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1 //===- iterator.h - Utilities for using and defining iterators --*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 
10 #ifndef LLVM_ADT_ITERATOR_H
11 #define LLVM_ADT_ITERATOR_H
12 
13 #include "llvm/ADT/iterator_range.h"
14 #include <algorithm>
15 #include <cstddef>
16 #include <iterator>
17 #include <type_traits>
18 #include <utility>
19 
20 namespace llvm {
21 
22 /// CRTP base class which implements the entire standard iterator facade
23 /// in terms of a minimal subset of the interface.
24 ///
25 /// Use this when it is reasonable to implement most of the iterator
26 /// functionality in terms of a core subset. If you need special behavior or
27 /// there are performance implications for this, you may want to override the
28 /// relevant members instead.
29 ///
30 /// Note, one abstraction that this does *not* provide is implementing
31 /// subtraction in terms of addition by negating the difference. Negation isn't
32 /// always information preserving, and I can see very reasonable iterator
33 /// designs where this doesn't work well. It doesn't really force much added
34 /// boilerplate anyways.
35 ///
36 /// Another abstraction that this doesn't provide is implementing increment in
37 /// terms of addition of one. These aren't equivalent for all iterator
38 /// categories, and respecting that adds a lot of complexity for little gain.
39 ///
40 /// Classes wishing to use `iterator_facade_base` should implement the following
41 /// methods:
42 ///
43 /// Forward Iterators:
44 ///   (All of the following methods)
45 ///   - DerivedT &operator=(const DerivedT &R);
46 ///   - bool operator==(const DerivedT &R) const;
47 ///   - const T &operator*() const;
48 ///   - T &operator*();
49 ///   - DerivedT &operator++();
50 ///
51 /// Bidirectional Iterators:
52 ///   (All methods of forward iterators, plus the following)
53 ///   - DerivedT &operator--();
54 ///
55 /// Random-access Iterators:
56 ///   (All methods of bidirectional iterators excluding the following)
57 ///   - DerivedT &operator++();
58 ///   - DerivedT &operator--();
59 ///   (and plus the following)
60 ///   - bool operator<(const DerivedT &RHS) const;
61 ///   - DifferenceTypeT operator-(const DerivedT &R) const;
62 ///   - DerivedT &operator+=(DifferenceTypeT N);
63 ///   - DerivedT &operator-=(DifferenceTypeT N);
64 ///
65 template <typename DerivedT, typename IteratorCategoryT, typename T,
66           typename DifferenceTypeT = std::ptrdiff_t, typename PointerT = T *,
67           typename ReferenceT = T &>
68 class iterator_facade_base
69     : public std::iterator<IteratorCategoryT, T, DifferenceTypeT, PointerT,
70                            ReferenceT> {
71 protected:
72   enum {
73     IsRandomAccess = std::is_base_of<std::random_access_iterator_tag,
74                                      IteratorCategoryT>::value,
75     IsBidirectional = std::is_base_of<std::bidirectional_iterator_tag,
76                                       IteratorCategoryT>::value,
77   };
78 
79   /// A proxy object for computing a reference via indirecting a copy of an
80   /// iterator. This is used in APIs which need to produce a reference via
81   /// indirection but for which the iterator object might be a temporary. The
82   /// proxy preserves the iterator internally and exposes the indirected
83   /// reference via a conversion operator.
84   class ReferenceProxy {
85     friend iterator_facade_base;
86 
87     DerivedT I;
88 
ReferenceProxy(DerivedT I)89     ReferenceProxy(DerivedT I) : I(std::move(I)) {}
90 
91   public:
ReferenceT()92     operator ReferenceT() const { return *I; }
93   };
94 
95 public:
96   DerivedT operator+(DifferenceTypeT n) const {
97     static_assert(std::is_base_of<iterator_facade_base, DerivedT>::value,
98                   "Must pass the derived type to this template!");
99     static_assert(
100         IsRandomAccess,
101         "The '+' operator is only defined for random access iterators.");
102     DerivedT tmp = *static_cast<const DerivedT *>(this);
103     tmp += n;
104     return tmp;
105   }
106   friend DerivedT operator+(DifferenceTypeT n, const DerivedT &i) {
107     static_assert(
108         IsRandomAccess,
109         "The '+' operator is only defined for random access iterators.");
110     return i + n;
111   }
112   DerivedT operator-(DifferenceTypeT n) const {
113     static_assert(
114         IsRandomAccess,
115         "The '-' operator is only defined for random access iterators.");
116     DerivedT tmp = *static_cast<const DerivedT *>(this);
117     tmp -= n;
118     return tmp;
119   }
120 
121   DerivedT &operator++() {
122     static_assert(std::is_base_of<iterator_facade_base, DerivedT>::value,
123                   "Must pass the derived type to this template!");
124     return static_cast<DerivedT *>(this)->operator+=(1);
125   }
126   DerivedT operator++(int) {
127     DerivedT tmp = *static_cast<DerivedT *>(this);
128     ++*static_cast<DerivedT *>(this);
129     return tmp;
130   }
131   DerivedT &operator--() {
132     static_assert(
133         IsBidirectional,
134         "The decrement operator is only defined for bidirectional iterators.");
135     return static_cast<DerivedT *>(this)->operator-=(1);
136   }
137   DerivedT operator--(int) {
138     static_assert(
139         IsBidirectional,
140         "The decrement operator is only defined for bidirectional iterators.");
141     DerivedT tmp = *static_cast<DerivedT *>(this);
142     --*static_cast<DerivedT *>(this);
143     return tmp;
144   }
145 
146   bool operator!=(const DerivedT &RHS) const {
147     return !static_cast<const DerivedT *>(this)->operator==(RHS);
148   }
149 
150   bool operator>(const DerivedT &RHS) const {
151     static_assert(
152         IsRandomAccess,
153         "Relational operators are only defined for random access iterators.");
154     return !static_cast<const DerivedT *>(this)->operator<(RHS) &&
155            !static_cast<const DerivedT *>(this)->operator==(RHS);
156   }
157   bool operator<=(const DerivedT &RHS) const {
158     static_assert(
159         IsRandomAccess,
160         "Relational operators are only defined for random access iterators.");
161     return !static_cast<const DerivedT *>(this)->operator>(RHS);
162   }
163   bool operator>=(const DerivedT &RHS) const {
164     static_assert(
165         IsRandomAccess,
166         "Relational operators are only defined for random access iterators.");
167     return !static_cast<const DerivedT *>(this)->operator<(RHS);
168   }
169 
170   PointerT operator->() { return &static_cast<DerivedT *>(this)->operator*(); }
171   PointerT operator->() const {
172     return &static_cast<const DerivedT *>(this)->operator*();
173   }
174   ReferenceProxy operator[](DifferenceTypeT n) {
175     static_assert(IsRandomAccess,
176                   "Subscripting is only defined for random access iterators.");
177     return ReferenceProxy(static_cast<DerivedT *>(this)->operator+(n));
178   }
179   ReferenceProxy operator[](DifferenceTypeT n) const {
180     static_assert(IsRandomAccess,
181                   "Subscripting is only defined for random access iterators.");
182     return ReferenceProxy(static_cast<const DerivedT *>(this)->operator+(n));
183   }
184 };
185 
186 /// CRTP base class for adapting an iterator to a different type.
187 ///
188 /// This class can be used through CRTP to adapt one iterator into another.
189 /// Typically this is done through providing in the derived class a custom \c
190 /// operator* implementation. Other methods can be overridden as well.
191 template <
192     typename DerivedT, typename WrappedIteratorT,
193     typename IteratorCategoryT =
194         typename std::iterator_traits<WrappedIteratorT>::iterator_category,
195     typename T = typename std::iterator_traits<WrappedIteratorT>::value_type,
196     typename DifferenceTypeT =
197         typename std::iterator_traits<WrappedIteratorT>::difference_type,
198     typename PointerT = typename std::conditional<
199         std::is_same<T, typename std::iterator_traits<
200                             WrappedIteratorT>::value_type>::value,
201         typename std::iterator_traits<WrappedIteratorT>::pointer, T *>::type,
202     typename ReferenceT = typename std::conditional<
203         std::is_same<T, typename std::iterator_traits<
204                             WrappedIteratorT>::value_type>::value,
205         typename std::iterator_traits<WrappedIteratorT>::reference, T &>::type,
206     // Don't provide these, they are mostly to act as aliases below.
207     typename WrappedTraitsT = std::iterator_traits<WrappedIteratorT>>
208 class iterator_adaptor_base
209     : public iterator_facade_base<DerivedT, IteratorCategoryT, T,
210                                   DifferenceTypeT, PointerT, ReferenceT> {
211   using BaseT = typename iterator_adaptor_base::iterator_facade_base;
212 
213 protected:
214   WrappedIteratorT I;
215 
216   iterator_adaptor_base() = default;
217 
iterator_adaptor_base(WrappedIteratorT u)218   explicit iterator_adaptor_base(WrappedIteratorT u) : I(std::move(u)) {
219     static_assert(std::is_base_of<iterator_adaptor_base, DerivedT>::value,
220                   "Must pass the derived type to this template!");
221   }
222 
wrapped()223   const WrappedIteratorT &wrapped() const { return I; }
224 
225 public:
226   using difference_type = DifferenceTypeT;
227 
228   DerivedT &operator+=(difference_type n) {
229     static_assert(
230         BaseT::IsRandomAccess,
231         "The '+=' operator is only defined for random access iterators.");
232     I += n;
233     return *static_cast<DerivedT *>(this);
234   }
235   DerivedT &operator-=(difference_type n) {
236     static_assert(
237         BaseT::IsRandomAccess,
238         "The '-=' operator is only defined for random access iterators.");
239     I -= n;
240     return *static_cast<DerivedT *>(this);
241   }
242   using BaseT::operator-;
243   difference_type operator-(const DerivedT &RHS) const {
244     static_assert(
245         BaseT::IsRandomAccess,
246         "The '-' operator is only defined for random access iterators.");
247     return I - RHS.I;
248   }
249 
250   // We have to explicitly provide ++ and -- rather than letting the facade
251   // forward to += because WrappedIteratorT might not support +=.
252   using BaseT::operator++;
253   DerivedT &operator++() {
254     ++I;
255     return *static_cast<DerivedT *>(this);
256   }
257   using BaseT::operator--;
258   DerivedT &operator--() {
259     static_assert(
260         BaseT::IsBidirectional,
261         "The decrement operator is only defined for bidirectional iterators.");
262     --I;
263     return *static_cast<DerivedT *>(this);
264   }
265 
266   bool operator==(const DerivedT &RHS) const { return I == RHS.I; }
267   bool operator<(const DerivedT &RHS) const {
268     static_assert(
269         BaseT::IsRandomAccess,
270         "Relational operators are only defined for random access iterators.");
271     return I < RHS.I;
272   }
273 
274   ReferenceT operator*() const { return *I; }
275 };
276 
277 /// An iterator type that allows iterating over the pointees via some
278 /// other iterator.
279 ///
280 /// The typical usage of this is to expose a type that iterates over Ts, but
281 /// which is implemented with some iterator over T*s:
282 ///
283 /// \code
284 ///   using iterator = pointee_iterator<SmallVectorImpl<T *>::iterator>;
285 /// \endcode
286 template <typename WrappedIteratorT,
287           typename T = typename std::remove_reference<
288               decltype(**std::declval<WrappedIteratorT>())>::type>
289 struct pointee_iterator
290     : iterator_adaptor_base<
291           pointee_iterator<WrappedIteratorT, T>, WrappedIteratorT,
292           typename std::iterator_traits<WrappedIteratorT>::iterator_category,
293           T> {
294   pointee_iterator() = default;
295   template <typename U>
pointee_iteratorpointee_iterator296   pointee_iterator(U &&u)
297       : pointee_iterator::iterator_adaptor_base(std::forward<U &&>(u)) {}
298 
299   T &operator*() const { return **this->I; }
300 };
301 
302 template <typename RangeT, typename WrappedIteratorT =
303                                decltype(std::begin(std::declval<RangeT>()))>
304 iterator_range<pointee_iterator<WrappedIteratorT>>
make_pointee_range(RangeT && Range)305 make_pointee_range(RangeT &&Range) {
306   using PointeeIteratorT = pointee_iterator<WrappedIteratorT>;
307   return make_range(PointeeIteratorT(std::begin(std::forward<RangeT>(Range))),
308                     PointeeIteratorT(std::end(std::forward<RangeT>(Range))));
309 }
310 
311 template <typename WrappedIteratorT,
312           typename T = decltype(&*std::declval<WrappedIteratorT>())>
313 class pointer_iterator
314     : public iterator_adaptor_base<pointer_iterator<WrappedIteratorT, T>,
315                                    WrappedIteratorT, T> {
316   mutable T Ptr;
317 
318 public:
319   pointer_iterator() = default;
320 
pointer_iterator(WrappedIteratorT u)321   explicit pointer_iterator(WrappedIteratorT u)
322       : pointer_iterator::iterator_adaptor_base(std::move(u)) {}
323 
324   T &operator*() { return Ptr = &*this->I; }
325   const T &operator*() const { return Ptr = &*this->I; }
326 };
327 
328 template <typename RangeT, typename WrappedIteratorT =
329                                decltype(std::begin(std::declval<RangeT>()))>
330 iterator_range<pointer_iterator<WrappedIteratorT>>
make_pointer_range(RangeT && Range)331 make_pointer_range(RangeT &&Range) {
332   using PointerIteratorT = pointer_iterator<WrappedIteratorT>;
333   return make_range(PointerIteratorT(std::begin(std::forward<RangeT>(Range))),
334                     PointerIteratorT(std::end(std::forward<RangeT>(Range))));
335 }
336 
337 } // end namespace llvm
338 
339 #endif // LLVM_ADT_ITERATOR_H
340