1 //===- llvm/Support/ErrorOr.h - Error Smart Pointer -----------------------===//
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
12 /// Provides ErrorOr<T> smart pointer.
14 //===----------------------------------------------------------------------===//
16 #ifndef LLVM_SUPPORT_ERROR_OR_H
17 #define LLVM_SUPPORT_ERROR_OR_H
19 #include "llvm/ADT/PointerIntPair.h"
20 #include "llvm/Support/AlignOf.h"
22 #include <system_error>
23 #include <type_traits>
26 template<class T, class V>
27 typename std::enable_if< std::is_constructible<T, V>::value
28 , typename std::remove_reference<V>::type>::type &&
29 moveIfMoveConstructible(V &Val) {
30 return std::move(Val);
33 template<class T, class V>
34 typename std::enable_if< !std::is_constructible<T, V>::value
35 , typename std::remove_reference<V>::type>::type &
36 moveIfMoveConstructible(V &Val) {
40 /// \brief Stores a reference that can be changed.
42 class ReferenceStorage {
46 ReferenceStorage(T &Ref) : Storage(&Ref) {}
48 operator T &() const { return *Storage; }
49 T &get() const { return *Storage; }
52 /// \brief Represents either an error or a value T.
54 /// ErrorOr<T> is a pointer-like class that represents the result of an
55 /// operation. The result is either an error, or a value of type T. This is
56 /// designed to emulate the usage of returning a pointer where nullptr indicates
57 /// failure. However instead of just knowing that the operation failed, we also
58 /// have an error_code and optional user data that describes why it failed.
60 /// It is used like the following.
62 /// ErrorOr<Buffer> getBuffer();
64 /// auto buffer = getBuffer();
65 /// if (error_code ec = buffer.getError())
67 /// buffer->write("adena");
71 /// An implicit conversion to bool provides a way to check if there was an
72 /// error. The unary * and -> operators provide pointer like access to the
73 /// value. Accessing the value when there is an error has undefined behavior.
75 /// When T is a reference type the behaivor is slightly different. The reference
76 /// is held in a std::reference_wrapper<std::remove_reference<T>::type>, and
77 /// there is special handling to make operator -> work as if T was not a
80 /// T cannot be a rvalue reference.
83 template <class OtherT> friend class ErrorOr;
84 static const bool isRef = std::is_reference<T>::value;
85 typedef ReferenceStorage<typename std::remove_reference<T>::type> wrap;
88 typedef typename std::conditional<isRef, wrap, T>::type storage_type;
91 typedef typename std::remove_reference<T>::type &reference;
92 typedef const typename std::remove_reference<T>::type &const_reference;
93 typedef typename std::remove_reference<T>::type *pointer;
98 typename std::enable_if<std::is_error_code_enum<E>::value ||
99 std::is_error_condition_enum<E>::value,
102 new (getErrorStorage()) std::error_code(make_error_code(ErrorCode));
105 ErrorOr(std::error_code EC) : HasError(true) {
106 new (getErrorStorage()) std::error_code(EC);
109 ErrorOr(T Val) : HasError(false) {
110 new (getStorage()) storage_type(moveIfMoveConstructible<storage_type>(Val));
113 ErrorOr(const ErrorOr &Other) {
114 copyConstruct(Other);
117 template <class OtherT>
118 ErrorOr(const ErrorOr<OtherT> &Other) {
119 copyConstruct(Other);
122 ErrorOr &operator =(const ErrorOr &Other) {
127 template <class OtherT>
128 ErrorOr &operator =(const ErrorOr<OtherT> &Other) {
133 ErrorOr(ErrorOr &&Other) {
134 moveConstruct(std::move(Other));
137 template <class OtherT>
138 ErrorOr(ErrorOr<OtherT> &&Other) {
139 moveConstruct(std::move(Other));
142 ErrorOr &operator =(ErrorOr &&Other) {
143 moveAssign(std::move(Other));
147 template <class OtherT>
148 ErrorOr &operator =(ErrorOr<OtherT> &&Other) {
149 moveAssign(std::move(Other));
155 getStorage()->~storage_type();
158 /// \brief Return false if there is an error.
159 LLVM_EXPLICIT operator bool() const {
163 reference get() { return *getStorage(); }
164 const_reference get() const { return const_cast<ErrorOr<T> >(this)->get(); }
166 std::error_code getError() const {
167 return HasError ? *getErrorStorage() : std::error_code();
170 pointer operator ->() {
171 return toPointer(getStorage());
174 reference operator *() {
175 return *getStorage();
179 template <class OtherT>
180 void copyConstruct(const ErrorOr<OtherT> &Other) {
181 if (!Other.HasError) {
182 // Get the other value.
184 new (getStorage()) storage_type(*Other.getStorage());
186 // Get other's error.
188 new (getErrorStorage()) std::error_code(Other.getError());
193 static bool compareThisIfSameType(const T1 &a, const T1 &b) {
197 template <class T1, class T2>
198 static bool compareThisIfSameType(const T1 &a, const T2 &b) {
202 template <class OtherT>
203 void copyAssign(const ErrorOr<OtherT> &Other) {
204 if (compareThisIfSameType(*this, Other))
208 new (this) ErrorOr(Other);
211 template <class OtherT>
212 void moveConstruct(ErrorOr<OtherT> &&Other) {
213 if (!Other.HasError) {
214 // Get the other value.
216 new (getStorage()) storage_type(std::move(*Other.getStorage()));
218 // Get other's error.
220 new (getErrorStorage()) std::error_code(Other.getError());
224 template <class OtherT>
225 void moveAssign(ErrorOr<OtherT> &&Other) {
226 if (compareThisIfSameType(*this, Other))
230 new (this) ErrorOr(std::move(Other));
233 pointer toPointer(pointer Val) {
237 pointer toPointer(wrap *Val) {
241 storage_type *getStorage() {
242 assert(!HasError && "Cannot get value when an error exists!");
243 return reinterpret_cast<storage_type*>(TStorage.buffer);
246 const storage_type *getStorage() const {
247 assert(!HasError && "Cannot get value when an error exists!");
248 return reinterpret_cast<const storage_type*>(TStorage.buffer);
251 std::error_code *getErrorStorage() {
252 assert(HasError && "Cannot get error when a value exists!");
253 return reinterpret_cast<std::error_code *>(ErrorStorage.buffer);
256 const std::error_code *getErrorStorage() const {
257 return const_cast<ErrorOr<T> *>(this)->getErrorStorage();
262 AlignedCharArrayUnion<storage_type> TStorage;
263 AlignedCharArrayUnion<std::error_code> ErrorStorage;
268 template <class T, class E>
269 typename std::enable_if<std::is_error_code_enum<E>::value ||
270 std::is_error_condition_enum<E>::value,
272 operator==(ErrorOr<T> &Err, E Code) {
273 return std::error_code(Err) == Code;
275 } // end namespace llvm