Re: [Boost-users] Sequence of Types, What For?

From: Arkadiy Vertleyb (vertleyb_at_[hidden])
Date: 2006-11-02 13:39:52

"David Abrahams" <dave_at_[hidden]> wrote

> "Arkadiy Vertleyb" <vertleyb_at_[hidden]> writes:
>
>> "Terry G" <tjgolubi_at_[hidden]> wrote
>>
>>> I've just finished reading C++ Template Metaprogramming again.
>>> I just don't get it.
>>
>> Should probably not be your first book on template metaprogramming.
>
> Really?  We designed the book to serve as a complete introduction.
>
>> Really, it's just like trying to learn STL without any idea what a
>> template is.
>
> It's not as though the book starts out assuming you know what a
> metafunction is.  What introductory material do you think we missed
> and should have covered?

Well, it's all about approach to the problem.  You present metaprogramming 
as a systematic discipline.  Andrei presents it as a number of tricks.  You 
define metafunction.  Andrei just shows how to select between types and 
implement typelist.  For some people your approach may be more natural, but 
I personaly prefer his as far as the introduction to the subject is 
concerned.  Once I get a good feel of what can be done, and why, then I am 
ready to talk about systematic approach.

Another thing that IMO doesn't help understanding of the material, either in 
the book or in the MPL docs, is the extensive usage of lambda.  IMO it just 
adds extra magic to the examples, making one feel that he doesn't really 
understand how things work.  I think examples with regular metafunction 
objects would be much easier to understand, at least initially.

>>> Why would someone use a type container?
>>
>> For example, one could implement a generic tuple class (although I don't
>> think boost::tuple uses this approach, but probably fusion does?).  Also 
>> see
>> below...
>
> Of course, that begs the question of why someone would want a tuple.

To implement a relational library ;-)

>>> I'd really like a reference to an MPL primer or some case studies.
>>
>> IMO, you should really start with template metaprogramming itself, and 
>> then
>> move to MPL.  Try Alexandrescu's "Modern C++ design" -- it also contains 
>> a
>> lot of examples why you would want to use a typelist.
>
> As great a book as "Modern C++ Design" is, it really surprises me to
> see it recommended as an introduction to template metaprogramming over
> "C++ Template Metaprogramming."  MC++D is not about specifically TMP,
> and doesn't even attempt to describe what TMP is.  It does contain
> numerous examples of basic TMP applied to traditional OO design
> patterns, which I think is the real point of that book, and IMO makes
> MC++D an ideal follow-on to C++TMP.

Not really, once I read you book I would like to use MPL, right ;-)

As for me personally, I read Alexandrescu first, then  Czarnecki (chapter 
10, I think), and then your book.  If I had to start now, I would use same 
order -- it works for me -- but I realize that it can be different for other 
people.

To summarize -- I think your book is great for the intermediate to advanced 
metaprogrammers, but as far as beginners are conserned, it may not always 
work.  As in the case with the OP.

Regards,
Arkadiy