Virtual Static Functions Revisited

From:

Al <two@haik.us>

Newsgroups:

comp.lang.c++.moderated

Date:

Sat, 25 Aug 2007 17:33:51 CST

Message-ID:

<1OmdnQaX1LjxP03bnZ2dnUVZ_hWdnZ2d@comcast.com>

Hi guys,

I have a problem that would be solved nicely with the use of static
virtual functions. I know that C++ doesn't directly support these. So,
I'm looking for "modern" ways of faking such a thing, if they exist.

For instance:

class File;

class Shape
{
     virtual void Draw() = 0;
     virtual ~Shape() {}

  // Wish I had this:
  // virtual static Shape& CreateFromFile(const File&) = 0;

};

So I took a look at [20.8] in the FAQ, but it doesn't really solve my
problem. In particular, I see two issues:

1) I don't see how the "clone" function qualifies as a constructor.
2) The "create" function *also* requires an *existing* instance!

E.g. in the example they have:

void userCode(Shape& s)
  {
    Shape* s2 = s.clone();
    Shape* s3 = s.create();
    ...
    delete s2; // You need a virtual destructor here
    delete s3;
  }

Where is that original `s' supposed to come from?

What I would like to accomplish is to create a brand-new Shape, from
scratch (i.e. given a File), and virtually. Is there any way to do this?
(I'm open to portable hacks, macros, etc. as well).

Thanks!
-Al-

PS: Is native support for virtual statics expected in C++0x or through
defect reports?

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