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

From: Jens Theisen (jens.theisen_at_[hidden])
Date: 2005-03-22 06:21:11

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.

> 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.

> That shouldn't be slow if there's plenty of inlining.

The indirections are runtime indirections.

Best regards,

Jens