2 * Copyright 2016 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>
64 namespace detail { struct NoneHelper {}; }
66 typedef int detail::NoneHelper::*None;
68 const None none = nullptr;
71 * gcc-4.7 warns about use of uninitialized memory around the use of storage_
72 * even though this is explicitly initialized at each point.
74 #if defined(__GNUC__) && !defined(__clang__)
75 # pragma GCC diagnostic push
76 # pragma GCC diagnostic ignored "-Wuninitialized"
77 # pragma GCC diagnostic ignored "-Wpragmas"
78 # pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
81 class OptionalEmptyException : public std::runtime_error {
83 OptionalEmptyException()
84 : std::runtime_error("Empty Optional cannot be unwrapped") {}
90 typedef Value value_type;
92 static_assert(!std::is_reference<Value>::value,
93 "Optional may not be used with reference types");
94 static_assert(!std::is_abstract<Value>::value,
95 "Optional may not be used with abstract types");
101 Optional(const Optional& src)
102 noexcept(std::is_nothrow_copy_constructible<Value>::value) {
104 if (src.hasValue()) {
105 construct(src.value());
111 Optional(Optional&& src)
112 noexcept(std::is_nothrow_move_constructible<Value>::value) {
114 if (src.hasValue()) {
115 construct(std::move(src.value()));
122 /* implicit */ Optional(const None&) noexcept
126 /* implicit */ Optional(Value&& newValue)
127 noexcept(std::is_nothrow_move_constructible<Value>::value) {
128 construct(std::move(newValue));
131 /* implicit */ Optional(const Value& newValue)
132 noexcept(std::is_nothrow_copy_constructible<Value>::value) {
136 ~Optional() noexcept {
140 void assign(const None&) {
144 void assign(Optional&& src) {
146 if (src.hasValue()) {
147 assign(std::move(src.value()));
155 void assign(const Optional& src) {
156 if (src.hasValue()) {
163 void assign(Value&& newValue) {
165 value_ = std::move(newValue);
167 construct(std::move(newValue));
171 void assign(const Value& newValue) {
180 Optional& operator=(Arg&& arg) {
181 assign(std::forward<Arg>(arg));
185 Optional& operator=(Optional &&other)
186 noexcept (std::is_nothrow_move_assignable<Value>::value) {
188 assign(std::move(other));
192 Optional& operator=(const Optional &other)
193 noexcept (std::is_nothrow_copy_assignable<Value>::value) {
199 template<class... Args>
200 void emplace(Args&&... args) {
202 construct(std::forward<Args>(args)...);
212 const Value& value() const& {
224 return std::move(value_);
227 const Value* get_pointer() const& { return hasValue_ ? &value_ : nullptr; }
228 Value* get_pointer() & { return hasValue_ ? &value_ : nullptr; }
229 Value* get_pointer() && = delete;
231 bool hasValue() const { return hasValue_; }
233 explicit operator bool() const {
237 const Value& operator*() const& { return value(); }
238 Value& operator*() & { return value(); }
239 Value operator*() && { return std::move(value()); }
241 const Value* operator->() const { return &value(); }
242 Value* operator->() { return &value(); }
244 // Return a copy of the value if set, or a given default if not.
246 Value value_or(U&& dflt) const& {
247 return hasValue_ ? value_ : std::forward<U>(dflt);
251 Value value_or(U&& dflt) && {
252 return hasValue_ ? std::move(value_) : std::forward<U>(dflt);
256 void require_value() const {
258 throw OptionalEmptyException();
262 template<class... Args>
263 void construct(Args&&... args) {
264 const void* ptr = &value_;
265 // for supporting const types
266 new(const_cast<void*>(ptr)) Value(std::forward<Args>(args)...);
271 union { Value value_; };
275 #if defined(__GNUC__) && !defined(__clang__)
276 #pragma GCC diagnostic pop
280 const T* get_pointer(const Optional<T>& opt) {
281 return opt.get_pointer();
285 T* get_pointer(Optional<T>& opt) {
286 return opt.get_pointer();
290 void swap(Optional<T>& a, Optional<T>& b) {
291 if (a.hasValue() && b.hasValue()) {
294 swap(a.value(), b.value());
295 } else if (a.hasValue() || b.hasValue()) {
296 std::swap(a, b); // fall back to default implementation if they're mixed.
301 class Opt = Optional<typename std::decay<T>::type>>
302 Opt make_optional(T&& v) {
303 return Opt(std::forward<T>(v));
306 ///////////////////////////////////////////////////////////////////////////////
310 bool operator==(const Optional<V>& a, const V& b) {
311 return a.hasValue() && a.value() == b;
315 bool operator!=(const Optional<V>& a, const V& b) {
320 bool operator==(const V& a, const Optional<V>& b) {
321 return b.hasValue() && b.value() == a;
325 bool operator!=(const V& a, const Optional<V>& b) {
330 bool operator==(const Optional<V>& a, const Optional<V>& b) {
331 if (a.hasValue() != b.hasValue()) { return false; }
332 if (a.hasValue()) { return a.value() == b.value(); }
337 bool operator!=(const Optional<V>& a, const Optional<V>& b) {
342 bool operator< (const Optional<V>& a, const Optional<V>& b) {
343 if (a.hasValue() != b.hasValue()) { return a.hasValue() < b.hasValue(); }
344 if (a.hasValue()) { return a.value() < b.value(); }
349 bool operator> (const Optional<V>& a, const Optional<V>& b) {
354 bool operator<=(const Optional<V>& a, const Optional<V>& b) {
359 bool operator>=(const Optional<V>& a, const Optional<V>& b) {
363 // Suppress comparability of Optional<T> with T, despite implicit conversion.
364 template<class V> bool operator< (const Optional<V>&, const V& other) = delete;
365 template<class V> bool operator<=(const Optional<V>&, const V& other) = delete;
366 template<class V> bool operator>=(const Optional<V>&, const V& other) = delete;
367 template<class V> bool operator> (const Optional<V>&, const V& other) = delete;
368 template<class V> bool operator< (const V& other, const Optional<V>&) = delete;
369 template<class V> bool operator<=(const V& other, const Optional<V>&) = delete;
370 template<class V> bool operator>=(const V& other, const Optional<V>&) = delete;
371 template<class V> bool operator> (const V& other, const Optional<V>&) = delete;
373 ///////////////////////////////////////////////////////////////////////////////
377 #endif // FOLLY_OPTIONAL_H_