Re: [Boost-users] question about fusion concepts/documentation

Subject: Re: [Boost-users] question about fusion concepts/documentation
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2011-05-29 18:23:33

AMDG

On 05/29/2011 03:05 PM, Robert Ramey wrote:
> I'm working on a personal project based on fusion.  This project builds a 
> library on
> 
> I'm working on an experimental library which builds on fusion.  As part of 
> that
> I want to implement concept checking for this experimental library.  Since
> this builds on fusion, I want to implement concept checking for fusion
> so my library can build on that.  Since fusion seems clearly documented, I 
> would
> hope that this would be a fairly straight forward exercise.
> 
> This will also give me a better feel for concept checking and archtypes.
> 
> Looking at fusion documentation on page: 
> ..libs/fusion/doc/html/fusion/iterator/concepts/forward_iterator.html
> I find:
> 
> Expression Requirments
> ======================
> next(i)
> ...
> 
> Meta Expressions
> ============
> result_of::next<I>::type
> ...
> 
> Expression Semantics
> ===============
> next(i) - An iterator to the element following i
> ...
> 
> So the quesion is: Why is result_of::next<I>::type necessary?  The 
> expression symantics
> only make sense if  next(i) if it returns the same type as it's argument. So 
> why is the
> result_of::next<I>::type needed at all.  That is won't it always be equal to 
> I.  If not,
> why not?
> 

The type of next(i) is not the same as
the type of i.  Every iterator to a fusion
sequence has a different type.  For example,
for fusion::vector, the iterators could look
something like:

template<class Sequence, int I>
struct vector_iterator;

template<class Sequence, int I>
vector_iterator<Sequence, I+1> next(vector_iterator<Sequence, I>);

> The question I have applies to other functions as well but next illustrates 
> my question.
> specifially I have the same question regarding advance, and advance_c.
> 
> Also the espressions i == j  should return a value convertible to bool. 
> Then what
> is result_of::equal_to<I, J>::type to be used for? distance raises similar 
> quesions.
> 

Actually, result_of::equal_to is used a lot
more when implementing algorithms than
operator==.  equality for fusion iterators
depends only on their types.  distance
can be computed at compile time.

> I'm actually having quite a bit of difficulty with this task - the above 
> quesions are
> only the easiest one to formulate.
> 

In Christ,
Steven Watanabe