[boost] Re: best way to make {reverse_}fold a sequence

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-03-01 17:00:42

Thanks for the help, David.

On 03/01/2005 08:25 AM, David Abrahams wrote:
[snip]
> Of course that is way too general.  Not every template that might
> match is meant to be a sequence.

OK, but I just want it to work with my fold<...>::type, and making
it as general as possible seemed the fasted way to just get a
compile to work.

> (Also: do you really mean your sequence iterators to be the same type
> as the sequence itself?)

It doesn't really matter to me as long as I get a clean compile
and some result from deref that I can use.  If the deref<...>::type
is inherit2<Left,Right>, then I can always get the Left or Right
parts with another template specialization.

>>template <
>>  template<typename,typename>class ForwardOpTmpl
>>, typename LeftArg
>>, typename RightArg
>>
>>struct next < ForwardOpTmpl<LeftArg,RightArg> >
>>{
>>    typedef RightArg type;
>>};
>>//}--------- cut-here --------
>>But intel-linux said next was ambiguous.  
>  
> As above, too general, but I'm curious anyway: ambiguous with what?  A
> single declaration can't be ambiguous with itself.

I was unclear.  The error message was about next:

fold_sequence_test.cpp(98): error: "next" is ambiguous
         typedef next<iter0>::type iter1;
                 ^
See the http://boost-sandbox.sourceforge.net/vault/
in cppljevans/mpl/fold_sequence.zip.  The compiler error messages
included in this zip only include the 1st error message.  The above
can be seen by simply mv'ing the nearby #if 0 down past the above
statement and compiling.

>>Also, the above
>>code assumes the last arg to fold was:
>>
>>  ForwardOpTmpl<arg<2>,arg<1> >
> 
> No it doesn't.  The last arg to fold has nothing to do with it.  It
> might have been a metafunction class.  In fact, fold has nothing to do
> with it.

Well, I've got to know the place_holder arg order in order to know
whether next should return the Left or right part of my_inherit<L,R>.

[snip]
> 
> !? *More* general?  That's definitely going down the wrong road.
> 

Yes, but for right now, I just want a compile the works on the
simplest type of Sequence arg to fold.

[snip]
> 
> template <class Left, class Right>
> struct next<my_inherit<Left,Right> >
> {
>   // ... whatever
> };
> 
> would be more appropriate.
> 

The fold_sequence_test.zip referred to above does this, but with
inherit2 instead of my_inherit; yet, it still get's a compile
error, as shown by the fold_sequence_test.intel within the zip.