Re: [boost] "any" suggestions

From: Corwin Joy (cjoy_at_[hidden])
Date: 2001-07-15 20:49:48

As disucussed below, my first thought was that getting generic casts out of
the "any" class would be extremely difficult - but it turns out to be not
that bad and pretty cool IMHO.  (There are some tricks related to how "any"
has to work, however.)   To simplify things for a first pass, I have also
made the assumption that the type held by "any" is default constructible - I
think this can actually be eliminated with a bit more work. Anyway, attached
is a 3k zip file with the code to do basic any casts.

Corwin

P.S. I've only tested this on VC6 so other compilers may complain a bit.

----- Original Message -----
From: "Peter Dimov" <pdimov_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Sunday, July 15, 2001 3:01 PM
Subject: Re: [boost] "any" suggestions

> From: "Corwin Joy" <cjoy_at_[hidden]>
> > --- In boost_at_y..., "Peter Dimov" <pdimov_at_m...> wrote:
> > > Could you please provide an example of such a design? This is quite
> > > interesting.
> >
> > Sure.  We have a variant class in the Database Template Library that I
> > co-authored (http://www.geocities.com/corwinjoy/dtl/index.htm).  What
> > happens is that at construction time, the program analyzes the table
being
> > bound to and determines the types of the columns such as date, string,
> > integer, wide-string etc.  Once these types have been determined a
variant
> > is constructed to hold each database column.  After initial
construction,
> > any writes into this variant class are not allowed to change the
variant's
> > type (otherwise the database binding would become invalid).  So, writes
> into
> > the variant are cast into whatever type is currently held by the object.
> If
> > the cast cannot be performed, a runtime exception is thrown.
>
> Aha, I see. This, too, can be done without touching the 'any' class, if
you
> register the inserters beforehand.
>
> > This sounds interesting - I'm not quite sure how the register
methodology
> > works with the held value type.  I'd be interested to see how you
> integrate
> > the register templates shown above with the virtual mechanism required
by
> > the holder class for any.
>
> It probably can be done if one insists, but a std::map of typeid's will
work
> just fine. Operations on 'any' aren't supposed to be time critical. :-)
>
> > Also, one other minor point, I think your
> > extractors above would need some kind of notion of a terminating
token(s)
> so
> > that string does not always win.
>
> The string is terminated by whitespace. The infrastructure should support
> multiple extractors for a given type, so it should be possible to register
> another std::string extractor that responds to \"[^\"]*\" that can read
> strings containing whitespace.
>
> > > convert_to<int>(a); // returns 1, type int
> > > convert_to<std::string>(a); // fails (std::bad_cast)
> >
> > This kind of thing would be really cool.  Here I *really* can't see how
> this
> > would work with the virtual function mechanism required by any.  How
> exactly
> > would you modify the placeholder class to do this.  (Or were you
planning
> on
> > a massive switch...case base on the type_info (shudder...) :-O ).
>
> Yep, this is double dispatch... std::map keyed on a pair of typeid's is
the
> usual solution for the dynamic case.
>
> > This would be awesome if it were possible.  In practice I fear that the
> > language constraints might make it hard to get here.
>
> It's possible. It will not be as efficient as a hand-crafted 'variant'
with
> switch-on-type, but it will work, which is the important thing. :-)
>
> Unfortunately you can't make a hash table keyed on std::type_info's. In
> C++0x, maybe.
>
> --
> Peter Dimov
> Multi Media Ltd.
>
>
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