2 * Copyright 2013 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)) {
60 #include <type_traits>
62 #include <boost/operators.hpp>
67 namespace detail { struct NoneHelper {}; }
69 typedef int detail::NoneHelper::*None;
71 const None none = nullptr;
74 * gcc-4.7 warns about use of uninitialized memory around the use of storage_
75 * even though this is explicitly initialized at each point.
77 #if defined(__GNUC__) && !defined(__clang__)
78 # pragma GCC diagnostic push
79 # pragma GCC diagnostic ignored "-Wuninitialized"
80 # pragma GCC diagnostic ignored "-Wpragmas"
81 # pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
87 static_assert(!std::is_reference<Value>::value,
88 "Optional may not be used with reference types");
94 Optional(const Optional& src)
95 noexcept(std::is_nothrow_copy_constructible<Value>::value) {
98 construct(src.value());
104 Optional(Optional&& src)
105 noexcept(std::is_nothrow_move_constructible<Value>::value) {
107 if (src.hasValue()) {
108 construct(std::move(src.value()));
115 /* implicit */ Optional(const None&)
119 /* implicit */ Optional(Value&& newValue) {
120 construct(std::move(newValue));
123 /* implicit */ Optional(const Value& newValue) {
127 ~Optional() noexcept {
131 void assign(const None&) {
135 void assign(Optional&& src) {
136 if (src.hasValue()) {
137 assign(std::move(src.value()));
144 void assign(const Optional& src) {
145 if (src.hasValue()) {
152 void assign(Value&& newValue) {
154 value_ = std::move(newValue);
156 construct(std::move(newValue));
160 void assign(const Value& newValue) {
169 Optional& operator=(Arg&& arg) {
170 assign(std::forward<Arg>(arg));
174 Optional& operator=(Optional &&other)
175 noexcept (std::is_nothrow_move_assignable<Value>::value) {
177 assign(std::move(other));
181 Optional& operator=(const Optional &other)
182 noexcept (std::is_nothrow_copy_assignable<Value>::value) {
188 template<class... Args>
189 void emplace(Args&&... args) {
191 construct(std::forward<Args>(args)...);
201 const Value& value() const {
211 bool hasValue() const { return hasValue_; }
213 explicit operator bool() const {
217 const Value& operator*() const { return value(); }
218 Value& operator*() { return value(); }
220 const Value* operator->() const { return &value(); }
221 Value* operator->() { return &value(); }
224 template<class... Args>
225 void construct(Args&&... args) {
226 const void* ptr = &value_;
227 // for supporting const types
228 new(const_cast<void*>(ptr)) Value(std::forward<Args>(args)...);
233 union { Value value_; };
237 #if defined(__GNUC__) && !defined(__clang__)
238 #pragma GCC diagnostic pop
242 const T* get_pointer(const Optional<T>& opt) {
243 return opt ? &opt.value() : nullptr;
247 T* get_pointer(Optional<T>& opt) {
248 return opt ? &opt.value() : nullptr;
252 void swap(Optional<T>& a, Optional<T>& b) {
253 if (a.hasValue() && b.hasValue()) {
256 swap(a.value(), b.value());
257 } else if (a.hasValue() || b.hasValue()) {
258 std::swap(a, b); // fall back to default implementation if they're mixed.
263 class Opt = Optional<typename std::decay<T>::type>>
264 Opt make_optional(T&& v) {
265 return Opt(std::forward<T>(v));
269 bool operator< (const Optional<V>& a, const Optional<V>& b) {
270 if (a.hasValue() != b.hasValue()) { return a.hasValue() < b.hasValue(); }
271 if (a.hasValue()) { return a.value() < b.value(); }
276 bool operator==(const Optional<V>& a, const Optional<V>& b) {
277 if (a.hasValue() != b.hasValue()) { return false; }
278 if (a.hasValue()) { return a.value() == b.value(); }
283 bool operator<=(const Optional<V>& a, const Optional<V>& b) {
288 bool operator!=(const Optional<V>& a, const Optional<V>& b) {
293 bool operator>=(const Optional<V>& a, const Optional<V>& b) {
298 bool operator> (const Optional<V>& a, const Optional<V>& b) {
302 // To supress comparability of Optional<T> with T, despite implicit conversion.
303 template<class V> bool operator< (const Optional<V>&, const V& other) = delete;
304 template<class V> bool operator<=(const Optional<V>&, const V& other) = delete;
305 template<class V> bool operator==(const Optional<V>&, const V& other) = delete;
306 template<class V> bool operator!=(const Optional<V>&, const V& other) = delete;
307 template<class V> bool operator>=(const Optional<V>&, const V& other) = delete;
308 template<class V> bool operator> (const Optional<V>&, const V& other) = delete;
309 template<class V> bool operator< (const V& other, const Optional<V>&) = delete;
310 template<class V> bool operator<=(const V& other, const Optional<V>&) = delete;
311 template<class V> bool operator==(const V& other, const Optional<V>&) = delete;
312 template<class V> bool operator!=(const V& other, const Optional<V>&) = delete;
313 template<class V> bool operator>=(const V& other, const Optional<V>&) = delete;
314 template<class V> bool operator> (const V& other, const Optional<V>&) = delete;
318 #endif//FOLLY_OPTIONAL_H_