Re: [Boost-users] [mpl] newbie question

From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2006-02-26 01:33:45

Top posting because I'm going to remark about this thread in general, but
this message is a fine example.

Andy seems to be speaking (mostly) about int_<>, integral_c, etc (I'll just
say the incorrect 'int<>')  and plus;
David is talking about transform.

I think, but can't say for sure, that Andy wants plus to work 'as expected'
and always return int<>, not just 'concept-identical' to int.  Maybe it
does, I'm not sure.  And/or he wants transform  ***when applied to int<>
with plus, etc*** to return int<>.

David, says, that ***in general*** transform can't make the return type
"match" (in Andy's stricter sense) the inputs.

Maybe it could for the special case of int<>?  IMO, that would be quite
hard, as the point of the templates is to be general.

Do I have this right, in any sense?

P.S. for David, below:

Date: Sat, 25 Feb 2006 10:22:57 -0500
> From: David Abrahams <dave_at_[hidden]>
> Subject: Re: [Boost-users] [mpl] newbie question
> To: boost-users_at_[hidden]
> Message-ID: <uoe0v799q.fsf_at_[hidden]>
> Content-Type: text/plain; charset=us-ascii
>
> "Andy Little" <andy_at_[hidden]> writes:
>
> > I think we need a little bit more context here.
> >
> >> > > OP
> >>> David Abrahams
> >>
> >> Andy Little (me)
> >>
> >> > > Just trying to use mpl::transform on a vector of int's and I can't
> seem to
> >> > > get
> >> > > it working properly. Can anyone see what's wrong?? I'm trying to
> perform
> >> > >
> >> > > vector_c<int, 1, 2, 3> + vector_c<int, 1, 1, 1> = vector_c<int, 2,
> 3 ,4>
> >> > >
> >> > > The is_same function always returns false when I compile and run
> it.
> >> >
> >> > The result of the transform is only required to be
> "concept-identical"

the line above says "the transform"

>> > to the result you're looking for.
> >> >
> >> IMO that behaviour is sloppy. I see no reason why (at least)
> >>
> >> boost::is_same < plus< int_<1> ,int_<1> >::type, int_<2> > shouldnt
> >> be true.
> >
> > Your  change from "the transform" to "being about transform" above is
> > problematic for me.

I don't know what you mean.  Nobody wrote "the transform" above.

as mentioned, he did, but that doesn't mean I understand the point he is
trying to make.

Don't make this complicated. It is very simple: I made a statemnt
> *about the behavior of transform*, and you replied "that behavior is
> sloppy," and then proceeded to go on about something only distantly
> related (the type of results of arithmetic operations).
>
> The behavior I was describing is not sloppy.  To make the library
> behave differently would introduce a huge overhead in implementation
> code that is more likely than not to slow down compilation of user
> programs.
>
> Let me be a little more explicit:
>
>      transform<vector_c<int, ...> >::type
>
> is likely to be a specialization of vectorN where N is a numeral.
> Therefore, testing it against a vector_c specialization using is_same
> will always fail.  That is the behavior I was referring to.
>
> I wouldn't care much what you said, except for that you're doing this
> in a thread where impressionable "newbies" are reading, and I don't
> want them to misunderstand the library design or the implementation
> decisions and their costs/benefits.  Your statement above not only
> unfairly maligns Aleksey's work, it is misleading.
>
> --
> Dave Abrahams
> Boost Consulting
> www.boost-consulting.com
>
>