Problem with is_writable and is_swappable in N1550

# Problem with `is_writable` and `is_swappable` in N1550

Author: | David Abrahams and Jeremy Siek
Contact: | dave@boost-consulting.com, jsiek@osl.iu.edu
Organization: | Boost Consulting, Indiana University Bloomington
date: | $Date: 2003/11/19 01:24:28 $
Copyright: | Copyright David Abrahams, Jeremy Siek 2003. Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

Table of Contents

- Introduction
- Proposed Resolution
- Rationale

# Introduction

The `is_writable` and `is_swappable` traits classes in N1550 provide a mechanism for determining at compile time if an iterator type is a model of the new Writable Iterator and Swappable Iterator concepts, analogous to `iterator_traits<X>::iterator_category` for the old iterator concepts. For backward compatibility, `is_writable` and `is_swappable` not only work with new iterators, but they also are intended to work for old iterators (iterators that meet the requirements for one of the iterator concepts in the current standard). In the case of old iterators, the writability and swapability is deduced based on the `iterator_category` and also the `reference` type. The specification for this deduction gives false positives for forward iterators that have non-assignable value types.

To review, the part of the `is_writable` trait definition which applies to old iterators is:

    if (cat is convertible to output_iterator_tag)
        return true;
    else if (cat is convertible to forward_iterator_tag
             and iterator_traits<Iterator>::reference is a
                 mutable reference)
        return true;
    else
        return false;

Suppose the `value_type` of the iterator `It` has a private assignment operator:

    class B {
    public:
      ...
    private:
      B& operator=(const B&);
    };

and suppose the `reference` type of the iterator is `B&`. In that case, `is_writable<It>::value` will be true when in fact attempting to write into `B` will cause an error.

The same problem applies to `is_swappable`.

# Proposed Resolution

1. Remove the `is_writable` and `is_swappable` traits, and remove the requirements in the Writable Iterator and Swappable Iterator concepts that require their models to support these traits.

2. Change the `is_readable` specification to be: `is_readable<X>::type` is `true_type` if the result type of `X::operator*` is convertible to `iterator_traits<X>::value_type` and is `false_type` otherwise. Also, `is_readable` is required to satisfy the requirements for the UnaryTypeTrait concept (defined in the type traits proposal).

   Remove the requirement for support of the `is_readable` trait from the Readable Iterator concept.

3. Remove the `iterator_tag` class.

4. Change the specification of `traversal_category` to:

       traversal-category(Iterator) =
           let cat = iterator_traits<Iterator>::iterator_category
           if (cat is convertible to incrementable_iterator_tag)
             return cat; // Iterator is a new iterator
           else if (cat is convertible to random_access_iterator_tag)
               return random_access_traversal_tag;
           else if (cat is convertible to bidirectional_iterator_tag)
               return bidirectional_traversal_tag;
           else if (cat is convertible to forward_iterator_tag)
               return forward_traversal_tag;
           else if (cat is convertible to input_iterator_tag)
               return single_pass_iterator_tag;
           else if (cat is convertible to output_iterator_tag)
               return incrementable_iterator_tag;
           else
               return null_category_tag;

# Rationale

1. There are two reasons for removing `is_writable` and `is_swappable`. The first is that we do not know of a way to fix the specification so that it gives the correct answer for all iterators. Second, there was only a weak motivation for having `is_writable` and `is_swappable` there in the first place. The main motivation was simply uniformity: we have tags for the old iterator categories so we should have tags for the new iterator categories. While having tags and the capability to dispatch based on the traversal categories is often used, we see less of a need for dispatching based on writability and swappability, since typically algorithms that need these capabilities have no alternative if they are not provided.
2. We discovered that the `is_readable` trait can be implemented using only the iterator type itself and its `value_type`. Therefore we remove the requirement for `is_readable` from the Readable Iterator concept, and change the definition of `is_readable` so that it works for any iterator type.
3. The purpose of the `iterator_tag` class was to bundle the traversal and access category tags into the `iterator_category` typedef. With `is_writable` and `is_swappable` gone, and `is_readable` no longer in need of special hints, there is no reason for iterators to provide information about the access capabilities of an iterator. Thus there is no need for the `iterator_tag`. The traversal tag can be directly used for the `iterator_category`. If a new iterator is intended to be backward compatible with old iterator concepts, a tag type that is convertible to both one of the new traversal tags and also to an old iterator tag can be created and use for the `iterator_category`.
4. The changes to the specification of `traversal_category` are a direct result of the removal of `iterator_tag`.