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2 | [library Boost.Iterator | |
3 | [/ version 1.0.1] | |
4 | [quickbook 1.6] | |
5 | [authors [Abrahams, David], [Siek, Jeremy], [Witt, Thomas]] | |
6 | [copyright 2003 2005 David Abrahams Jeremy Siek Thomas Witt] | |
7 | [category iterator] | |
8 | [id iterator] | |
9 | [dirname iterator] | |
10 | [purpose | |
11 | ] | |
12 | [license | |
13 | Distributed under the Boost Software License, Version 1.0. | |
14 | (See accompanying file LICENSE_1_0.txt or copy at | |
15 | <ulink url="http://www.boost.org/LICENSE_1_0.txt"> | |
16 | http://www.boost.org/LICENSE_1_0.txt | |
17 | </ulink>) | |
18 | ] | |
19 | ] | |
20 | ||
21 | [/ QuickBook Document version 1.0 ] | |
22 | ||
23 | [/ Images ] | |
24 | ||
25 | [def _note_ [$images/note.png]] | |
26 | [def _alert_ [$images/caution.png]] | |
27 | [def _detail_ [$images/note.png]] | |
28 | [def _tip_ [$images/tip.png]] | |
29 | ||
30 | [/ Links ] | |
31 | ||
32 | [def _iterator_ [@../../libs/iterator/doc/index.html Boost.Iterator]] | |
33 | ||
34 | [section:intro Introduction] | |
35 | ||
36 | [def _concepts_ [@http://www.boost.org/more/generic_programming.html#concept concepts]] | |
37 | ||
38 | The Boost Iterator Library contains two parts. The first | |
39 | is a system of _concepts_ which extend the C++ standard | |
40 | iterator requirements. The second is a framework of | |
41 | components for building iterators based on these | |
42 | extended concepts and includes several useful iterator | |
43 | adaptors. The extended iterator concepts have been | |
44 | carefully designed so that old-style iterators | |
45 | can fit in the new concepts and so that new-style | |
46 | iterators will be compatible with old-style algorithms, | |
47 | though algorithms may need to be updated if they want to | |
48 | take full advantage of the new-style iterator | |
49 | capabilities. Several components of this library have | |
50 | been accepted into the C++ standard technical report. | |
51 | The components of the Boost Iterator Library replace the | |
52 | older Boost Iterator Adaptor Library. | |
53 | ||
54 | ||
55 | [h2 New-Style Iterators] | |
56 | ||
57 | [def _N1185_ [@http://www.gotw.ca/publications/N1185.pdf N1185]] | |
58 | [def _N1211_ [@http://www.gotw.ca/publications/N1211.pdf N1211]] | |
59 | [def _GOTW_50_ [@http://www.gotw.ca/gotw/050.htm Guru of the Week]] | |
60 | ||
61 | The iterator categories defined in C++98 are extremely limiting | |
62 | because they bind together two orthogonal concepts: traversal and | |
63 | element access. For example, because a random access iterator is | |
64 | required to return a reference (and not a proxy) when dereferenced, | |
65 | it is impossible to capture the capabilities of | |
66 | `vector<bool>::iterator` using the C++98 categories. This is the | |
67 | infamous "`vector<bool>` is not a container, and its iterators | |
68 | aren't random access iterators", debacle about which Herb Sutter | |
69 | wrote two papers for the standards comittee (_N1185_ and _N1211_), | |
70 | and a _GOTW_50_. New-style iterators go well beyond | |
71 | patching up `vector<bool>`, though: there are lots of other | |
72 | iterators already in use which can't be adequately represented by | |
73 | the existing concepts. For details about the new iterator | |
74 | concepts, see our [@./new-iter-concepts.html Standard Proposal for New-Style Iterators]. | |
75 | ||
76 | [h2 Iterator Facade and Adaptor] | |
77 | ||
78 | [def _facade_ [@./iterator_facade.html facade]] | |
79 | [def _adaptor_ [@./iterator_adaptor.html adaptor]] | |
80 | ||
81 | Writing standard-conforming iterators is tricky, but the need comes | |
82 | up often. In order to ease the implementation of new iterators, | |
83 | the Boost.Iterator library provides the _facade_ class template, | |
84 | which implements many useful defaults and compile-time checks | |
85 | designed to help the iterator author ensure that his iterator is | |
86 | correct. | |
87 | ||
88 | It is also common to define a new iterator that is similar to some | |
89 | underlying iterator or iterator-like type, but that modifies some | |
90 | aspect of the underlying type's behavior. For that purpose, the | |
91 | library supplies the _adaptor_ class template, which is specially | |
92 | designed to take advantage of as much of the underlying type's | |
93 | behavior as possible. | |
94 | ||
95 | Both _facade_ and _adaptor_ as well as many of the `specialized | |
96 | adaptors`_ mentioned below have been proposed for standardization | |
97 | ([@./facade-and-adaptor.html Standard Proposal For Iterator Facade and Adaptor]). | |
98 | ||
99 | [h2 Specialized Adaptors] | |
100 | ||
101 | The iterator library supplies a useful suite of standard-conforming | |
102 | iterator templates based on the Boost [link | |
103 | iterator.intro.iterator_facade_and_adaptor iterator facade and adaptor] | |
104 | templates. | |
105 | ||
106 | [def _counting_ [@./counting_iterator.html `counting_iterator`]] | |
107 | [def _filter_ [@./filter_iterator.html `filter_iterator`]] | |
108 | [def _function_ [@./function_output_iterator.html `function_output_iterator`]] | |
109 | [def _indirect_ [@./indirect_iterator.html `indirect_iterator`]] | |
110 | [def _permutation_ [@./permutation_iterator.html `permutation_iterator`]] | |
111 | [def _reverse_ [@./reverse_iterator.html `reverse_iterator`]] | |
112 | [def _shared_ [@./shared_container_iterator.html `shared_container_iterator`]] | |
113 | [def _transform_ [@./transform_iterator.html `transform_iterator`]] | |
114 | [def _zip_ [@./zip_iterator.html `zip_iterator`]] | |
115 | ||
116 | [def _shared_ptr_ [@../../smart_ptr/shared_ptr.htm `shared_ptr`]] | |
117 | ||
118 | * _counting_: an iterator over a sequence of consecutive values. | |
119 | Implements a "lazy sequence" | |
120 | ||
121 | * _filter_: an iterator over the subset of elements of some | |
122 | sequence which satisfy a given predicate | |
123 | ||
124 | * _function_: an output iterator wrapping a unary function | |
125 | object; each time an element is written into the dereferenced | |
126 | iterator, it is passed as a parameter to the function object. | |
127 | ||
128 | * _indirect_: an iterator over the objects *pointed-to* by the | |
129 | elements of some sequence. | |
130 | ||
131 | * _permutation_: an iterator over the elements of some random-access | |
132 | sequence, rearranged according to some sequence of integer indices. | |
133 | ||
134 | * _reverse_: an iterator which traverses the elements of some | |
135 | bidirectional sequence in reverse. Corrects many of the | |
136 | shortcomings of C++98's ``std::reverse_iterator``. | |
137 | ||
138 | * _shared_: an iterator over elements of a container whose | |
139 | lifetime is maintained by a _shared_ptr_ stored in the iterator. | |
140 | ||
141 | * _transform_: an iterator over elements which are the result of | |
142 | applying some functional transformation to the elements of an | |
143 | underlying sequence. This component also replaces the old | |
144 | ``projection_iterator_adaptor``. | |
145 | ||
146 | * _zip_: an iterator over tuples of the elements at corresponding | |
147 | positions of heterogeneous underlying iterators. | |
148 | ||
149 | [h2 Iterator Utilities] | |
150 | ||
151 | [h3 Traits] | |
152 | ||
153 | [def _pointee_ [@./pointee.html `pointee.hpp`]] | |
154 | [def _iterator_traits_ [@./iterator_traits.html `iterator_traits.hpp`]] | |
155 | [def _interoperable_ [@./interoperable.html `interoperable.hpp`]] | |
156 | [def _MPL_ [@../../mpl/doc/index.html [*MPL]]] | |
157 | ||
158 | * _pointee_: Provides the capability to deduce the referent types | |
159 | of pointers, smart pointers and iterators in generic code. Used | |
160 | in _indirect_. | |
161 | ||
162 | * _iterator_traits_: Provides _MPL_ compatible metafunctions which | |
163 | retrieve an iterator's traits. Also corrects for the deficiencies | |
164 | of broken implementations of `std::iterator_traits`. | |
165 | ||
166 | [\ * |interoperable|_ (PDF__): Provides an _MPL_ compatible metafunction for | |
167 | testing iterator interoperability | |
168 | ] | |
169 | ||
170 | [h3 Testing and Concept Checking] | |
171 | ||
172 | [def _iterator_concepts_ [@./iterator_concepts.html `iterator_concepts.hpp`]] | |
173 | [def _iterator_archetypes_ [@./iterator_archetypes.html `iterator_archetypes.hpp`]] | |
174 | ||
175 | * _iterator_concepts_: Concept checking classes for the new iterator concepts. | |
176 | ||
177 | * _iterator_archetypes_: Concept archetype classes for the new iterators concepts. | |
178 | ||
179 | [endsect] | |
180 | ||
181 | [include concepts.qbk] | |
182 | ||
183 | [section:generic Generic Iterators] | |
184 | ||
185 | [include facade.qbk] | |
186 | ||
187 | [include adaptor.qbk] | |
188 | ||
189 | [endsect] | |
190 | ||
191 | [include specialized_adaptors.qbk] | |
192 | ||
193 | [section:utilities Utilities] | |
194 | ||
195 | [include archetypes.qbk] | |
196 | ||
197 | [include concept_checking.qbk] | |
198 | ||
199 | [include traits.qbk] | |
200 | ||
201 | [include utilities.qbk] | |
202 | ||
203 | [endsect] | |
204 | ||
205 | [section:upgrading Upgrading from the old Boost Iterator Adaptor Library] | |
206 | ||
207 | [def _type_generator_ [@http://www.boost.org/more/generic_programming.html#type_generator type generator]] | |
208 | ||
209 | If you have been using the old Boost Iterator Adaptor library to | |
210 | implement iterators, you probably wrote a `Policies` class which | |
211 | captures the core operations of your iterator. In the new library | |
212 | design, you'll move those same core operations into the body of the | |
213 | iterator class itself. If you were writing a family of iterators, | |
214 | you probably wrote a _type_generator_ to build the | |
215 | `iterator_adaptor` specialization you needed; in the new library | |
216 | design you don't need a type generator (though may want to keep it | |
217 | around as a compatibility aid for older code) because, due to the | |
218 | use of the Curiously Recurring Template Pattern (CRTP) [Cop95]_, | |
219 | you can now define the iterator class yourself and acquire | |
220 | functionality through inheritance from `iterator_facade` or | |
221 | `iterator_adaptor`. As a result, you also get much finer control | |
222 | over how your iterator works: you can add additional constructors, | |
223 | or even override the iterator functionality provided by the | |
224 | library. | |
225 | ||
226 | ||
227 | If you're looking for the old `projection_iterator` component, | |
228 | its functionality has been merged into _transform_iterator_: as | |
229 | long as the function object's `result_type` (or the `Reference` | |
230 | template argument, if explicitly specified) is a true reference | |
231 | type, _transform_iterator_ will behave like | |
232 | `projection_iterator` used to. | |
233 | ||
234 | [endsect] | |
235 | ||
236 | [section:history History] | |
237 | ||
238 | In 2000 Dave Abrahams was writing an iterator for a container of | |
239 | pointers, which would access the pointed-to elements when | |
240 | dereferenced. Naturally, being a library writer, he decided to | |
241 | generalize the idea and the Boost Iterator Adaptor library was born. | |
242 | Dave was inspired by some writings of Andrei Alexandrescu and chose a | |
243 | policy based design (though he probably didn't capture Andrei's idea | |
244 | very well - there was only one policy class for all the iterator's | |
245 | orthogonal properties). Soon Jeremy Siek realized he would need the | |
246 | library and they worked together to produce a "Boostified" version, | |
247 | which was reviewed and accepted into the library. They wrote a paper | |
248 | and made several important revisions of the code. | |
249 | ||
250 | Eventually, several shortcomings of the older library began to make | |
251 | the need for a rewrite apparent. Dave and Jeremy started working | |
252 | at the Santa Cruz C++ committee meeting in 2002, and had quickly | |
253 | generated a working prototype. At the urging of Mat Marcus, they | |
254 | decided to use the GenVoca/CRTP pattern approach, and moved the | |
255 | policies into the iterator class itself. Thomas Witt expressed | |
256 | interest and became the voice of strict compile-time checking for | |
257 | the project, adding uses of the SFINAE technique to eliminate false | |
258 | converting constructors and operators from the overload set. He | |
259 | also recognized the need for a separate `iterator_facade`, and | |
260 | factored it out of `iterator_adaptor`. Finally, after a | |
261 | near-complete rewrite of the prototype, they came up with the | |
262 | library you see today. | |
263 | ||
264 | [:\[Coplien, 1995\] Coplien, J., Curiously Recurring Template | |
265 | Patterns, C++ Report, February 1995, pp. 24-27.] | |
266 | ||
267 | [endsect] | |
268 | ||
269 |