| Author: | -David Abrahams and Jeremy Siek |
|---|---|
| Contact: | -dave@boost-consulting.com, jsiek@osl.iu.edu |
| Organization: | -Boost Consulting, Indiana University Bloomington |
| Date: | -2003-11-19 |
| 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) |
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.
-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.
-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.
-Remove the iterator_tag class.
-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; --