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

From: David Abrahams (dave_at_[hidden])
Date: 2006-02-25 10:22:57

"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"
>> > 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.  

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