NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/13191

Internet Message Format | 1996-08-05 | 2.6 KB

Path: mail2news.demon.co.uk!txwang
From: Wang TianXing <[e-mail]>
Newsgroups: comp.lang.c++
Subject: Re: object creation from an abstract base class
Date: Sun, 24 Mar 1996 16:47:51 GMT
Message-ID: <[e-mail]>
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On 24 Mar 1996 01:16:48 GMT, Philippe Verdy
<[e-mail]> wrote:

| Wang TianXing <[e-mail]> s'crit :
| > | >Michael Catello wrote:
| > | >>
| > | >> [code deleted]
| > | >>
| > | >> Obviously it is the CreateFoo() function that I am wondering about. In
| > | >> the actual implementation I had multiple static "Create" functions for
| > | >> the base class that would allow me to create a new object: one based
| > | >> on an enumerated token (shown above), another an existing object, as
| > | >> well as one based on the format of a datafile. My application never
| > | >> references any of the derived classes directly, except in their
| > | >> creation and definition.
| > | >>
| > | >> Is there another/better/more appropriate way to handle this type of
| > | >> object creation? Thanks for your assistance,
| > | >>
| >
| > So, the original poster should write code like this:
| >
| >     CFoo *interface = new CBar;
| >
| > and remove the static function CFoo::CreateFoo(FooType);
| >
| No ! We need such a function in many C++ programs which have
| to manage presistent collections of objects. However, such
| applications do not use an enumeration but a registration
| base class which is called by adding a static member to each
| derived class, which will register themselves and obtain a
| valid Id collected on a persistent stream. But on the final
| result, managing such collections require that we have a
| CFoo::CreateFoo(FooType) function in order to recreate the
| collection of objects from a source which only enumerates
| FooTypes. This is useful to create containers of graphic
| objects with common attributes, and varying parameters
| stored on a persistent stream, so that we can load and draw
| the collection of objects without knowing anything on them.
| Your code is legal Wang but you can generalize it to avoid
| the switch() and the enumeration.
| Philippe Verdy

Well, Michael seems really interested in how to handle persistent
objects elegantly.  There's a good example in Borland C++: TStreamable
and its related classes.  It can write pointers into a stream.
Michael may want to hack in the implementation to get some ideas.

---
Wang TianXing