2 * Copyright 2015 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
17 #ifndef FOLLY_OPTIONAL_H_
18 #define FOLLY_OPTIONAL_H_
21 * Optional - For conditional initialization of values, like boost::optional,
22 * but with support for move semantics and emplacement. Reference type support
23 * has not been included due to limited use cases and potential confusion with
24 * semantics of assignment: Assigning to an optional reference could quite
25 * reasonably copy its value or redirect the reference.
27 * Optional can be useful when a variable might or might not be needed:
29 * Optional<Logger> maybeLogger = ...;
31 * maybeLogger->log("hello");
34 * Optional enables a 'null' value for types which do not otherwise have
35 * nullability, especially useful for parameter passing:
37 * void testIterator(const unique_ptr<Iterator>& it,
38 * initializer_list<int> idsExpected,
39 * Optional<initializer_list<int>> ranksExpected = none) {
40 * for (int i = 0; it->next(); ++i) {
41 * EXPECT_EQ(it->doc().id(), idsExpected[i]);
42 * if (ranksExpected) {
43 * EXPECT_EQ(it->doc().rank(), (*ranksExpected)[i]);
48 * Optional models OptionalPointee, so calling 'get_pointer(opt)' will return a
49 * pointer to nullptr if the 'opt' is empty, and a pointer to the value if it is
52 * Optional<int> maybeInt = ...;
53 * if (int* v = get_pointer(maybeInt)) {
59 #include <type_traits>
62 #include <boost/operators.hpp>
64 #include <folly/Portability.h>
68 namespace detail { struct NoneHelper {}; }
70 typedef int detail::NoneHelper::*None;
72 const None none = nullptr;
75 * gcc-4.7 warns about use of uninitialized memory around the use of storage_
76 * even though this is explicitly initialized at each point.
78 #if defined(__GNUC__) && !defined(__clang__)
79 # pragma GCC diagnostic push
80 # pragma GCC diagnostic ignored "-Wuninitialized"
81 # pragma GCC diagnostic ignored "-Wpragmas"
82 # pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
88 static_assert(!std::is_reference<Value>::value,
89 "Optional may not be used with reference types");
90 static_assert(!std::is_abstract<Value>::value,
91 "Optional may not be used with abstract types");
97 Optional(const Optional& src)
98 noexcept(std::is_nothrow_copy_constructible<Value>::value) {
100 if (src.hasValue()) {
101 construct(src.value());
107 Optional(Optional&& src)
108 noexcept(std::is_nothrow_move_constructible<Value>::value) {
110 if (src.hasValue()) {
111 construct(std::move(src.value()));
118 /* implicit */ Optional(const None&) noexcept
122 /* implicit */ Optional(Value&& newValue)
123 noexcept(std::is_nothrow_move_constructible<Value>::value) {
124 construct(std::move(newValue));
127 /* implicit */ Optional(const Value& newValue)
128 noexcept(std::is_nothrow_copy_constructible<Value>::value) {
132 ~Optional() noexcept {
136 void assign(const None&) {
140 void assign(Optional&& src) {
142 if (src.hasValue()) {
143 assign(std::move(src.value()));
151 void assign(const Optional& src) {
152 if (src.hasValue()) {
159 void assign(Value&& newValue) {
161 value_ = std::move(newValue);
163 construct(std::move(newValue));
167 void assign(const Value& newValue) {
176 Optional& operator=(Arg&& arg) {
177 assign(std::forward<Arg>(arg));
181 Optional& operator=(Optional &&other)
182 noexcept (std::is_nothrow_move_assignable<Value>::value) {
184 assign(std::move(other));
188 Optional& operator=(const Optional &other)
189 noexcept (std::is_nothrow_copy_assignable<Value>::value) {
195 template<class... Args>
196 void emplace(Args&&... args) {
198 construct(std::forward<Args>(args)...);
208 const Value& value() const {
218 bool hasValue() const { return hasValue_; }
220 explicit operator bool() const {
224 const Value& operator*() const { return value(); }
225 Value& operator*() { return value(); }
227 const Value* operator->() const { return &value(); }
228 Value* operator->() { return &value(); }
230 // Return a copy of the value if set, or a given default if not.
232 Value value_or(U&& dflt) const& {
233 return hasValue_ ? value_ : std::forward<U>(dflt);
237 Value value_or(U&& dflt) && {
238 return hasValue_ ? std::move(value_) : std::forward<U>(dflt);
242 template<class... Args>
243 void construct(Args&&... args) {
244 const void* ptr = &value_;
245 // for supporting const types
246 new(const_cast<void*>(ptr)) Value(std::forward<Args>(args)...);
251 union { Value value_; };
255 #if defined(__GNUC__) && !defined(__clang__)
256 #pragma GCC diagnostic pop
260 const T* get_pointer(const Optional<T>& opt) {
261 return opt ? &opt.value() : nullptr;
265 T* get_pointer(Optional<T>& opt) {
266 return opt ? &opt.value() : nullptr;
270 void swap(Optional<T>& a, Optional<T>& b) {
271 if (a.hasValue() && b.hasValue()) {
274 swap(a.value(), b.value());
275 } else if (a.hasValue() || b.hasValue()) {
276 std::swap(a, b); // fall back to default implementation if they're mixed.
281 class Opt = Optional<typename std::decay<T>::type>>
282 Opt make_optional(T&& v) {
283 return Opt(std::forward<T>(v));
286 ///////////////////////////////////////////////////////////////////////////////
290 bool operator==(const Optional<V>& a, const V& b) {
291 return a.hasValue() && a.value() == b;
295 bool operator!=(const Optional<V>& a, const V& b) {
300 bool operator==(const V& a, const Optional<V>& b) {
301 return b.hasValue() && b.value() == a;
305 bool operator!=(const V& a, const Optional<V>& b) {
310 bool operator==(const Optional<V>& a, const Optional<V>& b) {
311 if (a.hasValue() != b.hasValue()) { return false; }
312 if (a.hasValue()) { return a.value() == b.value(); }
317 bool operator!=(const Optional<V>& a, const Optional<V>& b) {
322 bool operator< (const Optional<V>& a, const Optional<V>& b) {
323 if (a.hasValue() != b.hasValue()) { return a.hasValue() < b.hasValue(); }
324 if (a.hasValue()) { return a.value() < b.value(); }
329 bool operator> (const Optional<V>& a, const Optional<V>& b) {
334 bool operator<=(const Optional<V>& a, const Optional<V>& b) {
339 bool operator>=(const Optional<V>& a, const Optional<V>& b) {
343 // Suppress comparability of Optional<T> with T, despite implicit conversion.
344 template<class V> bool operator< (const Optional<V>&, const V& other) = delete;
345 template<class V> bool operator<=(const Optional<V>&, const V& other) = delete;
346 template<class V> bool operator>=(const Optional<V>&, const V& other) = delete;
347 template<class V> bool operator> (const Optional<V>&, const V& other) = delete;
348 template<class V> bool operator< (const V& other, const Optional<V>&) = delete;
349 template<class V> bool operator<=(const V& other, const Optional<V>&) = delete;
350 template<class V> bool operator>=(const V& other, const Optional<V>&) = delete;
351 template<class V> bool operator> (const V& other, const Optional<V>&) = delete;
353 ///////////////////////////////////////////////////////////////////////////////
357 #endif // FOLLY_OPTIONAL_H_