class hierarchy problem (do I need MI?)

Hello
Can anyone assist with the following class hierarcy problem?
I have a series of window classes, the object model currently being as such:

Window
/ | \
/ | \
MDIClientWindow | TreeViewWindow
WndProcWindow
/ \
/ \
DualPaneWindow SplitterWindow

i.e., the base class is Window. It has three direct subclasses,
MDIClientWindow, WndProcWindow, and TreeViewWindow. WndProcWindow also has
two further classes deriving from it, DualPaneWindow and SplitterWindow.
(MDIClient- and TreeView- don't have their own WndProcs)
Now, I am having the following problem: I want to create a few pure virtual
methods on the base Window class, in order that I can create a function that
will take a Window* and call a certain method on a [Something]Window,
knowing that it will have it - e.g. Create()=0. This will make Window a pure
abstract class, this is fine. However, I have been scratching my head as to
the best way to implement this, because WndProcWindow is already an abstract
class - it has a pure virtual method, WndProc.
I'd like to make Window an abstract class, with some methods e.g. Create(),
as pure virtual methods. But this would mean that WndProcWindow would have
to implement them, and it wouldn't be appropriate for it to. Is there any
neat way of getting around this, like some way of telling the compiler that
WndProcWindow won't actually implement the Size method, but any
instantiatable class deriving from it must, because it also derives from
Window?
e.g.
// (Window.h)
void Create(LPCTSTR class, DWORD style);
{
....(generic CreateWindow code here)
}
void Create() = 0; // ???

//WndProcWindow.h
LRESULT WndProc(...) = 0;
void Create(LPCTSTR, DWORD);
void Create(); // ???

//SplitterWindow.cpp
SplitterWindow::Create()
{
WndProcWindow::Create(RegisterClass(... "splitterclass", ...), WS_CHILD |
WS_VISIBLE);
}

//TreeViewWindow.cpp
TreeViewWindow::Create()
{
WndProcWindow::Create(WC_TREEVIEW, WS_CHILD | WS_VISIBLE);
}

//WndProcWindow.cpp
WndProcWindow::Create(LPCTSTR class, DWORD style)
{
Window::Create(class, style);
}

WndProcWindow::Create() //??? help! what goes here?....
{
}


I thought of one solution - multiple inheritance. This would solve it - as I
could have an abstract class, say CreatableWindow, that had the pure virtual
methods e.g. Size(...) = 0, and then DualPane-, MDIClient-, TreeView- and
SplitterWindow could all multiply inherit from WndProcWindow or Window, and
then CreatableWindow aswell. But I've heard horror stories about multiple
inheritance - and that MS says it's one of those "if you really must"
things. A hunch tells me a multiply-inheriting class is like an inbred. Is
my hunch right, will it slow down my code or lead to spaghetti code without
any real benefit, or is it a little-discovered, rarely used technology
that's perfect for this kind of scenario?

On this particular project the porting to other compilers isn't a problem as
I've used loads of VS specific features anyway, but I would like to know if
it is to be avoided or not? Is it a performance hit?

Thanks for any guidance on how to accomplish the above or suggestions on how
I can better structure this without ripping the design apart completely! (It
seems good apart from this...)

(I hope this doesn't seem irrelevant to c.l.c++ as it's really about classes
not windows...)

re: class hierarchy problem (do I need MI?)

Bonj schrieb:[color=blue]
> Hello
> Can anyone assist with the following class hierarcy problem?
> I have a series of window classes, the object model currently being as such:
>
> Window
> / | \
> / | \
> MDIClientWindow | TreeViewWindow
> WndProcWindow
> / \
> / \
> DualPaneWindow SplitterWindow
>
> i.e., the base class is Window. It has three direct subclasses,
> MDIClientWindow, WndProcWindow, and TreeViewWindow. WndProcWindow also has
> two further classes deriving from it, DualPaneWindow and SplitterWindow.
> (MDIClient- and TreeView- don't have their own WndProcs)
> Now, I am having the following problem: I want to create a few pure virtual
> methods on the base Window class, in order that I can create a function that
> will take a Window* and call a certain method on a [Something]Window,
> knowing that it will have it - e.g. Create()=0. This will make Window a pure
> abstract class, this is fine. However, I have been scratching my head as to
> the best way to implement this, because WndProcWindow is already an abstract
> class - it has a pure virtual method, WndProc.
> I'd like to make Window an abstract class, with some methods e.g. Create(),
> as pure virtual methods. But this would mean that WndProcWindow would have
> to implement them, and it wouldn't be appropriate for it to. Is there any
> neat way of getting around this, like some way of telling the compiler that
> WndProcWindow won't actually implement the Size method, but any
> instantiatable class deriving from it must, because it also derives from
> Window?[/color]

What makes you think that this is a problem?

class Window
{
public:
virtual ~Window() {}
virtual void Create() = 0;
};

class WndProcWindow : public Window
{
public:
virtual void WndProc() = 0;
};

class SplitterWindow : public WndProcWindow
{
public:
virtual void Create() { /* stuff */ }
virtual void WndProc() { /* stuff */ }
};

is perfectly valid. No need for WndProcWindow to implement any of
Window's pure virtuals as long as you don't want to instantiate
WndProcWindow - which you can't do anyway because of the pure virtual
WndProc().
[color=blue]
> I thought of one solution - multiple inheritance. This would solve it - as I
> could have an abstract class, say CreatableWindow, that had the pure virtual
> methods e.g. Size(...) = 0, and then DualPane-, MDIClient-, TreeView- and
> SplitterWindow could all multiply inherit from WndProcWindow or Window, and
> then CreatableWindow aswell. But I've heard horror stories about multiple
> inheritance - and that MS says it's one of those "if you really must"
> things. A hunch tells me a multiply-inheriting class is like an inbred. Is
> my hunch right, will it slow down my code or lead to spaghetti code without
> any real benefit, or is it a little-discovered, rarely used technology
> that's perfect for this kind of scenario?[/color]

*Never* trust any generalisations :-)
MI is a powerful tool for some situations and should be avoided for
others, I personally don't subscribe to the "if you really must" theory
nor a general ban on it. I would say you shouldn't use it unless you
really understand it though - but that goes for just about any
programming technique ;-)
Either way, there isn't much of a reason for you to use MI unless I
totally misunderstood your problem - it would be the perfect option to
go for if there e.g. might be a subclass of TreeViewWindow that also has
a WndProc.

Cheers,
Malte

re: class hierarchy problem (do I need MI?)

"Malte Starostik" <malte@starostik.de> wrote in message
news:423cb5ed@olaf.komtel.net...[color=blue]
> Bonj schrieb:[color=green]
>> Hello
>> Can anyone assist with the following class hierarcy problem?
>> I have a series of window classes, the object model currently being as
>> such:
>>
>> Window
>> / | \
>> / | \
>> MDIClientWindow | TreeViewWindow
>> WndProcWindow
>> / \
>> / \
>> DualPaneWindow SplitterWindow
>>
>> i.e., the base class is Window. It has three direct subclasses,
>> MDIClientWindow, WndProcWindow, and TreeViewWindow. WndProcWindow also
>> has
>> two further classes deriving from it, DualPaneWindow and SplitterWindow.
>> (MDIClient- and TreeView- don't have their own WndProcs)
>> Now, I am having the following problem: I want to create a few pure
>> virtual
>> methods on the base Window class, in order that I can create a function
>> that
>> will take a Window* and call a certain method on a [Something]Window,
>> knowing that it will have it - e.g. Create()=0. This will make Window a
>> pure
>> abstract class, this is fine. However, I have been scratching my head as
>> to
>> the best way to implement this, because WndProcWindow is already an
>> abstract
>> class - it has a pure virtual method, WndProc.
>> I'd like to make Window an abstract class, with some methods e.g.
>> Create(),
>> as pure virtual methods. But this would mean that WndProcWindow would
>> have
>> to implement them, and it wouldn't be appropriate for it to. Is there any
>> neat way of getting around this, like some way of telling the compiler
>> that
>> WndProcWindow won't actually implement the Size method, but any
>> instantiatable class deriving from it must, because it also derives from
>> Window?[/color]
>
> What makes you think that this is a problem?
>
> class Window
> {
> public:
> virtual ~Window() {}
> virtual void Create() = 0;
> };
>
> class WndProcWindow : public Window
> {
> public:
> virtual void WndProc() = 0;
> };
>
> class SplitterWindow : public WndProcWindow
> {
> public:
> virtual void Create() { /* stuff */ }
> virtual void WndProc() { /* stuff */ }
> };
>
> is perfectly valid. No need for WndProcWindow to implement any of
> Window's pure virtuals as long as you don't want to instantiate
> WndProcWindow - which you can't do anyway because of the pure virtual
> WndProc().[/color]

ah, yes - cheers, i didn't realise that you could do that, so that's pretty
much exactly what I've gone for.
[color=blue]
>[color=green]
>> I thought of one solution - multiple inheritance. This would solve it -
>> as I
>> could have an abstract class, say CreatableWindow, that had the pure
>> virtual
>> methods e.g. Size(...) = 0, and then DualPane-, MDIClient-, TreeView- and
>> SplitterWindow could all multiply inherit from WndProcWindow or Window,
>> and
>> then CreatableWindow aswell. But I've heard horror stories about multiple
>> inheritance - and that MS says it's one of those "if you really must"
>> things. A hunch tells me a multiply-inheriting class is like an inbred.
>> Is
>> my hunch right, will it slow down my code or lead to spaghetti code
>> without
>> any real benefit, or is it a little-discovered, rarely used technology
>> that's perfect for this kind of scenario?[/color]
>
> *Never* trust any generalisations :-)
> MI is a powerful tool for some situations and should be avoided for
> others, I personally don't subscribe to the "if you really must" theory
> nor a general ban on it. I would say you shouldn't use it unless you
> really understand it though - but that goes for just about any
> programming technique ;-)[/color]


I've gone for a class 'ChildWindow' but this doesn't derive from the main
class, it is essentially just an interface, so it's like a 'V' shape but not
a diamond shape.
The SplitterWindow derives from this and Window, but I can see clearly that
only one 'copy' of each is being pulled in.

thanks

[color=blue]
> Either way, there isn't much of a reason for you to use MI unless I
> totally misunderstood your problem - it would be the perfect option to
> go for if there e.g. might be a subclass of TreeViewWindow that also has[/color]

I didn't know whether a TreeView could even have its own WndProc, but even
if it can I'm not going for that, I'm instead going to put something like
'HandleNotifier(code)=0' in the ChildWindow abstract class. However there
will be another class derived from TreeViewWindow - the treeview that's
specific to this particular application.
[color=blue]
> a WndProc.
>
> Cheers,
> Malte[/color]