3
DK-2750 Ballerup
+45 4477 4477
Dave Lorde -- dlorde@cix.compulink.co.uk
Friday, January 31, 1997
[Mini-digest: 7 responses]
In-Reply-To: <199701291244.NAA01555@inet.uni-c.dk>
Check out TechNote 016: Using C++ Multiple Inheritance with MFC
This explains the limitations. Virtual inheritance does not resolve all
ambiguities, you need to take some precautions. In particular:
"In order for the MFC message map system to work correctly, there are
two additional requirements:
·There must be only one CWnd-derived base class.
·The CWnd-derived base class must be the first (or left-most)
base class."
Dave
-----From: Kyle Green
Mike,
In general you have to explicitly resolve ambiguities with over-rides. With
a diamond inheritance pattern, there are two paths to to the ancestor, and
by over-riding the ambigous (dual) functions, you explicity control the
path back to the ancestors function.
In addition, CCommandTarget would need to be inherited virtually by both
class A and class B, class C would not need to inherit anything virtually.
virtual inheritance is required for objects which may appear more than once
in an ancestor tree.
These are general comments about multiple inheritance, but I don't know if
they apply at all to your CCommandTarget situation. There may be other
factors involved.
Kyle
At 01:44 PM 1/29/97 +0100, Siemens, TA wrote:
>Environment: MSVC 4.0, Windows 95
>Can the message map problem with multiple inheritance involving
>CCommandTarget be solved using virtual inheritance?
>Clarification of question by example:
>Suppose you have 2 classes, Class A and Class B, both derived at some point
>from CCommandTarget. Deriving a class, Class C, from A and B using simple
>inheritance will result in ambiguities for CCommandTarget when selecting
>the proper handler for a message, but if C is inherited from A and B using
>virtual inhertance, will the ambiguities be solved, then?
>I've read som KB articles found by the search string "multiple NEAR
>inheritance AND MFC" that stated that I might get problems with the
>compiler due to some compiler options like /vms /vmb and /vmg, but I was
>not able to determine if had anything directly to do with this question
>(I'm sorry, but I don't remember the ID's of the articles).
-----From: Mike Blaszczak
At 13:44 1/29/97 +0100, Siemens, TA wrote:
>Environment: MSVC 4.0, Windows 95
>Clarification of question by example:
>Suppose you have 2 classes, Class A and Class B, both derived at some point
>from CCommandTarget. Deriving a class, Class C, from A and B using simple
>inheritance will result in ambiguities for CCommandTarget when selecting
>the proper handler for a message, but if C is inherited from A and B using
>virtual inhertance, will the ambiguities be solved, then?
No.
.B ekiM
http://www.nwlink.com/~mikeblas/
These words are my own. I do not speak on behalf of Microsoft.
This performance was not lip-synched.
-----From: Stephane Larocque
Hi,
if I were you, I would avoid to use this kind of inheritance. I dont know
how it is called in english, but in french we call it "Heritage en
losange". Instead of this, try aggregation, like this:
class C (: public CCommandTarget)
{
public:
class A : public CCommandTarget
{
...
}
class B : public CCommandTarget
{
...
}
...
}
That's it. I think this will solve your problem.
Stephane.
-----From: hou@tfn.com (Bing Hou)
This involves both C++ and MFC.
C++ first. Ambiguity in this case is caused by functions with same names in
different bases, the compiler doesn't know which one you are using. Virtual
inheritance guarantees that only one copy of an common base is created at
run time, in this case, the CCmdTarget class, it doesn't help you to solve
ambiguous functions that you implement in class A and B at compile time. So
the answer is no. To overcome this, you have to carefully re-implement each
ambiguous function in your derived class by explictly delegating to one
base's implementation. For example, Derived::AmbFunc() { A::AmbFunc(); }.
Then MFC. In order for the message map mechanism to work, you must have
only one CWnd base. Because MFC chains the message map from bottom to top,
you must keep this chain very clean when you inheriting away from the CWnd
base. Also the CWnd type base must be the first parent in your inheritance
construction.(The left-most parent)
Bing Hou
hou@tfn.com
------------------------------------------------------------------------
The shortest distance between two points is always under construction.
- Noelie Alito
______________________________ Reply Separator _________________________________
Subject: Multiple inheritance with MFC and especially CCommandTarget
Author: "Siemens; TA" at Internet
Date: 1/29/97 1:44 PM
Environment: MSVC 4.0, Windows 95
I have a question on the inheritance issues with MFC that I hope someone
could help me out with:
Can the message map problem with multiple inheritance involving
CCommandTarget be solved using virtual inheritance?
Clarification of question by example:
Suppose you have 2 classes, Class A and Class B, both derived at some point
from CCommandTarget. Deriving a class, Class C, from A and B using simple
inheritance will result in ambiguities for CCommandTarget when selecting
the proper handler for a message, but if C is inherited from A and B using
virtual inhertance, will the ambiguities be solved, then?
I've read som KB articles found by the search string "multiple NEAR
inheritance AND MFC" that stated that I might get problems with the
compiler due to some compiler options like /vms /vmb and /vmg, but I was
not able to determine if had anything directly to do with this question
(I'm sorry, but I don't remember the ID's of the articles).
Best regards,
Mike Jakobsen
Siemens@inet.uni-c.dk
Siemens A/S
Borupvang 3
DK-2750 Ballerup
+45 4477 4477
-----From: Roma
Siemens, TA wrote:
>
> Environment: MSVC 4.0, Windows 95
>
> I have a question on the inheritance issues with MFC that I hope someone
> could help me out with:
> Can the message map problem with multiple inheritance involving
> CCommandTarget be solved using virtual inheritance?
>
> Clarification of question by example:
> Suppose you have 2 classes, Class A and Class B, both derived at some point
> from CCommandTarget. Deriving a class, Class C, from A and B using simple
> inheritance will result in ambiguities for CCommandTarget when selecting
> the proper handler for a message, but if C is inherited from A and B using
> virtual inhertance, will the ambiguities be solved, then?
Hi again!
A bit more thoughts on this issue:
This type of virtual inheritance:
class A : public CCmdTarget {};
class B : public CCmdTarget {};
class C : public virtual A, public virtual B {};
will not help you, because virtual keywords in class C declaration means
nothing
until you start derive from class C.
Object of class C will have size >= sizeof(A)+sizeof(B), and there will
be two instances
of the CObject and two instances of CCmdTarget.
If you want to have only one instance of CCmdTarget in the class C
object, you have to
declare CCmdTarget class as virtual base for both class A and class B,
but I believe
this will not help you either:
#include
class A : public virtual CCmdTarget
{
//...
DECLARE_MESSAGE_MAP();
};
class B : public virtual CCmdTarget
{
//...
DECLARE_MESSAGE_MAP();
};
class C : public A, public B
{
//...
};
//At some point you will have:
BEGIN_MESSAGE_MAP(A, CCmdTarget)
END_MESSAGE_MAP
//and
BEGIN_MESSAGE_MAP(B, CCmdTarget)
END_MESSAGE_MAP
//One note DECLARE_MESSAGE_MAP() declares two static data members in the
class
//( _messageEntries, messageMap ) and one virtual function
GetMessageMap().
This way you will get THREE declarations of 'messageMap' and
_messageEntries in the C:
one from CCmdTarget (only one - due to the virtual inheritance), one
from class A, and one
from class B. Also, there will be a problem with virtual function which
is declared in
the CCmdTarget. It's overriden in the A and B, and now both of them are
present in the C, that's why if you'll compile above code (that's
possible) compiler will give you the following output:
error C2250: 'C' : ambiguous inheritance of 'A::GetMessageMap'
error C2250: 'C' : ambiguous inheritance of 'B::GetMessageMap'
And even if there will be no virtual function, virtual base classes will
not help you to avoid
ambiguities in accessing messageMap and _messageEntries members of the
class C, because these
members are declared not only in CCmdTarget, but in the derived classes
too.
-Roma
-----From: Roma
Siemens, TA wrote:
>
> Environment: MSVC 4.0, Windows 95
>
> I have a question on the inheritance issues with MFC that I hope someone
> could help me out with:
> Can the message map problem with multiple inheritance involving
> CCommandTarget be solved using virtual inheritance?
>
> Clarification of question by example:
> Suppose you have 2 classes, Class A and Class B, both derived at some point
> from CCommandTarget. Deriving a class, Class C, from A and B using simple
> inheritance will result in ambiguities for CCommandTarget when selecting
> the proper handler for a message, but if C is inherited from A and B using
> virtual inhertance, will the ambiguities be solved, then?
>
> I've read som KB articles found by the search string "multiple NEAR
> inheritance AND MFC" that stated that I might get problems with the
> compiler due to some compiler options like /vms /vmb and /vmg, but I was
> not able to determine if had anything directly to do with this question
> (I'm sorry, but I don't remember the ID's of the articles).
Check MFC technical note #016 - it's called "Using C++ Multiple
Inheritance
with MFC"
At the bottom of the article there is an example of "Hello, world!"
MFC-based
program, which uses multiple inheritance.
Main class in the program is declared like this:
class CHelloAppAndFrame : public CFrameWnd, public CWinApp
{
...
};
Both of the base classes are CCmdTarget-derived, so, at least in some
cases, multiple inheritance from CCmdTarget based classes are possible
without virtual inheritance.
>
> Best regards,
> Mike Jakobsen
> Siemens@inet.uni-c.dk
> Siemens A/S
> Borupvang 3
> DK-2750 Ballerup
> +45 4477 4477
Hth,
-Roma
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