[Boost-users] Re: boost::iterator_facade problem with postfix increment proxy

From: David Abrahams (dave_at_[hidden])
Date: 2005-03-22 08:03:28

Jens Theisen <jens.theisen_at_[hidden]> writes:

> David Abrahams wrote:
>> Jens Theisen <jens.theisen_at_[hidden]> writes:
>>> My reference type is a simple reference, I want to implement an
>>> lvalue iterator (at least in some cases).
>> And why is it only single-pass?
>> 
>>> I am trying to make a proxy iterator 
>>                         ^^^^^^^^^^^^^^
>> What do you mean by that?
>
> It is a class templated on a value_type to with implementations can be 
> attached (at run time). For example:
>
> List< char >  l = makeList(some_std_string) + makeList(some_char_array);
>
> l.begin(), l.end() give you an iterator which type depends solely on the 
> value_type (char in this case), rather than the iterator types used as 
> backends (std::string::iterator and pointers). These iterators will 
> iterate over the std::string first and after this over the char
> array.

We call that "any_iterator" (see
http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?LibrariesUnderConstruction)

>> I don't see why that would be incompatible with forward traversal.  We
>> don't even seem to have a requirement that a copy of an iterator
>> compares equal with it.  Perhaps we should; then you might be in
>> trouble [Jeremy, Thomas, your thoughts?]
>
> Oh, you're right. I somehow assumed that would be the case. However, 
> this is a requirement for Forward iterators, and iterator_faced gives 
> the forward_traversal_tag to Forward traversal iterators with reference 
> reference type. So it maybe a good idea to add this requirement.

Yeah, I agree.

>> That shouldn't be slow if there's plenty of inlining.
>
> The indirections are runtime indirections.

I see.

Well, a generalized single-pass iterator can't be writable unless it
has a proxy reference, IIRC.  Otherwise, how does

       *x = a;

work?

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com