[Boost-users] question about fusion concepts/documentation

Subject: [Boost-users] question about fusion concepts/documentation
From: Robert Ramey (ramey_at_[hidden])
Date: 2011-05-29 18:05:04

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 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.

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

Robert Ramey