1 //===- llvm/Support/ErrorOr.h - Error Smart Pointer -------------*- 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 /// \file
10 ///
11 /// Provides ErrorOr<T> smart pointer.
12 ///
13 //===----------------------------------------------------------------------===//
14 
15 #ifndef LLVM_SUPPORT_ERROROR_H
16 #define LLVM_SUPPORT_ERROROR_H
17 
18 #include "llvm/Support/AlignOf.h"
19 #include <cassert>
20 #include <system_error>
21 #include <type_traits>
22 #include <utility>
23 
24 namespace llvm {
25 
26 /// Represents either an error or a value T.
27 ///
28 /// ErrorOr<T> is a pointer-like class that represents the result of an
29 /// operation. The result is either an error, or a value of type T. This is
30 /// designed to emulate the usage of returning a pointer where nullptr indicates
31 /// failure. However instead of just knowing that the operation failed, we also
32 /// have an error_code and optional user data that describes why it failed.
33 ///
34 /// It is used like the following.
35 /// \code
36 ///   ErrorOr<Buffer> getBuffer();
37 ///
38 ///   auto buffer = getBuffer();
39 ///   if (error_code ec = buffer.getError())
40 ///     return ec;
41 ///   buffer->write("adena");
42 /// \endcode
43 ///
44 ///
45 /// Implicit conversion to bool returns true if there is a usable value. The
46 /// unary * and -> operators provide pointer like access to the value. Accessing
47 /// the value when there is an error has undefined behavior.
48 ///
49 /// When T is a reference type the behavior is slightly different. The reference
50 /// is held in a std::reference_wrapper<std::remove_reference<T>::type>, and
51 /// there is special handling to make operator -> work as if T was not a
52 /// reference.
53 ///
54 /// T cannot be a rvalue reference.
55 template<class T>
56 class ErrorOr {
57   template <class OtherT> friend class ErrorOr;
58 
59   static constexpr bool isRef = std::is_reference_v<T>;
60 
61   using wrap = std::reference_wrapper<std::remove_reference_t<T>>;
62 
63 public:
64   using storage_type = std::conditional_t<isRef, wrap, T>;
65 
66 private:
67   using reference = std::remove_reference_t<T> &;
68   using const_reference = const std::remove_reference_t<T> &;
69   using pointer = std::remove_reference_t<T> *;
70   using const_pointer = const std::remove_reference_t<T> *;
71 
72 public:
73   template <class E>
74   ErrorOr(E ErrorCode,
75           std::enable_if_t<std::is_error_code_enum<E>::value ||
76                                std::is_error_condition_enum<E>::value,
77                            void *> = nullptr)
78       : HasError(true) {
79     new (getErrorStorage()) std::error_code(make_error_code(ErrorCode));
80   }
81 
82   ErrorOr(std::error_code EC) : HasError(true) {
83     new (getErrorStorage()) std::error_code(EC);
84   }
85 
86   template <class OtherT>
87   ErrorOr(OtherT &&Val,
88           std::enable_if_t<std::is_convertible_v<OtherT, T>> * = nullptr)
89       : HasError(false) {
90     new (getStorage()) storage_type(std::forward<OtherT>(Val));
91   }
92 
93   ErrorOr(const ErrorOr &Other) {
94     copyConstruct(Other);
95   }
96 
97   template <class OtherT>
98   ErrorOr(const ErrorOr<OtherT> &Other,
99           std::enable_if_t<std::is_convertible_v<OtherT, T>> * = nullptr) {
100     copyConstruct(Other);
101   }
102 
103   template <class OtherT>
104   explicit ErrorOr(
105       const ErrorOr<OtherT> &Other,
106       std::enable_if_t<!std::is_convertible_v<OtherT, const T &>> * = nullptr) {
107     copyConstruct(Other);
108   }
109 
110   ErrorOr(ErrorOr &&Other) {
111     moveConstruct(std::move(Other));
112   }
113 
114   template <class OtherT>
115   ErrorOr(ErrorOr<OtherT> &&Other,
116           std::enable_if_t<std::is_convertible_v<OtherT, T>> * = nullptr) {
117     moveConstruct(std::move(Other));
118   }
119 
120   // This might eventually need SFINAE but it's more complex than is_convertible
121   // & I'm too lazy to write it right now.
122   template <class OtherT>
123   explicit ErrorOr(
124       ErrorOr<OtherT> &&Other,
125       std::enable_if_t<!std::is_convertible_v<OtherT, T>> * = nullptr) {
126     moveConstruct(std::move(Other));
127   }
128 
129   ErrorOr &operator=(const ErrorOr &Other) {
130     copyAssign(Other);
131     return *this;
132   }
133 
134   ErrorOr &operator=(ErrorOr &&Other) {
135     moveAssign(std::move(Other));
136     return *this;
137   }
138 
139   ~ErrorOr() {
140     if (!HasError)
141       getStorage()->~storage_type();
142   }
143 
144   /// Return false if there is an error.
145   explicit operator bool() const {
146     return !HasError;
147   }
148 
149   reference get() { return *getStorage(); }
150   const_reference get() const { return const_cast<ErrorOr<T> *>(this)->get(); }
151 
152   std::error_code getError() const {
153     return HasError ? *getErrorStorage() : std::error_code();
154   }
155 
156   pointer operator ->() {
157     return toPointer(getStorage());
158   }
159 
160   const_pointer operator->() const { return toPointer(getStorage()); }
161 
162   reference operator *() {
163     return *getStorage();
164   }
165 
166   const_reference operator*() const { return *getStorage(); }
167 
168 private:
169   template <class OtherT>
170   void copyConstruct(const ErrorOr<OtherT> &Other) {
171     if (!Other.HasError) {
172       // Get the other value.
173       HasError = false;
174       new (getStorage()) storage_type(*Other.getStorage());
175     } else {
176       // Get other's error.
177       HasError = true;
178       new (getErrorStorage()) std::error_code(Other.getError());
179     }
180   }
181 
182   template <class T1>
183   static bool compareThisIfSameType(const T1 &a, const T1 &b) {
184     return &a == &b;
185   }
186 
187   template <class T1, class T2>
188   static bool compareThisIfSameType(const T1 &a, const T2 &b) {
189     return false;
190   }
191 
192   template <class OtherT>
193   void copyAssign(const ErrorOr<OtherT> &Other) {
194     if (compareThisIfSameType(*this, Other))
195       return;
196 
197     this->~ErrorOr();
198     new (this) ErrorOr(Other);
199   }
200 
201   template <class OtherT>
202   void moveConstruct(ErrorOr<OtherT> &&Other) {
203     if (!Other.HasError) {
204       // Get the other value.
205       HasError = false;
206       new (getStorage()) storage_type(std::move(*Other.getStorage()));
207     } else {
208       // Get other's error.
209       HasError = true;
210       new (getErrorStorage()) std::error_code(Other.getError());
211     }
212   }
213 
214   template <class OtherT>
215   void moveAssign(ErrorOr<OtherT> &&Other) {
216     if (compareThisIfSameType(*this, Other))
217       return;
218 
219     this->~ErrorOr();
220     new (this) ErrorOr(std::move(Other));
221   }
222 
223   pointer toPointer(pointer Val) {
224     return Val;
225   }
226 
227   const_pointer toPointer(const_pointer Val) const { return Val; }
228 
229   pointer toPointer(wrap *Val) {
230     return &Val->get();
231   }
232 
233   const_pointer toPointer(const wrap *Val) const { return &Val->get(); }
234 
235   storage_type *getStorage() {
236     assert(!HasError && "Cannot get value when an error exists!");
237     return reinterpret_cast<storage_type *>(&TStorage);
238   }
239 
240   const storage_type *getStorage() const {
241     assert(!HasError && "Cannot get value when an error exists!");
242     return reinterpret_cast<const storage_type *>(&TStorage);
243   }
244 
245   std::error_code *getErrorStorage() {
246     assert(HasError && "Cannot get error when a value exists!");
247     return reinterpret_cast<std::error_code *>(&ErrorStorage);
248   }
249 
250   const std::error_code *getErrorStorage() const {
251     return const_cast<ErrorOr<T> *>(this)->getErrorStorage();
252   }
253 
254   union {
255     AlignedCharArrayUnion<storage_type> TStorage;
256     AlignedCharArrayUnion<std::error_code> ErrorStorage;
257   };
258   bool HasError : 1;
259 };
260 
261 template <class T, class E>
262 std::enable_if_t<std::is_error_code_enum<E>::value ||
263                      std::is_error_condition_enum<E>::value,
264                  bool>
265 operator==(const ErrorOr<T> &Err, E Code) {
266   return Err.getError() == Code;
267 }
268 
269 } // end namespace llvm
270 
271 #endif // LLVM_SUPPORT_ERROROR_H
272