libs/iterator/doc/facade-and-adaptor.rst
.. Distributed under 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) +++++++++++++++++++++++++++++ Iterator Facade and Adaptor +++++++++++++++++++++++++++++ :Author: David Abrahams, Jeremy Siek, Thomas Witt :Contact: dave@boost-consulting.com, jsiek@osl.iu.edu, witt@styleadvisor.com :organization: `Boost Consulting`_, Indiana University `Open Systems Lab`_, `Zephyr Associates, Inc.`_ :date: $Date: 2006-09-11 18:27:29 -0400 (Mon, 11 Sep 2006) $ :Number: This is a revised version of N1530_\ =03-0113, which was accepted for Technical Report 1 by the C++ standard committee's library working group. .. Version 1.9 of this ReStructuredText document corresponds to n1530_, the paper accepted by the LWG. .. _n1530: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1530.html :copyright: Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. .. _`Boost Consulting`: http://www.boost-consulting.com .. _`Open Systems Lab`: http://www.osl.iu.edu .. _`Zephyr Associates, Inc.`: http://www.styleadvisor.com :abstract: We propose a set of class templates that help programmers build standard-conforming iterators, both from scratch and by adapting other iterators. .. contents:: Table of Contents ============ Motivation ============ Iterators play an important role in modern C++ programming. The iterator is the central abstraction of the algorithms of the Standard Library, allowing algorithms to be re-used in in a wide variety of contexts. The C++ Standard Library contains a wide variety of useful iterators. Every one of the standard containers comes with constant and mutable iterators [#mutable]_, and also reverse versions of those same iterators which traverse the container in the opposite direction. The Standard also supplies ``istream_iterator`` and ``ostream_iterator`` for reading from and writing to streams, ``insert_iterator``, ``front_insert_iterator`` and ``back_insert_iterator`` for inserting elements into containers, and ``raw_storage_iterator`` for initializing raw memory [7]. Despite the many iterators supplied by the Standard Library, obvious and useful iterators are missing, and creating new iterator types is still a common task for C++ programmers. The literature documents several of these, for example line_iterator [3] and Constant_iterator [9]. The iterator abstraction is so powerful that we expect programmers will always need to invent new iterator types. Although it is easy to create iterators that *almost* conform to the standard, the iterator requirements contain subtleties which can make creating an iterator which *actually* conforms quite difficult. Further, the iterator interface is rich, containing many operators that are technically redundant and tedious to implement. To automate the repetitive work of constructing iterators, we propose ``iterator_facade``, an iterator base class template which provides the rich interface of standard iterators and delegates its implementation to member functions of the derived class. In addition to reducing the amount of code necessary to create an iterator, the ``iterator_facade`` also provides compile-time error detection. Iterator implementation mistakes that often go unnoticed are turned into compile-time errors because the derived class implementation must match the expectations of the ``iterator_facade``. A common pattern of iterator construction is the adaptation of one iterator to form a new one. The functionality of an iterator is composed of four orthogonal aspects: traversal, indirection, equality comparison and distance measurement. Adapting an old iterator to create a new one often saves work because one can reuse one aspect of functionality while redefining the other. For example, the Standard provides ``reverse_iterator``, which adapts any Bidirectional Iterator by inverting its direction of traversal. As with plain iterators, iterator adaptors defined outside the Standard have become commonplace in the literature: * Checked iter[13] adds bounds-checking to an existing iterator. * The iterators of the View Template Library[14], which adapts containers, are themselves adaptors over the underlying iterators. * Smart iterators [5] adapt an iterator's dereferencing behavior by applying a function object to the object being referenced and returning the result. * Custom iterators [4], in which a variety of adaptor types are enumerated. * Compound iterators [1], which access a slice out of a container of containers. * Several iterator adaptors from the MTL [12]. The MTL contains a strided iterator, where each call to ``operator++()`` moves the iterator ahead by some constant factor, and a scaled iterator, which multiplies the dereferenced value by some constant. .. [#concept] We use the term concept to mean a set of requirements that a type must satisfy to be used with a particular template parameter. .. [#mutable] The term mutable iterator refers to iterators over objects that can be changed by assigning to the dereferenced iterator, while constant iterator refers to iterators over objects that cannot be modified. To fulfill the need for constructing adaptors, we propose the ``iterator_adaptor`` class template. Instantiations of ``iterator_adaptor`` serve as a base classes for new iterators, providing the default behavior of forwarding all operations to the underlying iterator. The user can selectively replace these features in the derived iterator class. This proposal also includes a number of more specialized adaptors, such as the ``transform_iterator`` that applies some user-specified function during the dereference of the iterator. ======================== Impact on the Standard ======================== This proposal is purely an addition to the C++ standard library. However, note that this proposal relies on the proposal for New Iterator Concepts. ======== Design ======== Iterator Concepts ================= This proposal is formulated in terms of the new ``iterator concepts`` as proposed in n1550_, since user-defined and especially adapted iterators suffer from the well known categorization problems that are inherent to the current iterator categories. .. _n1550: http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html This proposal does not strictly depend on proposal n1550_, as there is a direct mapping between new and old categories. This proposal could be reformulated using this mapping if n1550_ was not accepted. Interoperability ================ The question of iterator interoperability is poorly addressed in the current standard. There are currently two defect reports that are concerned with interoperability issues. Issue 179_ concerns the fact that mutable container iterator types are only required to be convertible to the corresponding constant iterator types, but objects of these types are not required to interoperate in comparison or subtraction expressions. This situation is tedious in practice and out of line with the way built in types work. This proposal implements the proposed resolution to issue 179_, as most standard library implementations do nowadays. In other words, if an iterator type A has an implicit or user defined conversion to an iterator type B, the iterator types are interoperable and the usual set of operators are available. Issue 280_ concerns the current lack of interoperability between reverse iterator types. The proposed new reverse_iterator template fixes the issues raised in 280. It provides the desired interoperability without introducing unwanted overloads. .. _179: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#179 .. _280: http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#280 Iterator Facade =============== .. include:: iterator_facade_body.rst Iterator Adaptor ================ .. include:: iterator_adaptor_body.rst Specialized Adaptors ==================== This proposal also contains several examples of specialized adaptors which were easily implemented using ``iterator_adaptor``: * ``indirect_iterator``, which iterates over iterators, pointers, or smart pointers and applies an extra level of dereferencing. * A new ``reverse_iterator``, which inverts the direction of a Base iterator's motion, while allowing adapted constant and mutable iterators to interact in the expected ways (unlike those in most implementations of C++98). * ``transform_iterator``, which applies a user-defined function object to the underlying values when dereferenced. * ``filter_iterator``, which provides a view of an iterator range in which some elements of the underlying range are skipped. .. _counting: * ``counting_iterator``, which adapts any incrementable type (e.g. integers, iterators) so that incrementing/decrementing the adapted iterator and dereferencing it produces successive values of the Base type. * ``function_output_iterator``, which makes it easier to create custom output iterators. Based on examples in the Boost library, users have generated many new adaptors, among them a permutation adaptor which applies some permutation to a random access iterator, and a strided adaptor, which adapts a random access iterator by multiplying its unit of motion by a constant factor. In addition, the Boost Graph Library (BGL) uses iterator adaptors to adapt other graph libraries, such as LEDA [10] and Stanford GraphBase [8], to the BGL interface (which requires C++ Standard compliant iterators). =============== Proposed Text =============== Header ``<iterator_helper>`` synopsis [lib.iterator.helper.synopsis] ======================================================================= :: struct use_default; struct iterator_core_access { /* implementation detail */ }; template < class Derived , class Value , class CategoryOrTraversal , class Reference = Value& , class Difference = ptrdiff_t > class iterator_facade; template < class Derived , class Base , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class iterator_adaptor; template < class Iterator , class Value = use_default , class CategoryOrTraversal = use_default , class Reference = use_default , class Difference = use_default > class indirect_iterator; template <class Dereferenceable> struct pointee; template <class Dereferenceable> struct indirect_reference; template <class Iterator> class reverse_iterator; template < class UnaryFunction , class Iterator , class Reference = use_default , class Value = use_default > class transform_iterator; template <class Predicate, class Iterator> class filter_iterator; template < class Incrementable , class CategoryOrTraversal = use_default , class Difference = use_default > class counting_iterator; template <class UnaryFunction> class function_output_iterator; Iterator facade [lib.iterator.facade] ===================================== .. include:: iterator_facade_abstract.rst Class template ``iterator_facade`` ---------------------------------- .. include:: iterator_facade_ref.rst Iterator adaptor [lib.iterator.adaptor] ======================================= .. include:: iterator_adaptor_abstract.rst Class template ``iterator_adaptor`` ----------------------------------- .. include:: iterator_adaptor_ref.rst Specialized adaptors [lib.iterator.special.adaptors] ==================================================== The ``enable_if_convertible<X,Y>::type`` expression used in this section is for exposition purposes. The converting constructors for specialized adaptors should be only be in an overload set provided that an object of type ``X`` is implicitly convertible to an object of type ``Y``. The signatures involving ``enable_if_convertible`` should behave *as-if* ``enable_if_convertible`` were defined to be:: template <bool> enable_if_convertible_impl {}; template <> enable_if_convertible_impl<true> { struct type; }; template<typename From, typename To> struct enable_if_convertible : enable_if_convertible_impl<is_convertible<From,To>::value> {}; If an expression other than the default argument is used to supply the value of a function parameter whose type is written in terms of ``enable_if_convertible``, the program is ill-formed, no diagnostic required. [*Note:* The ``enable_if_convertible`` approach uses SFINAE to take the constructor out of the overload set when the types are not implicitly convertible. ] Indirect iterator ----------------- .. include:: indirect_iterator_abstract.rst Class template ``pointee`` .................................... .. include:: pointee_ref.rst Class template ``indirect_reference`` ..................................... .. include:: indirect_reference_ref.rst Class template ``indirect_iterator`` .................................... .. include:: indirect_iterator_ref.rst Reverse iterator ---------------- .. include:: reverse_iterator_abstract.rst Class template ``reverse_iterator`` ................................... .. include:: reverse_iterator_ref.rst Transform iterator ------------------ .. include:: transform_iterator_abstract.rst Class template ``transform_iterator`` ..................................... .. include:: transform_iterator_ref.rst Filter iterator --------------- .. include:: filter_iterator_abstract.rst Class template ``filter_iterator`` .................................. .. include:: filter_iterator_ref.rst Counting iterator ----------------- .. include:: counting_iterator_abstract.rst Class template ``counting_iterator`` .................................... .. include:: counting_iterator_ref.rst Function output iterator ------------------------ .. include:: func_output_iter_abstract.rst Class template ``function_output_iterator`` ........................................... .. include:: func_output_iter_ref.rst .. LocalWords: Abrahams Siek Witt istream ostream iter MTL strided interoperate LocalWords: CRTP metafunctions inlining lvalue JGS incrementable BGL LEDA cv LocalWords: GraphBase struct ptrdiff UnaryFunction const int typename bool pp LocalWords: lhs rhs SFINAE markup iff tmp OtherDerived OtherIterator DWA foo LocalWords: dereferenceable subobject AdaptableUnaryFunction impl pre ifdef'd LocalWords: OtherIncrementable Coplien