Re: MFC and c++ problems

In article news:<fa63u1pin4p62hk7umrn7bvrjvhlm3tk29@xxxxxxx>, Joseph M.
Newcomer wrote:
So a control subclass becomes a completely self-contained class in my
world.... It cannot look into any other class that might contain it,
including CWinApp, CView, or CDialog classes that might contain it.
GetParent()->SendMessage is the interface of choice, and this can
usually be concealed with some reasonable programming techniques.

I agree with most of what you wrote in that long post, joe, but I have to
play Devil's Advocate with you here for a moment ...

You say that a control must make no assumptions about its parent.

But, if a control sends a message to its parent, is it not making the
assumption that its parent can handle that message? How is this any better
than calling a method defined by the parent?

Semantically, of course, the parent is free not to handle any message that
it doesn't understand. Your program containing a control that sends
messages to its parent will compile and run ... but may not WORK, because
the control may require the parent to handle the messages it sends, and
make some response.

I agree wholeheartedly with you that it is a great strength of OO
programming that there is no coupling between unrelated program components
... but here we're dealing with components which do have some logical
(i.e. real-world) coupling. We can't hide from that, and we should be
prepared to allow it to be manifest in our expressions of the program.

If MyControl calls methods in its parent directly there is a visible
connection between control and parent that documents the real-world
connection between their behaviours. If you place that control in a parent
that doesn't provide the methods that the control needs to call your
program will fail to compile ... which is rather better, in my book, than
having it compile and run but not work.

One could write an interface class that supported the method(s) needed by
the control, and make sure that any parent class using that control
inherited from the interface. One would pass the control's constructor a
pointer/reference to the parent *as an interface object* through which the
calls could be made ... that way the control does not need to know any
more about the parent class than is necessary, and the compiler can still
catch the error if an attempt is made to place the control in an
inappropriate parent.

The trouble with using SendMessage is firstly that it *cannot* be made
typesafe in this way, and secondly that it imposes a (small) runtime
overhead.

Having made those points ... I must say that for most scenarios I can
think of I would actually use the SendMessage technique; for the usual
commonplace control-notifies-parent-of-user-action situations it is what
is expected. Windows uses messages for notifications of this kind all the
time, and allows messages to be spied upon, hooked, redirected, etc.; and
supports subclassing of the message sender/receiver without breaking the
overall model.

It's not a completely cut-and-dry issue, though ...

Cheers,
Daniel.

.

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