/* * Copyright 2017-present Facebook, Inc. * * 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. */ /* * @author Eric Niebler (eniebler@fb.com), Sven Over (over@fb.com) * Acknowledgements: Giuseppe Ottaviano (ott@fb.com) */ /** * @class Function * * @brief A polymorphic function wrapper that is not copyable and does not * require the wrapped function to be copy constructible. * * `folly::Function` is a polymorphic function wrapper, similar to * `std::function`. The template parameters of the `folly::Function` define * the parameter signature of the wrapped callable, but not the specific * type of the embedded callable. E.g. a `folly::Function` * can wrap callables that return an `int` when passed an `int`. This can be a * function pointer or any class object implementing one or both of * * int operator(int); * int operator(int) const; * * If both are defined, the non-const one takes precedence. * * Unlike `std::function`, a `folly::Function` can wrap objects that are not * copy constructible. As a consequence of this, `folly::Function` itself * is not copyable, either. * * Another difference is that, unlike `std::function`, `folly::Function` treats * const-ness of methods correctly. While a `std::function` allows to wrap * an object that only implements a non-const `operator()` and invoke * a const-reference of the `std::function`, `folly::Function` requires you to * declare a function type as const in order to be able to execute it on a * const-reference. * * For example: * * class Foo { * public: * void operator()() { * // mutates the Foo object * } * }; * * class Bar { * std::function foo_; // wraps a Foo object * public: * void mutateFoo() const * { * foo_(); * } * }; * * Even though `mutateFoo` is a const-method, so it can only reference `foo_` * as const, it is able to call the non-const `operator()` of the Foo * object that is embedded in the foo_ function. * * `folly::Function` will not allow you to do that. You will have to decide * whether you need to invoke your wrapped callable from a const reference * (like in the example above), in which case it will only wrap a * `operator() const`. If your functor does not implement that, * compilation will fail. If you do not require to be able to invoke the * wrapped function in a const context, you can wrap any functor that * implements either or both of const and non-const `operator()`. * * The template parameter of `folly::Function`, the `FunctionType`, can be * const-qualified. Be aware that the const is part of the function signature. * It does not mean that the function type is a const type. * * using FunctionType = R(Args...); * using ConstFunctionType = R(Args...) const; * * In this example, `FunctionType` and `ConstFunctionType` are different * types. `ConstFunctionType` is not the same as `const FunctionType`. * As a matter of fact, trying to use the latter should emit a compiler * warning or error, because it has no defined meaning. * * // This will not compile: * folly::Function func = Foo(); * // because Foo does not have a member function of the form: * // void operator()() const; * * // This will compile just fine: * folly::Function func = Foo(); * // and it will wrap the existing member function: * // void operator()(); * * When should a const function type be used? As a matter of fact, you will * probably not need to use const function types very often. See the following * example: * * class Bar { * folly::Function func_; * folly::Function constFunc_; * * void someMethod() { * // Can call func_. * func_(); * // Can call constFunc_. * constFunc_(); * } * * void someConstMethod() const { * // Can call constFunc_. * constFunc_(); * // However, cannot call func_ because a non-const method cannot * // be called from a const one. * } * }; * * As you can see, whether the `folly::Function`'s function type should * be declared const or not is identical to whether a corresponding method * would be declared const or not. * * You only require a `folly::Function` to hold a const function type, if you * intend to invoke it from within a const context. This is to ensure that * you cannot mutate its inner state when calling in a const context. * * This is how the const/non-const choice relates to lambda functions: * * // Non-mutable lambdas: can be stored in a non-const... * folly::Function print_number = * [] (int number) { std::cout << number << std::endl; }; * * // ...as well as in a const folly::Function * folly::Function print_number_const = * [] (int number) { std::cout << number << std::endl; }; * * // Mutable lambda: can only be stored in a non-const folly::Function: * int number = 0; * folly::Function print_number = * [number] () mutable { std::cout << ++number << std::endl; }; * // Trying to store the above mutable lambda in a * // `folly::Function` would lead to a compiler error: * // error: no viable conversion from '(lambda at ...)' to * // 'folly::Function' * * Casting between const and non-const `folly::Function`s: * conversion from const to non-const signatures happens implicitly. Any * function that takes a `folly::Function` can be passed * a `folly::Function` without explicit conversion. * This is safe, because casting from const to non-const only entails giving * up the ability to invoke the function from a const context. * Casting from a non-const to a const signature is potentially dangerous, * as it means that a function that may change its inner state when invoked * is made possible to call from a const context. Therefore this cast does * not happen implicitly. The function `folly::constCastFunction` can * be used to perform the cast. * * // Mutable lambda: can only be stored in a non-const folly::Function: * int number = 0; * folly::Function print_number = * [number] () mutable { std::cout << ++number << std::endl; }; * * // const-cast to a const folly::Function: * folly::Function print_number_const = * constCastFunction(std::move(print_number)); * * When to use const function types? * Generally, only when you need them. When you use a `folly::Function` as a * member of a struct or class, only use a const function signature when you * need to invoke the function from const context. * When passing a `folly::Function` to a function, the function should accept * a non-const `folly::Function` whenever possible, i.e. when it does not * need to pass on or store a const `folly::Function`. This is the least * possible constraint: you can always pass a const `folly::Function` when * the function accepts a non-const one. * * How does the const behaviour compare to `std::function`? * `std::function` can wrap object with non-const invokation behaviour but * exposes them as const. The equivalent behaviour can be achieved with * `folly::Function` like so: * * std::function stdfunc = someCallable; * * folly::Function uniqfunc = constCastFunction( * folly::Function(someCallable) * ); * * You need to wrap the callable first in a non-const `folly::Function` to * select a non-const invoke operator (or the const one if no non-const one is * present), and then move it into a const `folly::Function` using * `constCastFunction`. * The name of `constCastFunction` should warn you that something * potentially dangerous is happening. As a matter of fact, using * `std::function` always involves this potentially dangerous aspect, which * is why it is not considered fully const-safe or even const-correct. * However, in most of the cases you will not need the dangerous aspect at all. * Either you do not require invokation of the function from a const context, * in which case you do not need to use `constCastFunction` and just * use the inner `folly::Function` in the example above, i.e. just use a * non-const `folly::Function`. Or, you may need invokation from const, but * the callable you are wrapping does not mutate its state (e.g. it is a class * object and implements `operator() const`, or it is a normal, * non-mutable lambda), in which case you can wrap the callable in a const * `folly::Function` directly, without using `constCastFunction`. * Only if you require invokation from a const context of a callable that * may mutate itself when invoked you have to go through the above procedure. * However, in that case what you do is potentially dangerous and requires * the equivalent of a `const_cast`, hence you need to call * `constCastFunction`. */ #pragma once #include #include #include #include #include #include #include #include #include namespace folly { template class Function; template Function constCastFunction( Function&&) noexcept; #if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE template Function constCastFunction( Function&&) noexcept; #endif namespace detail { namespace function { enum class Op { MOVE, NUKE, FULL, HEAP }; union Data { void* big; std::aligned_storage<6 * sizeof(void*)>::type tiny; }; template using IsSmall = Conjunction< std::integral_constant, std::is_nothrow_move_constructible>; using SmallTag = std::true_type; using HeapTag = std::false_type; template struct NotFunction : std::true_type {}; template struct NotFunction> : std::false_type {}; template using EnableIfNotFunction = typename std::enable_if::value>::type; struct CoerceTag {}; template bool isNullPtrFn(T* p) { return p == nullptr; } template std::false_type isNullPtrFn(T&&) { return {}; } inline bool uninitNoop(Op, Data*, Data*) { return false; } template using CallableResult = decltype(std::declval()(std::declval()...)); template < typename From, typename To, typename = typename std::enable_if< !std::is_reference::value || std::is_reference::value>::type> using SafeResultOf = decltype(static_cast(std::declval())); template struct FunctionTraits; template struct FunctionTraits { using Call = ReturnType (*)(Data&, Args&&...); using IsConst = std::false_type; using ConstSignature = ReturnType(Args...) const; using NonConstSignature = ReturnType(Args...); using OtherSignature = ConstSignature; template using ResultOf = SafeResultOf>&, Args...>, ReturnType>; template static ReturnType callSmall(Data& p, Args&&... args) { return static_cast((*static_cast( static_cast(&p.tiny)))(static_cast(args)...)); } template static ReturnType callBig(Data& p, Args&&... args) { return static_cast( (*static_cast(p.big))(static_cast(args)...)); } static ReturnType uninitCall(Data&, Args&&...) { throw std::bad_function_call(); } ReturnType operator()(Args... args) { auto& fn = *static_cast*>(this); return fn.call_(fn.data_, static_cast(args)...); } class SharedProxy { std::shared_ptr> sp_; public: explicit SharedProxy(Function&& func) : sp_(std::make_shared>(std::move(func))) {} ReturnType operator()(Args&&... args) const { return (*sp_)(static_cast(args)...); } }; }; template struct FunctionTraits { using Call = ReturnType (*)(Data&, Args&&...); using IsConst = std::true_type; using ConstSignature = ReturnType(Args...) const; using NonConstSignature = ReturnType(Args...); using OtherSignature = NonConstSignature; template using ResultOf = SafeResultOf< CallableResult>&, Args...>, ReturnType>; template static ReturnType callSmall(Data& p, Args&&... args) { return static_cast((*static_cast( static_cast(&p.tiny)))(static_cast(args)...)); } template static ReturnType callBig(Data& p, Args&&... args) { return static_cast( (*static_cast(p.big))(static_cast(args)...)); } static ReturnType uninitCall(Data&, Args&&...) { throw std::bad_function_call(); } ReturnType operator()(Args... args) const { auto& fn = *static_cast*>(this); return fn.call_(fn.data_, static_cast(args)...); } class SharedProxy { std::shared_ptr> sp_; public: explicit SharedProxy(Function&& func) : sp_(std::make_shared>(std::move(func))) {} ReturnType operator()(Args&&... args) const { return (*sp_)(static_cast(args)...); } }; }; #if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE template struct FunctionTraits { using Call = ReturnType (*)(Data&, Args&&...) noexcept; using IsConst = std::false_type; using ConstSignature = ReturnType(Args...) const noexcept; using NonConstSignature = ReturnType(Args...) noexcept; using OtherSignature = ConstSignature; template using ResultOf = SafeResultOf>&, Args...>, ReturnType>; template static ReturnType callSmall(Data& p, Args&&... args) noexcept { return static_cast((*static_cast( static_cast(&p.tiny)))(static_cast(args)...)); } template static ReturnType callBig(Data& p, Args&&... args) noexcept { return static_cast( (*static_cast(p.big))(static_cast(args)...)); } static ReturnType uninitCall(Data&, Args&&...) noexcept { throw std::bad_function_call(); } ReturnType operator()(Args... args) noexcept { auto& fn = *static_cast*>(this); return fn.call_(fn.data_, static_cast(args)...); } class SharedProxy { std::shared_ptr> sp_; public: explicit SharedProxy(Function&& func) : sp_(std::make_shared>(std::move(func))) {} ReturnType operator()(Args&&... args) const { return (*sp_)(static_cast(args)...); } }; }; template struct FunctionTraits { using Call = ReturnType (*)(Data&, Args&&...) noexcept; using IsConst = std::true_type; using ConstSignature = ReturnType(Args...) const noexcept; using NonConstSignature = ReturnType(Args...) noexcept; using OtherSignature = NonConstSignature; template using ResultOf = SafeResultOf< CallableResult>&, Args...>, ReturnType>; template static ReturnType callSmall(Data& p, Args&&... args) noexcept { return static_cast((*static_cast( static_cast(&p.tiny)))(static_cast(args)...)); } template static ReturnType callBig(Data& p, Args&&... args) noexcept { return static_cast( (*static_cast(p.big))(static_cast(args)...)); } static ReturnType uninitCall(Data&, Args&&...) noexcept { throw std::bad_function_call(); } ReturnType operator()(Args... args) const noexcept { auto& fn = *static_cast*>(this); return fn.call_(fn.data_, static_cast(args)...); } class SharedProxy { std::shared_ptr> sp_; public: explicit SharedProxy(Function&& func) : sp_(std::make_shared>(std::move(func))) {} ReturnType operator()(Args&&... args) const { return (*sp_)(static_cast(args)...); } }; }; #endif template bool execSmall(Op o, Data* src, Data* dst) { switch (o) { case Op::MOVE: ::new (static_cast(&dst->tiny)) Fun(std::move(*static_cast(static_cast(&src->tiny)))); FOLLY_FALLTHROUGH; case Op::NUKE: static_cast(static_cast(&src->tiny))->~Fun(); break; case Op::FULL: return true; case Op::HEAP: break; } return false; } template bool execBig(Op o, Data* src, Data* dst) { switch (o) { case Op::MOVE: dst->big = src->big; src->big = nullptr; break; case Op::NUKE: delete static_cast(src->big); break; case Op::FULL: case Op::HEAP: break; } return true; } } // namespace function } // namespace detail template class Function final : private detail::function::FunctionTraits { // These utility types are defined outside of the template to reduce // the number of instantiations, and then imported in the class // namespace for convenience. using Data = detail::function::Data; using Op = detail::function::Op; using SmallTag = detail::function::SmallTag; using HeapTag = detail::function::HeapTag; using CoerceTag = detail::function::CoerceTag; using Traits = detail::function::FunctionTraits; using Call = typename Traits::Call; using Exec = bool (*)(Op, Data*, Data*); template using IsSmall = detail::function::IsSmall; // The `data_` member is mutable to allow `constCastFunction` to work without // invoking undefined behavior. Const-correctness is only violated when // `FunctionType` is a const function type (e.g., `int() const`) and `*this` // is the result of calling `constCastFunction`. mutable Data data_{}; Call call_{&Traits::uninitCall}; Exec exec_{&detail::function::uninitNoop}; friend Traits; friend Function folly::constCastFunction<>( Function&&) noexcept; friend class Function; template Function(Fun&& fun, SmallTag) noexcept { using FunT = typename std::decay::type; if (!detail::function::isNullPtrFn(fun)) { ::new (static_cast(&data_.tiny)) FunT(static_cast(fun)); call_ = &Traits::template callSmall; exec_ = &detail::function::execSmall; } } template Function(Fun&& fun, HeapTag) { using FunT = typename std::decay::type; data_.big = new FunT(static_cast(fun)); call_ = &Traits::template callBig; exec_ = &detail::function::execBig; } template Function(Function&& that, CoerceTag) : Function(static_cast&&>(that), HeapTag{}) {} Function( Function&& that, CoerceTag) noexcept { that.exec_(Op::MOVE, &that.data_, &data_); std::swap(call_, that.call_); std::swap(exec_, that.exec_); } public: /** * Default constructor. Constructs an empty Function. */ Function() = default; // not copyable Function(const Function&) = delete; #if __OBJC__ // Make sure Objective C blocks are copied template /*implicit*/ Function(ReturnType (^objCBlock)(Args... args)) : Function([blockCopy = (ReturnType (^)(Args...))[objCBlock copy]]( Args... args) { return blockCopy(args...); }){}; #endif /** * Move constructor */ Function(Function&& that) noexcept { that.exec_(Op::MOVE, &that.data_, &data_); std::swap(call_, that.call_); std::swap(exec_, that.exec_); } /** * Constructs an empty `Function`. */ /* implicit */ Function(std::nullptr_t) noexcept {} /** * Constructs a new `Function` from any callable object that is _not_ a * `folly::Function`. This handles function pointers, pointers to static * member functions, `std::reference_wrapper` objects, `std::function` * objects, and arbitrary objects that implement `operator()` if the parameter * signature matches (i.e. it returns an object convertible to `R` when called * with `Args...`). * * \note `typename Traits::template ResultOf` prevents this overload * from being selected by overload resolution when `fun` is not a compatible * function. * * \note The noexcept requires some explanation. `IsSmall` is true when the * decayed type fits within the internal buffer and is noexcept-movable. But * this ctor might copy, not move. What we need here, if this ctor does a * copy, is that this ctor be noexcept when the copy is noexcept. That is not * checked in `IsSmall`, and shouldn't be, because once the `Function` is * constructed, the contained object is never copied. This check is for this * ctor only, in the case that this ctor does a copy. */ template < typename Fun, typename = detail::function::EnableIfNotFunction, typename = typename Traits::template ResultOf> /* implicit */ Function(Fun fun) noexcept( IsSmall::value && noexcept(Fun(std::declval()))) : Function(std::move(fun), IsSmall{}) {} /** * For move-constructing from a `folly::Function`. * For a `Function` with a `const` function type, the object must be * callable from a `const`-reference, i.e. implement `operator() const`. * For a `Function` with a non-`const` function type, the object will * be called from a non-const reference, which means that it will execute * a non-const `operator()` if it is defined, and falls back to * `operator() const` otherwise. */ template < typename Signature, typename = typename Traits::template ResultOf>> Function(Function&& that) noexcept( noexcept(Function(std::move(that), CoerceTag{}))) : Function(std::move(that), CoerceTag{}) {} /** * If `ptr` is null, constructs an empty `Function`. Otherwise, * this constructor is equivalent to `Function(std::mem_fn(ptr))`. */ template < typename Member, typename Class, // Prevent this overload from being selected when `ptr` is not a // compatible member function pointer. typename = decltype(Function(std::mem_fn((Member Class::*)0)))> /* implicit */ Function(Member Class::*ptr) noexcept { if (ptr) { *this = std::mem_fn(ptr); } } ~Function() { exec_(Op::NUKE, &data_, nullptr); } Function& operator=(const Function&) = delete; #if __OBJC__ // Make sure Objective C blocks are copied template /* implicit */ Function &operator=(ReturnType (^objCBlock)(Args... args)) { (*this) = [blockCopy = (ReturnType (^)(Args...))[objCBlock copy]]( Args... args) { return blockCopy(args...); }; return *this; } #endif /** * Move assignment operator * * \note Leaves `that` in a valid but unspecified state. If `&that == this` * then `*this` is left in a valid but unspecified state. */ Function& operator=(Function&& that) noexcept { // Q: Why is it safe to destroy and reconstruct this object in place? // A: Two reasons: First, `Function` is a final class, so in doing this // we aren't slicing off any derived parts. And second, the move // operation is guaranteed not to throw so we always leave the object // in a valid state. // In the case of self-move (this == &that), this leaves the object in // a default-constructed state. First the object is destroyed, then we // pass the destroyed object to the move constructor. The first thing the // move constructor does is default-construct the object. That object is // "moved" into itself, which is a no-op for a default-constructed Function. this->~Function(); ::new (this) Function(std::move(that)); return *this; } /** * Assigns a callable object to this `Function`. If the operation fails, * `*this` is left unmodified. * * \note `typename = decltype(Function(std::declval()))` prevents this * overload from being selected by overload resolution when `fun` is not a * compatible function. */ template ()))> Function& operator=(Fun fun) noexcept( noexcept(/* implicit */ Function(std::declval()))) { // Doing this in place is more efficient when we can do so safely. if (noexcept(/* implicit */ Function(std::declval()))) { // Q: Why is is safe to destroy and reconstruct this object in place? // A: See the explanation in the move assignment operator. this->~Function(); ::new (this) Function(std::move(fun)); } else { // Construct a temporary and (nothrow) swap. Function(std::move(fun)).swap(*this); } return *this; } /** * For assigning from a `Function`. */ template < typename Signature, typename = typename Traits::template ResultOf>> Function& operator=(Function&& that) noexcept( noexcept(Function(std::move(that)))) { return (*this = Function(std::move(that))); } /** * Clears this `Function`. */ Function& operator=(std::nullptr_t) noexcept { return (*this = Function()); } /** * If `ptr` is null, clears this `Function`. Otherwise, this assignment * operator is equivalent to `*this = std::mem_fn(ptr)`. */ template auto operator=(Member Class::*ptr) noexcept // Prevent this overload from being selected when `ptr` is not a // compatible member function pointer. -> decltype(operator=(std::mem_fn(ptr))) { return ptr ? (*this = std::mem_fn(ptr)) : (*this = Function()); } /** * Call the wrapped callable object with the specified arguments. */ using Traits::operator(); /** * Exchanges the callable objects of `*this` and `that`. */ void swap(Function& that) noexcept { std::swap(*this, that); } /** * Returns `true` if this `Function` contains a callable, i.e. is * non-empty. */ explicit operator bool() const noexcept { return exec_(Op::FULL, nullptr, nullptr); } /** * Returns `true` if this `Function` stores the callable on the * heap. If `false` is returned, there has been no additional memory * allocation and the callable is stored inside the `Function` * object itself. */ bool hasAllocatedMemory() const noexcept { return exec_(Op::HEAP, nullptr, nullptr); } using typename Traits::SharedProxy; /** * Move this `Function` into a copyable callable object, of which all copies * share the state. */ SharedProxy asSharedProxy() && { return SharedProxy{std::move(*this)}; } /** * Construct a `std::function` by moving in the contents of this `Function`. * Note that the returned `std::function` will share its state (i.e. captured * data) across all copies you make of it, so be very careful when copying. */ std::function asStdFunction() && { return std::move(*this).asSharedProxy(); } }; template void swap(Function& lhs, Function& rhs) noexcept { lhs.swap(rhs); } template bool operator==(const Function& fn, std::nullptr_t) { return !fn; } template bool operator==(std::nullptr_t, const Function& fn) { return !fn; } template bool operator!=(const Function& fn, std::nullptr_t) { return !(fn == nullptr); } template bool operator!=(std::nullptr_t, const Function& fn) { return !(nullptr == fn); } /** * NOTE: See detailed note about `constCastFunction` at the top of the file. * This is potentially dangerous and requires the equivalent of a `const_cast`. */ template Function constCastFunction( Function&& that) noexcept { return Function{std::move(that), detail::function::CoerceTag{}}; } template Function constCastFunction( Function&& that) noexcept { return std::move(that); } #if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE template Function constCastFunction( Function&& that) noexcept { return Function{ std::move(that), detail::function::CoerceTag{}}; } template Function constCastFunction( Function&& that) noexcept { return std::move(that); } #endif namespace detail { namespace function { template struct IsCallableAsImpl : std::false_type {}; template struct IsCallableAsImpl< Fun, ReturnType(Args...), void_t::type>> : std::is_convertible< typename std::result_of::type, ReturnType> {}; template struct IsCallableAs : IsCallableAsImpl {}; } // namespace function } // namespace detail /** * @class FunctionRef * * @brief A reference wrapper for callable objects * * FunctionRef is similar to std::reference_wrapper, but the template parameter * is the function signature type rather than the type of the referenced object. * A folly::FunctionRef is cheap to construct as it contains only a pointer to * the referenced callable and a pointer to a function which invokes the * callable. * * The user of FunctionRef must be aware of the reference semantics: storing a * copy of a FunctionRef is potentially dangerous and should be avoided unless * the referenced object definitely outlives the FunctionRef object. Thus any * function that accepts a FunctionRef parameter should only use it to invoke * the referenced function and not store a copy of it. Knowing that FunctionRef * itself has reference semantics, it is generally okay to use it to reference * lambdas that capture by reference. */ template class FunctionRef; template class FunctionRef final { using Call = ReturnType (*)(void*, Args&&...); static ReturnType uninitCall(void*, Args&&...) { throw std::bad_function_call(); } template static ReturnType call(void* object, Args&&... args) { using Pointer = _t>; return static_cast(invoke( static_cast(*static_cast(object)), static_cast(args)...)); } void* object_{nullptr}; Call call_{&FunctionRef::uninitCall}; public: /** * Default constructor. Constructs an empty FunctionRef. * * Invoking it will throw std::bad_function_call. */ FunctionRef() = default; /** * Construct a FunctionRef from a reference to a callable object. */ template < typename Fun, typename std::enable_if< Conjunction< Negation>>>, detail::function::IsCallableAs>::value, int>::type = 0> constexpr /* implicit */ FunctionRef(Fun&& fun) noexcept // `Fun` may be a const type, in which case we have to do a const_cast // to store the address in a `void*`. This is safe because the `void*` // will be cast back to `Fun*` (which is a const pointer whenever `Fun` // is a const type) inside `FunctionRef::call` : object_( const_cast(static_cast(std::addressof(fun)))), call_(&FunctionRef::call) {} ReturnType operator()(Args... args) const { return call_(object_, static_cast(args)...); } constexpr explicit operator bool() const { return object_; } }; } // namespace folly