/* * Copyright (c) Meta Platforms, Inc. and affiliates. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ #pragma once /* * Optional - For conditional initialization of values, like boost::optional, * but with support for move semantics and emplacement. Reference type support * has not been included due to limited use cases and potential confusion with * semantics of assignment: Assigning to an optional reference could quite * reasonably copy its value or redirect the reference. * * Optional can be useful when a variable might or might not be needed: * * Optional<Logger> maybeLogger = ...; * if (maybeLogger) { * maybeLogger->log("hello"); * } * * Optional enables a 'null' value for types which do not otherwise have * nullability, especially useful for parameter passing: * * void testIterator(const unique_ptr<Iterator>& it, * initializer_list<int> idsExpected, * Optional<initializer_list<int>> ranksExpected = none) { * for (int i = 0; it->next(); ++i) { * EXPECT_EQ(it->doc().id(), idsExpected[i]); * if (ranksExpected) { * EXPECT_EQ(it->doc().rank(), (*ranksExpected)[i]); * } * } * } * * Optional models OptionalPointee, so calling 'get_pointer(opt)' will return a * pointer to nullptr if the 'opt' is empty, and a pointer to the value if it is * not: * * Optional<int> maybeInt = ...; * if (int* v = get_pointer(maybeInt)) { * cout << *v << endl; * } */ #include <cstddef> #include <functional> #include <new> #include <stdexcept> #include <type_traits> #include <utility> #if __cplusplus >= 201703L && __has_include(<optional>) #include <optional> #endif #include <folly/Portability.h> #include <folly/Traits.h> #include <folly/Utility.h> #include <folly/lang/Exception.h> namespace folly { template <class Value> class Optional; namespace detail { template <class Value> struct OptionalPromise; template <class Value> struct OptionalPromiseReturn; } // namespace detail struct None { enum class _secret { _token }; /** * No default constructor to support both `op = {}` and `op = none` * as syntax for clearing an Optional, just like std::nullopt_t. */ explicit constexpr None(_secret) {} }; constexpr None none{None::_secret::_token}; class FOLLY_EXPORT OptionalEmptyException : public std::runtime_error { public: OptionalEmptyException() : std::runtime_error("Empty Optional cannot be unwrapped") {} }; template <class Value> class Optional { public: typedef Value value_type; using promise_type = detail::OptionalPromise<Value>; static_assert( !std::is_reference<Value>::value, "Optional may not be used with reference types"); static_assert( !std::is_abstract<Value>::value, "Optional may not be used with abstract types"); constexpr Optional() noexcept {} Optional(const Optional& src) noexcept( std::is_nothrow_copy_constructible<Value>::value) { if (src.hasValue()) { construct(src.value()); } } Optional(Optional&& src) noexcept( std::is_nothrow_move_constructible<Value>::value) { if (src.hasValue()) { construct(std::move(src.value())); src.reset(); } } constexpr /* implicit */ Optional(const None&) noexcept {} constexpr /* implicit */ Optional(Value&& newValue) noexcept( std::is_nothrow_move_constructible<Value>::value) { construct(std::move(newValue)); } constexpr /* implicit */ Optional(const Value& newValue) noexcept( std::is_nothrow_copy_constructible<Value>::value) { construct(newValue); } template <typename... Args> constexpr explicit Optional(in_place_t, Args&&... args) noexcept( std::is_nothrow_constructible<Value, Args...>::value) : Optional{PrivateConstructor{}, std::forward<Args>(args)...} {} template <typename U, typename... Args> constexpr explicit Optional( in_place_t, std::initializer_list<U> il, Args&&... args) noexcept(std:: is_nothrow_constructible< Value, std::initializer_list<U>, Args...>::value) : Optional{PrivateConstructor{}, il, std::forward<Args>(args)...} {} // Used only when an Optional is used with coroutines on MSVC /* implicit */ Optional(const detail::OptionalPromiseReturn<Value>& p) : Optional{} { p.promise_->value_ = this; } // Conversions to ease migration to std::optional #if __cplusplus >= 201703L && __has_include(<optional>) template < typename U, typename = std::enable_if_t<std::is_same_v<U, std::optional<Value>>>> explicit Optional(U&& newValue) noexcept( std::is_nothrow_move_constructible<Value>::value) { if (newValue.has_value()) { construct(std::move(*newValue)); newValue.reset(); } } template < typename U, typename = std::enable_if_t<std::is_same_v<U, std::optional<Value>>>> explicit Optional(const U& newValue) noexcept( std::is_nothrow_copy_constructible<Value>::value) { if (newValue.has_value()) { construct(*newValue); } } explicit operator std::optional<Value>() && noexcept( std::is_nothrow_move_constructible<Value>::value) { std::optional<Value> ret = storage_.hasValue ? std::optional<Value>(std::move(storage_.value)) : std::nullopt; reset(); return ret; } explicit operator std::optional<Value>() const& noexcept( std::is_nothrow_copy_constructible<Value>::value) { return storage_.hasValue ? std::optional<Value>(storage_.value) : std::nullopt; } #endif void assign(const None&) { reset(); } void assign(Optional&& src) { if (this != &src) { if (src.hasValue()) { assign(std::move(src.value())); src.reset(); } else { reset(); } } } void assign(const Optional& src) { if (src.hasValue()) { assign(src.value()); } else { reset(); } } void assign(Value&& newValue) { if (hasValue()) { storage_.value = std::move(newValue); } else { construct(std::move(newValue)); } } void assign(const Value& newValue) { if (hasValue()) { storage_.value = newValue; } else { construct(newValue); } } Optional& operator=(None) noexcept { reset(); return *this; } template <class Arg> Optional& operator=(Arg&& arg) { assign(std::forward<Arg>(arg)); return *this; } Optional& operator=(Optional&& other) noexcept( std::is_nothrow_move_assignable<Value>::value) { assign(std::move(other)); return *this; } Optional& operator=(const Optional& other) noexcept( std::is_nothrow_copy_assignable<Value>::value) { assign(other); return *this; } template <class... Args> Value& emplace(Args&&... args) { reset(); construct(std::forward<Args>(args)...); return value(); } template <class U, class... Args> typename std::enable_if< std::is_constructible<Value, std::initializer_list<U>&, Args&&...>::value, Value&>::type emplace(std::initializer_list<U> ilist, Args&&... args) { reset(); construct(ilist, std::forward<Args>(args)...); return value(); } void reset() noexcept { storage_.clear(); } void clear() noexcept { reset(); } void swap(Optional& that) noexcept(IsNothrowSwappable<Value>::value) { if (hasValue() && that.hasValue()) { using std::swap; swap(value(), that.value()); } else if (hasValue()) { that.emplace(std::move(value())); reset(); } else if (that.hasValue()) { emplace(std::move(that.value())); that.reset(); } } constexpr const Value& value() const& { require_value(); return storage_.value; } constexpr Value& value() & { require_value(); return storage_.value; } constexpr Value&& value() && { require_value(); return std::move(storage_.value); } constexpr const Value&& value() const&& { require_value(); return std::move(storage_.value); } const Value* get_pointer() const& { return storage_.hasValue ? &storage_.value : nullptr; } Value* get_pointer() & { return storage_.hasValue ? &storage_.value : nullptr; } Value* get_pointer() && = delete; constexpr bool has_value() const noexcept { return storage_.hasValue; } constexpr bool hasValue() const noexcept { return has_value(); } constexpr explicit operator bool() const noexcept { return has_value(); } constexpr const Value& operator*() const& { return value(); } constexpr Value& operator*() & { return value(); } constexpr const Value&& operator*() const&& { return std::move(value()); } constexpr Value&& operator*() && { return std::move(value()); } constexpr const Value* operator->() const { return &value(); } constexpr Value* operator->() { return &value(); } // Return a copy of the value if set, or a given default if not. template <class U> constexpr Value value_or(U&& dflt) const& { if (storage_.hasValue) { return storage_.value; } return std::forward<U>(dflt); } template <class U> constexpr Value value_or(U&& dflt) && { if (storage_.hasValue) { return std::move(storage_.value); } return std::forward<U>(dflt); } private: template <class T> friend constexpr Optional<std::decay_t<T>> make_optional(T&&); template <class T, class... Args> friend constexpr Optional<T> make_optional(Args&&... args); template <class T, class U, class... As> friend constexpr Optional<T> make_optional(std::initializer_list<U>, As&&...); /** * Construct the optional in place, this is duplicated as a non-explicit * constructor to allow returning values that are non-movable from * make_optional using list initialization. * * Until C++17, at which point this will become unnecessary because of * specified prvalue elision. */ struct PrivateConstructor { explicit PrivateConstructor() = default; }; template <typename... Args> constexpr Optional(PrivateConstructor, Args&&... args) noexcept( std::is_constructible<Value, Args&&...>::value) { construct(std::forward<Args>(args)...); } void require_value() const { if (!storage_.hasValue) { throw_exception<OptionalEmptyException>(); } } template <class... Args> void construct(Args&&... args) { const void* ptr = &storage_.value; // For supporting const types. new (const_cast<void*>(ptr)) Value(std::forward<Args>(args)...); storage_.hasValue = true; } struct StorageTriviallyDestructible { union { char emptyState; Value value; }; bool hasValue; constexpr StorageTriviallyDestructible() : emptyState('\0'), hasValue{false} {} void clear() { hasValue = false; } }; struct StorageNonTriviallyDestructible { union { char emptyState; Value value; }; bool hasValue; StorageNonTriviallyDestructible() : hasValue{false} {} ~StorageNonTriviallyDestructible() { clear(); } void clear() { if (hasValue) { hasValue = false; value.~Value(); } } }; using Storage = typename std::conditional< std::is_trivially_destructible<Value>::value, StorageTriviallyDestructible, StorageNonTriviallyDestructible>::type; Storage storage_; }; template <class T> const T* get_pointer(const Optional<T>& opt) { return opt.get_pointer(); } template <class T> T* get_pointer(Optional<T>& opt) { return opt.get_pointer(); } template <class T> void swap(Optional<T>& a, Optional<T>& b) noexcept(noexcept(a.swap(b))) { a.swap(b); } template <class T> constexpr Optional<std::decay_t<T>> make_optional(T&& v) { using PrivateConstructor = typename folly::Optional<std::decay_t<T>>::PrivateConstructor; return {PrivateConstructor{}, std::forward<T>(v)}; } template <class T, class... Args> constexpr folly::Optional<T> make_optional(Args&&... args) { using PrivateConstructor = typename folly::Optional<T>::PrivateConstructor; return {PrivateConstructor{}, std::forward<Args>(args)...}; } template <class T, class U, class... Args> constexpr folly::Optional<T> make_optional( std::initializer_list<U> il, Args&&... args) { using PrivateConstructor = typename folly::Optional<T>::PrivateConstructor; return {PrivateConstructor{}, il, std::forward<Args>(args)...}; } /////////////////////////////////////////////////////////////////////////////// // Comparisons. template <class U, class V> constexpr bool operator==(const Optional<U>& a, const V& b) { return a.hasValue() && a.value() == b; } template <class U, class V> constexpr bool operator!=(const Optional<U>& a, const V& b) { return !(a == b); } template <class U, class V> constexpr bool operator==(const U& a, const Optional<V>& b) { return b.hasValue() && b.value() == a; } template <class U, class V> constexpr bool operator!=(const U& a, const Optional<V>& b) { return !(a == b); } template <class U, class V> constexpr bool operator==(const Optional<U>& a, const Optional<V>& b) { if (a.hasValue() != b.hasValue()) { return false; } if (a.hasValue()) { return a.value() == b.value(); } return true; } template <class U, class V> constexpr bool operator!=(const Optional<U>& a, const Optional<V>& b) { return !(a == b); } template <class U, class V> constexpr bool operator<(const Optional<U>& a, const Optional<V>& b) { if (a.hasValue() != b.hasValue()) { return a.hasValue() < b.hasValue(); } if (a.hasValue()) { return a.value() < b.value(); } return false; } template <class U, class V> constexpr bool operator>(const Optional<U>& a, const Optional<V>& b) { return b < a; } template <class U, class V> constexpr bool operator<=(const Optional<U>& a, const Optional<V>& b) { return !(b < a); } template <class U, class V> constexpr bool operator>=(const Optional<U>& a, const Optional<V>& b) { return !(a < b); } // Suppress comparability of Optional<T> with T, despite implicit conversion. template <class V> bool operator<(const Optional<V>&, const V& other) = delete; template <class V> bool operator<=(const Optional<V>&, const V& other) = delete; template <class V> bool operator>=(const Optional<V>&, const V& other) = delete; template <class V> bool operator>(const Optional<V>&, const V& other) = delete; template <class V> bool operator<(const V& other, const Optional<V>&) = delete; template <class V> bool operator<=(const V& other, const Optional<V>&) = delete; template <class V> bool operator>=(const V& other, const Optional<V>&) = delete; template <class V> bool operator>(const V& other, const Optional<V>&) = delete; // Comparisons with none template <class V> constexpr bool operator==(const Optional<V>& a, None) noexcept { return !a.hasValue(); } template <class V> constexpr bool operator==(None, const Optional<V>& a) noexcept { return !a.hasValue(); } template <class V> constexpr bool operator<(const Optional<V>&, None) noexcept { return false; } template <class V> constexpr bool operator<(None, const Optional<V>& a) noexcept { return a.hasValue(); } template <class V> constexpr bool operator>(const Optional<V>& a, None) noexcept { return a.hasValue(); } template <class V> constexpr bool operator>(None, const Optional<V>&) noexcept { return false; } template <class V> constexpr bool operator<=(None, const Optional<V>&) noexcept { return true; } template <class V> constexpr bool operator<=(const Optional<V>& a, None) noexcept { return !a.hasValue(); } template <class V> constexpr bool operator>=(const Optional<V>&, None) noexcept { return true; } template <class V> constexpr bool operator>=(None, const Optional<V>& a) noexcept { return !a.hasValue(); } /////////////////////////////////////////////////////////////////////////////// } // namespace folly // Allow usage of Optional<T> in std::unordered_map and std::unordered_set FOLLY_NAMESPACE_STD_BEGIN template <class T> struct hash<folly::Optional<T>> { size_t operator()(folly::Optional<T> const& obj) const { if (!obj.hasValue()) { return 0; } return hash<typename remove_const<T>::type>()(*obj); } }; FOLLY_NAMESPACE_STD_END // Enable the use of folly::Optional with `co_await` // Inspired by https://github.com/toby-allsopp/coroutine_monad #if FOLLY_HAS_COROUTINES #include <folly/experimental/coro/Coroutine.h> namespace folly { namespace detail { template <typename Value> struct OptionalPromise; template <typename Value> struct OptionalPromiseReturn { Optional<Value> storage_; OptionalPromise<Value>* promise_; /* implicit */ OptionalPromiseReturn(OptionalPromise<Value>& promise) noexcept : promise_(&promise) { promise.value_ = &storage_; } OptionalPromiseReturn(OptionalPromiseReturn&& that) noexcept : OptionalPromiseReturn{*that.promise_} {} ~OptionalPromiseReturn() {} /* implicit */ operator Optional<Value>() & { return std::move(storage_); } }; template <typename Value> struct OptionalPromise { Optional<Value>* value_ = nullptr; OptionalPromise() = default; OptionalPromise(OptionalPromise const&) = delete; // This should work regardless of whether the compiler generates: // folly::Optional<Value> retobj{ p.get_return_object(); } // MSVC // or: // auto retobj = p.get_return_object(); // clang OptionalPromiseReturn<Value> get_return_object() noexcept { return *this; } coro::suspend_never initial_suspend() const noexcept { return {}; } coro::suspend_never final_suspend() const noexcept { return {}; } template <typename U = Value> void return_value(U&& u) { *value_ = static_cast<U&&>(u); } void unhandled_exception() { // Technically, throwing from unhandled_exception is underspecified: // https://github.com/GorNishanov/CoroutineWording/issues/17 rethrow_current_exception(); } }; template <typename Value> struct OptionalAwaitable { Optional<Value> o_; bool await_ready() const noexcept { return o_.hasValue(); } Value await_resume() { return std::move(o_.value()); } // Explicitly only allow suspension into an OptionalPromise template <typename U> void await_suspend(coro::coroutine_handle<OptionalPromise<U>> h) const { // Abort the rest of the coroutine. resume() is not going to be called h.destroy(); } }; } // namespace detail template <typename Value> detail::OptionalAwaitable<Value> /* implicit */ operator co_await(Optional<Value> o) { return {std::move(o)}; } } // namespace folly #endif // FOLLY_HAS_COROUTINES