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"
21 #include "llvm/Support/system_error.h"
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 using std::make_error_code;
103 new (getErrorStorage()) error_code(make_error_code(ErrorCode));
106 ErrorOr(std::error_code EC) : HasError(true) {
107 new (getErrorStorage()) error_code(EC);
110 ErrorOr(T Val) : HasError(false) {
111 new (getStorage()) storage_type(moveIfMoveConstructible<storage_type>(Val));
114 ErrorOr(const ErrorOr &Other) {
115 copyConstruct(Other);
118 template <class OtherT>
119 ErrorOr(const ErrorOr<OtherT> &Other) {
120 copyConstruct(Other);
123 ErrorOr &operator =(const ErrorOr &Other) {
128 template <class OtherT>
129 ErrorOr &operator =(const ErrorOr<OtherT> &Other) {
134 ErrorOr(ErrorOr &&Other) {
135 moveConstruct(std::move(Other));
138 template <class OtherT>
139 ErrorOr(ErrorOr<OtherT> &&Other) {
140 moveConstruct(std::move(Other));
143 ErrorOr &operator =(ErrorOr &&Other) {
144 moveAssign(std::move(Other));
148 template <class OtherT>
149 ErrorOr &operator =(ErrorOr<OtherT> &&Other) {
150 moveAssign(std::move(Other));
156 getStorage()->~storage_type();
159 /// \brief Return false if there is an error.
160 LLVM_EXPLICIT operator bool() const {
164 reference get() { return *getStorage(); }
165 const_reference get() const { return const_cast<ErrorOr<T> >(this)->get(); }
167 error_code getError() const {
168 return HasError ? *getErrorStorage() : error_code();
171 pointer operator ->() {
172 return toPointer(getStorage());
175 reference operator *() {
176 return *getStorage();
180 template <class OtherT>
181 void copyConstruct(const ErrorOr<OtherT> &Other) {
182 if (!Other.HasError) {
183 // Get the other value.
185 new (getStorage()) storage_type(*Other.getStorage());
187 // Get other's error.
189 new (getErrorStorage()) error_code(Other.getError());
194 static bool compareThisIfSameType(const T1 &a, const T1 &b) {
198 template <class T1, class T2>
199 static bool compareThisIfSameType(const T1 &a, const T2 &b) {
203 template <class OtherT>
204 void copyAssign(const ErrorOr<OtherT> &Other) {
205 if (compareThisIfSameType(*this, Other))
209 new (this) ErrorOr(Other);
212 template <class OtherT>
213 void moveConstruct(ErrorOr<OtherT> &&Other) {
214 if (!Other.HasError) {
215 // Get the other value.
217 new (getStorage()) storage_type(std::move(*Other.getStorage()));
219 // Get other's error.
221 new (getErrorStorage()) error_code(Other.getError());
225 template <class OtherT>
226 void moveAssign(ErrorOr<OtherT> &&Other) {
227 if (compareThisIfSameType(*this, Other))
231 new (this) ErrorOr(std::move(Other));
234 pointer toPointer(pointer Val) {
238 pointer toPointer(wrap *Val) {
242 storage_type *getStorage() {
243 assert(!HasError && "Cannot get value when an error exists!");
244 return reinterpret_cast<storage_type*>(TStorage.buffer);
247 const storage_type *getStorage() const {
248 assert(!HasError && "Cannot get value when an error exists!");
249 return reinterpret_cast<const storage_type*>(TStorage.buffer);
252 error_code *getErrorStorage() {
253 assert(HasError && "Cannot get error when a value exists!");
254 return reinterpret_cast<error_code*>(ErrorStorage.buffer);
257 const error_code *getErrorStorage() const {
258 return const_cast<ErrorOr<T> *>(this)->getErrorStorage();
263 AlignedCharArrayUnion<storage_type> TStorage;
264 AlignedCharArrayUnion<error_code> ErrorStorage;
269 template <class T, class E>
270 typename std::enable_if<std::is_error_code_enum<E>::value ||
271 std::is_error_condition_enum<E>::value,
273 operator==(ErrorOr<T> &Err, E Code) {
274 return error_code(Err) == Code;
276 } // end namespace llvm