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2 | [section:utilities Iterator Utilities] | |
3 | ||
4 | [section:utilities_traits Traits] | |
5 | ||
6 | [h2 Overview] | |
7 | ||
8 | Have you ever wanted to write a generic function that can operate | |
9 | on any kind of dereferenceable object? If you have, you've | |
10 | probably run into the problem of how to determine the type that the | |
11 | object "points at": | |
12 | ||
13 | template <class Dereferenceable> | |
14 | void f(Dereferenceable p) | |
15 | { | |
16 | *what-goes-here?* value = \*p; | |
17 | ... | |
18 | } | |
19 | ||
20 | ||
21 | [h2 `pointee`] | |
22 | ||
23 | It turns out to be impossible to come up with a fully-general | |
24 | algorithm to do determine *what-goes-here* directly, but it is | |
25 | possible to require that `pointee<Dereferenceable>::type` is | |
26 | correct. Naturally, `pointee` has the same difficulty: it can't | |
27 | determine the appropriate `::type` reliably for all | |
28 | `Dereferenceable`\ s, but it makes very good guesses (it works | |
29 | for all pointers, standard and boost smart pointers, and | |
30 | iterators), and when it guesses wrongly, it can be specialized as | |
31 | necessary: | |
32 | ||
33 | namespace boost | |
34 | { | |
35 | template <class T> | |
36 | struct pointee<third_party_lib::smart_pointer<T> > | |
37 | { | |
38 | typedef T type; | |
39 | }; | |
40 | } | |
41 | ||
42 | [h2 `indirect_reference`] | |
43 | ||
44 | `indirect_reference<T>::type` is rather more specialized than | |
45 | `pointee`, and is meant to be used to forward the result of | |
46 | dereferencing an object of its argument type. Most dereferenceable | |
47 | types just return a reference to their pointee, but some return | |
48 | proxy references or return the pointee by value. When that | |
49 | information is needed, call on `indirect_reference`. | |
50 | ||
51 | Both of these templates are essential to the correct functioning of | |
52 | [link iterator.specialized.indirect `indirect_iterator`]. | |
53 | ||
54 | [h2 `minimum_category`] | |
55 | ||
56 | `minimum_category` takes two iterator categories or two iterator traversal tags | |
57 | and returns the one that is the weakest (i.e. least advanced). For example: | |
58 | ||
59 | static_assert( | |
60 | is_same< | |
61 | minimum_category< | |
62 | std::forward_iterator_tag, | |
63 | std::random_access_iterator_tag | |
64 | >::type, | |
65 | std::forward_iterator_tag | |
66 | >::value, | |
67 | "Unexpected minimum_category result" | |
68 | ); | |
69 | ||
70 | [h2 Iterator category and traversal tags manipulation] | |
71 | ||
72 | The library provides several utilities to simplify conversions between iterator categories | |
73 | and traversal tags: | |
74 | ||
75 | * `iterator_category_to_traversal<C>::type` - the metafunction takes an iterator category `C` and returns | |
76 | the corresponding traversal tag. | |
77 | * `iterator_traversal<T>::type` - a shorthand for `iterator_category_to_traversal<iterator_category<T>::type>::type`. | |
78 | * `pure_traversal_tag<T>::type` - the metafunction takes a tag `T` which derives from one of the iterator traversal tags | |
79 | and returns that traversal tag. `T` may also derive from other tags describing the iterator (e.g. whether this is a `const`-iterator | |
80 | or not), these additional tags are not considered. | |
81 | * `pure_iterator_traversal<T>::type` - a shorthand for `pure_traversal_tag<iterator_traversal<T>::type>::type`. | |
82 | ||
83 | [h2 Reference] | |
84 | ||
85 | [h3 `pointee`] | |
86 | ||
87 | template <class Dereferenceable> | |
88 | struct pointee | |
89 | { | |
90 | typedef /* see below */ type; | |
91 | }; | |
92 | ||
93 | [*Requires:] For an object `x` of type `Dereferenceable`, `*x` | |
94 | is well-formed. If `++x` is ill-formed it shall neither be | |
95 | ambiguous nor shall it violate access control, and | |
96 | `Dereferenceable::element_type` shall be an accessible type. | |
97 | Otherwise `iterator_traits<Dereferenceable>::value_type` shall | |
98 | be well formed. \[Note: These requirements need not apply to | |
99 | explicit or partial specializations of `pointee`\] | |
100 | ||
101 | `type` is determined according to the following algorithm, where | |
102 | `x` is an object of type `Dereferenceable`: | |
103 | ||
104 | if ( ++x is ill-formed ) | |
105 | { | |
106 | return Dereferenceable::element_type | |
107 | } | |
108 | else if (*x is a mutable reference to | |
109 | std::iterator_traits<Dereferenceable>::value_type) | |
110 | { | |
111 | return iterator_traits<Dereferenceable>::value_type | |
112 | } | |
113 | else | |
114 | { | |
115 | return iterator_traits<Dereferenceable>::value_type const | |
116 | } | |
117 | ||
118 | [h3 `indirect_reference`] | |
119 | ||
120 | template <class Dereferenceable> | |
121 | struct indirect_reference | |
122 | { | |
123 | typedef /* see below */ type; | |
124 | }; | |
125 | ||
126 | [*Requires:] For an object `x` of type `Dereferenceable`, `*x` | |
127 | is well-formed. If `++x` is ill-formed it shall neither be | |
128 | ambiguous nor shall it violate access control, and | |
129 | `pointee<Dereferenceable>::type&` shall be well-formed. | |
130 | Otherwise `iterator_traits<Dereferenceable>::reference` shall | |
131 | be well formed. \[Note: These requirements need not apply to | |
132 | explicit or partial specializations of `indirect_reference`\] | |
133 | ||
134 | `type` is determined according to the following algorithm, where | |
135 | `x` is an object of type `Dereferenceable`: | |
136 | ||
137 | if ( ++x is ill-formed ) | |
138 | return pointee<Dereferenceable>::type& | |
139 | else | |
140 | std::iterator_traits<Dereferenceable>::reference | |
141 | ||
142 | [h3 `minimum_category`] | |
143 | ||
144 | template <typename C1, typename C2> | |
145 | struct minimum_category | |
146 | { | |
147 | typedef /* see below */ type; | |
148 | }; | |
149 | ||
150 | [*Requires:] Both `C1` and `C2` shall be standard iterator categories or | |
151 | iterator traversal tags. | |
152 | ||
153 | `type` is determined according to the following algorithm, where `c1` is an | |
154 | object of type `C1` and `c2` is an object of type `C2`: | |
155 | ||
156 | if (c1 is convertible to c2) | |
157 | return C2; | |
158 | else | |
159 | return C1; | |
160 | ||
161 | [note The above definition relies on the fact that the more restricting categories | |
162 | and traversal tags are convertible to the less restricting ones.] | |
163 | ||
164 | [h3 `iterator_category_to_traversal`] | |
165 | ||
166 | template <typename C> | |
167 | struct iterator_category_to_traversal | |
168 | { | |
169 | typedef /* see below */ type; | |
170 | }; | |
171 | ||
172 | [*Requires:] `C` shall be a standard iterator category or an | |
173 | iterator traversal tag. | |
174 | ||
175 | If `C` is an iterator traversal tag or convertible to one, `type` equivalent to `C`. | |
176 | Otherwise, `type` is defined to the closest iterator traversal tag matching `C`. | |
177 | ||
178 | [h3 `iterator_traversal`] | |
179 | ||
180 | template <typename Iterator> | |
181 | struct iterator_traversal | |
182 | { | |
183 | typedef typename iterator_category_to_traversal< | |
184 | typename iterator_category<Iterator>::type | |
185 | >::type type; | |
186 | }; | |
187 | ||
188 | [*Requires:] `Iterator` shall be an iterator. | |
189 | ||
190 | [h3 `pure_traversal_tag`] | |
191 | ||
192 | template <typename T> | |
193 | struct pure_traversal_tag | |
194 | { | |
195 | typedef /* see below */ type; | |
196 | }; | |
197 | ||
198 | [*Requires:] `T` shall be convertible to an iterator traversal tag. | |
199 | ||
200 | `type` is defined to be the most advanced traversal tag `Tag` so that `T` is convertible to `Tag`. | |
201 | ||
202 | [h3 `pure_iterator_traversal`] | |
203 | ||
204 | template <typename Iterator> | |
205 | struct pure_iterator_traversal | |
206 | { | |
207 | typedef typename pure_traversal_tag< | |
208 | typename iterator_traversal<Iterator>::type | |
209 | >::type type; | |
210 | }; | |
211 | ||
212 | [*Requires:] `Iterator` shall be an iterator. | |
213 | ||
214 | [endsect] | |
215 | ||
216 | [section:utilities_testing Testing and Concept Checking] | |
217 | ||
218 | The iterator concept checking classes provide a mechanism for a | |
219 | template to report better error messages when a user instantiates | |
220 | the template with a type that does not meet the requirements of the | |
221 | template. | |
222 | ||
223 | For an introduction to using concept checking classes, see | |
224 | the documentation for the | |
225 | [@../../concept_check/index.html `boost::concept_check`] library. | |
226 | ||
227 | ||
228 | [h2 Reference] | |
229 | ||
230 | [h3 Iterator Access Concepts] | |
231 | ||
232 | * |Readable|_ | |
233 | * |Writable|_ | |
234 | * |Swappable|_ | |
235 | * |Lvalue|_ | |
236 | ||
237 | [/ .. |Readable| replace:: *Readable Iterator* ] | |
238 | [/ .. _Readable: ReadableIterator.html ] | |
239 | [/ ] | |
240 | [/ .. |Writable| replace:: *Writable Iterator* ] | |
241 | [/ .. _Writable: WritableIterator.html ] | |
242 | [/ ] | |
243 | [/ .. |Swappable| replace:: *Swappable Iterator* ] | |
244 | [/ .. _Swappable: SwappableIterator.html ] | |
245 | [/ ] | |
246 | [/ .. |Lvalue| replace:: *Lvalue Iterator* ] | |
247 | [/ .. _Lvalue: LvalueIterator.html ] | |
248 | ||
249 | ||
250 | Iterator Traversal Concepts | |
251 | ........................... | |
252 | ||
253 | * |Incrementable|_ | |
254 | * |SinglePass|_ | |
255 | * |Forward|_ | |
256 | * |Bidir|_ | |
257 | * |Random|_ | |
258 | ||
259 | ||
260 | [/ .. |Incrementable| replace:: *Incrementable Iterator* ] | |
261 | [/ .. _Incrementable: IncrementableIterator.html ] | |
262 | [/ ] | |
263 | [/ .. |SinglePass| replace:: *Single Pass Iterator* ] | |
264 | [/ .. _SinglePass: SinglePassIterator.html ] | |
265 | [/ ] | |
266 | [/ .. |Forward| replace:: *Forward Traversal* ] | |
267 | [/ .. _Forward: ForwardTraversal.html ] | |
268 | [/ ] | |
269 | [/ .. |Bidir| replace:: *Bidirectional Traversal* ] | |
270 | [/ .. _Bidir: BidirectionalTraversal.html ] | |
271 | [/ ] | |
272 | [/ .. |Random| replace:: *Random Access Traversal* ] | |
273 | [/ .. _Random: RandomAccessTraversal.html ] | |
274 | ||
275 | ||
276 | ||
277 | [h3 `iterator_concepts.hpp` Synopsis] | |
278 | ||
279 | namespace boost_concepts { | |
280 | ||
281 | // Iterator Access Concepts | |
282 | ||
283 | template <typename Iterator> | |
284 | class ReadableIteratorConcept; | |
285 | ||
286 | template < | |
287 | typename Iterator | |
288 | , typename ValueType = std::iterator_traits<Iterator>::value_type | |
289 | > | |
290 | class WritableIteratorConcept; | |
291 | ||
292 | template <typename Iterator> | |
293 | class SwappableIteratorConcept; | |
294 | ||
295 | template <typename Iterator> | |
296 | class LvalueIteratorConcept; | |
297 | ||
298 | // Iterator Traversal Concepts | |
299 | ||
300 | template <typename Iterator> | |
301 | class IncrementableIteratorConcept; | |
302 | ||
303 | template <typename Iterator> | |
304 | class SinglePassIteratorConcept; | |
305 | ||
306 | template <typename Iterator> | |
307 | class ForwardTraversalConcept; | |
308 | ||
309 | template <typename Iterator> | |
310 | class BidirectionalTraversalConcept; | |
311 | ||
312 | template <typename Iterator> | |
313 | class RandomAccessTraversalConcept; | |
314 | ||
315 | // Interoperability | |
316 | ||
317 | template <typename Iterator, typename ConstIterator> | |
318 | class InteroperableIteratorConcept; | |
319 | ||
320 | } | |
321 | ||
322 | [endsect] | |
323 | ||
324 | [endsect] |