NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/comp/lang/cplus/12354

Text File | 1992-08-14 | 2.9 KB | 75 lines

Newsgroups: comp.lang.c++
From: [e-mail] (Nikki Locke)
Path: sparky!uunet!pipex!demon!trmphrst.demon.co.uk!nikki
Distribution: world
Subject: Re: run-time type checking)
References: <[e-mail]>
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Organization: Trumphurst Ltd.
Lines: 61
Date: Fri, 14 Aug 1992 13:25:17 +0000
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In article <[e-mail]> [e-mail] (Johan Bengtsson) writes:

> [e-mail] (John MAX Skaller) writes:
> :
> :There are other ways than tagged pointers to make downcasting
> :sound. One might be to declare the derived classes that you want
> :to downcast to in the base class FROM which you want to downcast:
> :    class Base { //....
> :        operator Derived1*() =0;
> :        operator Derived2*() =0;
> :    };
> :Now what does this do? Well, it preserved th open/closed principle
> :and encapsulation and the explicit interfaces principle.
>
> What if someone using your classes decides to add a Derived3 class
> to which downcasting should be possible?  He must modifiy your
> class Base, for which he may not have source code.
>
> Does not (help) preserve the open/closed principle at all, IMHO.
Perhaps I misunderstand the meaning of the "open/closed principle", but I
understood that it implies that every operation which can be applied to an
object through a reference or pointer to a Base class should be visible in
the Base class declaration. An excellent example was presented earlier in
this thread (but I have expired it, so I'll have to make up a new one).

class Base {
protected:
    int i;
public:
    int get() const { return i; }
    };

class Derived {
public:
    void set(int j) { i = j; }
    };

Now, if downcasting is not allowed, we can guarantee that any operation on
a Base pointer or reference will not modify i. I understood this to be one
of the requirements of the open/closed principle.

As soon as you allow downcasting to class Derived, i may be modified. This
violates (what I understand to be) the principle.

Of course, if you state clearly in the declaration of Base that it may be
converted into a Derived, you are also stating that i may be modified
through a Base pointer/reference.

The original poster suggested that some new syntax be invented to state
this intention. Max's post explained (albeit with a slightly incorrect
example :-) that no new syntax was needed, just a few (virtual) functions
in the Base class.

Corrections of any incorrect understanding on my part welcome.
---
Nikki Locke              |                        | [e-mail]
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