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fallible_iterator.h
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1 //===--- fallible_iterator.h - Wrapper for fallible iterators ---*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #ifndef LLVM_ADT_FALLIBLE_ITERATOR_H
10 #define LLVM_ADT_FALLIBLE_ITERATOR_H
11 
14 #include "llvm/Support/Error.h"
15 
16 #include <type_traits>
17 
18 namespace llvm {
19 
20 /// A wrapper class for fallible iterators.
21 ///
22 /// The fallible_iterator template wraps an underlying iterator-like class
23 /// whose increment and decrement operations are replaced with fallible versions
24 /// like:
25 ///
26 /// @code{.cpp}
27 /// Error inc();
28 /// Error dec();
29 /// @endcode
30 ///
31 /// It produces an interface that is (mostly) compatible with a traditional
32 /// c++ iterator, including ++ and -- operators that do not fail.
33 ///
34 /// Instances of the wrapper are constructed with an instance of the
35 /// underlying iterator and (for non-end iterators) a reference to an Error
36 /// instance. If the underlying increment/decrement operations fail, the Error
37 /// is returned via this reference, and the resulting iterator value set to an
38 /// end-of-range sentinel value. This enables the following loop idiom:
39 ///
40 /// @code{.cpp}
41 /// class Archive { // E.g. Potentially malformed on-disk archive
42 /// public:
43 /// fallible_iterator<ArchiveChildItr> children_begin(Error &Err);
44 /// fallible_iterator<ArchiveChildItr> children_end();
45 /// iterator_range<fallible_iterator<ArchiveChildItr>>
46 /// children(Error &Err) {
47 /// return make_range(children_begin(Err), children_end());
48 /// //...
49 /// };
50 ///
51 /// void walk(Archive &A) {
52 /// Error Err = Error::success();
53 /// for (auto &C : A.children(Err)) {
54 /// // Loop body only entered when increment succeeds.
55 /// }
56 /// if (Err) {
57 /// // handle error.
58 /// }
59 /// }
60 /// @endcode
61 ///
62 /// The wrapper marks the referenced Error as unchecked after each increment
63 /// and/or decrement operation, and clears the unchecked flag when a non-end
64 /// value is compared against end (since, by the increment invariant, not being
65 /// an end value proves that there was no error, and is equivalent to checking
66 /// that the Error is success). This allows early exits from the loop body
67 /// without requiring redundant error checks.
68 template <typename Underlying> class fallible_iterator {
69 private:
70  template <typename T>
71  using enable_if_struct_deref_supported = std::enable_if<
72  !std::is_void<decltype(std::declval<T>().operator->())>::value,
73  decltype(std::declval<T>().operator->())>;
74 
75 public:
76  /// Construct a fallible iterator that *cannot* be used as an end-of-range
77  /// value.
78  ///
79  /// A value created by this method can be dereferenced, incremented,
80  /// decremented and compared, providing the underlying type supports it.
81  ///
82  /// The error that is passed in will be initially marked as checked, so if the
83  /// iterator is not used at all the Error need not be checked.
85  (void)!!Err;
86  return fallible_iterator(std::move(I), &Err);
87  }
88 
89  /// Construct a fallible iteratro that can be used as an end-of-range value.
90  ///
91  /// A value created by this method can be dereferenced (if the underlying
92  /// value points at a valid value) and compared, but not incremented or
93  /// decremented.
95  return fallible_iterator(std::move(I), nullptr);
96  }
97 
98  /// Forward dereference to the underlying iterator.
99  auto operator*() -> decltype(*std::declval<Underlying>()) { return *I; }
100 
101  /// Forward const dereference to the underlying iterator.
102  auto operator*() const -> decltype(*std::declval<const Underlying>()) {
103  return *I;
104  }
105 
106  /// Forward structure dereference to the underlying iterator (if the
107  /// underlying iterator supports it).
108  template <typename T = Underlying>
109  typename enable_if_struct_deref_supported<T>::type operator->() {
110  return I.operator->();
111  }
112 
113  /// Forward const structure dereference to the underlying iterator (if the
114  /// underlying iterator supports it).
115  template <typename T = Underlying>
116  typename enable_if_struct_deref_supported<const T>::type operator->() const {
117  return I.operator->();
118  }
119 
120  /// Increment the fallible iterator.
121  ///
122  /// If the underlying 'inc' operation fails, this will set the Error value
123  /// and update this iterator value to point to end-of-range.
124  ///
125  /// The Error value is marked as needing checking, regardless of whether the
126  /// 'inc' operation succeeds or fails.
128  assert(getErrPtr() && "Cannot increment end iterator");
129  if (auto Err = I.inc())
130  handleError(std::move(Err));
131  else
132  resetCheckedFlag();
133  return *this;
134  }
135 
136  /// Decrement the fallible iterator.
137  ///
138  /// If the underlying 'dec' operation fails, this will set the Error value
139  /// and update this iterator value to point to end-of-range.
140  ///
141  /// The Error value is marked as needing checking, regardless of whether the
142  /// 'dec' operation succeeds or fails.
144  assert(getErrPtr() && "Cannot decrement end iterator");
145  if (auto Err = I.dec())
146  handleError(std::move(Err));
147  else
148  resetCheckedFlag();
149  return *this;
150  }
151 
152  /// Compare fallible iterators for equality.
153  ///
154  /// Returns true if both LHS and RHS are end-of-range values, or if both are
155  /// non-end-of-range values whose underlying iterator values compare equal.
156  ///
157  /// If this is a comparison between an end-of-range iterator and a
158  /// non-end-of-range iterator, then the Error (referenced by the
159  /// non-end-of-range value) is marked as checked: Since all
160  /// increment/decrement operations result in an end-of-range value, comparing
161  /// false against end-of-range is equivalent to checking that the Error value
162  /// is success. This flag management enables early returns from loop bodies
163  /// without redundant Error checks.
164  friend bool operator==(const fallible_iterator &LHS,
165  const fallible_iterator &RHS) {
166  // If both iterators are in the end state they compare
167  // equal, regardless of whether either is valid.
168  if (LHS.isEnd() && RHS.isEnd())
169  return true;
170 
171  assert(LHS.isValid() && RHS.isValid() &&
172  "Invalid iterators can only be compared against end");
173 
174  bool Equal = LHS.I == RHS.I;
175 
176  // If the iterators differ and this is a comparison against end then mark
177  // the Error as checked.
178  if (!Equal) {
179  if (LHS.isEnd())
180  (void)!!*RHS.getErrPtr();
181  else
182  (void)!!*LHS.getErrPtr();
183  }
184 
185  return Equal;
186  }
187 
188  /// Compare fallible iterators for inequality.
189  ///
190  /// See notes for operator==.
191  friend bool operator!=(const fallible_iterator &LHS,
192  const fallible_iterator &RHS) {
193  return !(LHS == RHS);
194  }
195 
196 private:
198  : I(std::move(I)), ErrState(Err, false) {}
199 
200  Error *getErrPtr() const { return ErrState.getPointer(); }
201 
202  bool isEnd() const { return getErrPtr() == nullptr; }
203 
204  bool isValid() const { return !ErrState.getInt(); }
205 
206  void handleError(Error Err) {
207  *getErrPtr() = std::move(Err);
208  ErrState.setPointer(nullptr);
209  ErrState.setInt(true);
210  }
211 
212  void resetCheckedFlag() {
213  *getErrPtr() = Error::success();
214  }
215 
216  Underlying I;
217  mutable PointerIntPair<Error *, 1> ErrState;
218 };
219 
220 /// Convenience wrapper to make a fallible_iterator value from an instance
221 /// of an underlying iterator and an Error reference.
222 template <typename Underlying>
224  return fallible_iterator<Underlying>::itr(std::move(I), Err);
225 }
226 
227 /// Convenience wrapper to make a fallible_iterator end value from an instance
228 /// of an underlying iterator.
229 template <typename Underlying>
231  return fallible_iterator<Underlying>::end(std::move(E));
232 }
233 
234 template <typename Underlying>
237  return make_range(make_fallible_itr(std::move(I), Err),
238  make_fallible_end(std::move(E)));
239 }
240 
241 } // end namespace llvm
242 
243 #endif // LLVM_ADT_FALLIBLE_ITERATOR_H
static fallible_iterator itr(Underlying I, Error &Err)
Construct a fallible iterator that cannot be used as an end-of-range value.
auto operator*() const -> decltype(*std::declval< const Underlying >())
Forward const dereference to the underlying iterator.
This class represents lattice values for constants.
Definition: AllocatorList.h:23
fallible_iterator & operator++()
Increment the fallible iterator.
auto operator*() -> decltype(*std::declval< Underlying >())
Forward dereference to the underlying iterator.
This provides a very simple, boring adaptor for a begin and end iterator into a range type...
static fallible_iterator end(Underlying I)
Construct a fallible iteratro that can be used as an end-of-range value.
Definition: BitVector.h:937
enable_if_struct_deref_supported< const T >::type operator->() const
Forward const structure dereference to the underlying iterator (if the underlying iterator supports i...
friend bool operator==(const fallible_iterator &LHS, const fallible_iterator &RHS)
Compare fallible iterators for equality.
fallible_iterator & operator--()
Decrement the fallible iterator.
friend bool operator!=(const fallible_iterator &LHS, const fallible_iterator &RHS)
Compare fallible iterators for inequality.
PointerIntPair - This class implements a pair of a pointer and small integer.
static GCRegistry::Add< CoreCLRGC > E("coreclr", "CoreCLR-compatible GC")
A wrapper class for fallible iterators.
std::underlying_type< E >::type Underlying(E Val)
Check that Val is in range for E, and return Val cast to E&#39;s underlying type.
Definition: BitmaskEnum.h:90
iterator_range< fallible_iterator< Underlying > > make_fallible_range(Underlying I, Underlying E, Error &Err)
fallible_iterator< Underlying > make_fallible_end(Underlying E)
Convenience wrapper to make a fallible_iterator end value from an instance of an underlying iterator...
fallible_iterator< Underlying > make_fallible_itr(Underlying I, Error &Err)
Convenience wrapper to make a fallible_iterator value from an instance of an underlying iterator and ...
iterator_range< T > make_range(T x, T y)
Convenience function for iterating over sub-ranges.
static ErrorSuccess success()
Create a success value.
Definition: Error.h:326
A range adaptor for a pair of iterators.
enable_if_struct_deref_supported< T >::type operator->()
Forward structure dereference to the underlying iterator (if the underlying iterator supports it)...
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
aarch64 promote const
Lightweight error class with error context and mandatory checking.
Definition: Error.h:157