Re: [boost] Cycle Iterator Adapter (was: Re: Help with iterator helpers)

From: Thomas Witt (witt_at_[hidden])
Date: 2002-02-01 06:16:26

Gennadiy,

I think there are a few problems with the code as it is right now.

1. The use of %= is non-portable as the result for negative n (I think we can 
reasonably assume the size being positive) is implementation defined. 
Furthermore behaviour for m_cycle_size == 0 is undefined.

    template <typename CycleIterator>
    void    advance( CycleIterator& x, difference_type n ) {
        n %= m_cycle_size;

2. The different implementations for advance depending on the iterator 
category are unneeded. In my opinion advance should only be provided for 
random access iterators, the only exception beinng an conversion adaptor that 
explicitly strives to raise the iterator category. The stdlib approach as I 
understand it is: provide only those functions that are reasonably efficient. 
Boost should take the same approach as users are used to it.

If advance for other then random access iterators is removed the size ctor 
parameter can be removed. To me specifying a range and its size is error 
prone and should be avoided.

3. The policies object stores base_iterators. As a result 
cyclic_iterator<iterator> and cyclic_iterator<const_iterator> are not 
interoperable (see msg #24803). Users will not expect it to be so. This is a 
result of the current iterator_adaptor design and cannot be avoided at no 
cost.

I've uploaded cyclic_arry.hpp in dir other. It internally uses a 
cyclic_iterator implementation. Though this code might not be perfect it 
tries to avoid the problems mentioned above. To make cyclic_iterator 
interoperable it always stores non const iterators in the policies object.
This is no problem with cyclic_array but may well be a problem for other uses.

Thomas

-- 
Dipl.-Ing. Thomas Witt
Institut fuer Verkehrswesen, Eisenbahnbau und -betrieb, Universitaet Hannover
voice: [phone] - 4273, fax: [phone]
http://www.ive.uni-hannover.de