2 * Copyright 2017-present 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 * @author Eric Niebler (eniebler@fb.com), Sven Over (over@fb.com)
18 * Acknowledgements: Giuseppe Ottaviano (ott@fb.com)
24 * @brief A polymorphic function wrapper that is not copyable and does not
25 * require the wrapped function to be copy constructible.
27 * `folly::Function` is a polymorphic function wrapper, similar to
28 * `std::function`. The template parameters of the `folly::Function` define
29 * the parameter signature of the wrapped callable, but not the specific
30 * type of the embedded callable. E.g. a `folly::Function<int(int)>`
31 * can wrap callables that return an `int` when passed an `int`. This can be a
32 * function pointer or any class object implementing one or both of
35 * int operator(int) const;
37 * If both are defined, the non-const one takes precedence.
39 * Unlike `std::function`, a `folly::Function` can wrap objects that are not
40 * copy constructible. As a consequence of this, `folly::Function` itself
41 * is not copyable, either.
43 * Another difference is that, unlike `std::function`, `folly::Function` treats
44 * const-ness of methods correctly. While a `std::function` allows to wrap
45 * an object that only implements a non-const `operator()` and invoke
46 * a const-reference of the `std::function`, `folly::Function` requires you to
47 * declare a function type as const in order to be able to execute it on a
55 * // mutates the Foo object
60 * std::function<void(void)> foo_; // wraps a Foo object
62 * void mutateFoo() const
68 * Even though `mutateFoo` is a const-method, so it can only reference `foo_`
69 * as const, it is able to call the non-const `operator()` of the Foo
70 * object that is embedded in the foo_ function.
72 * `folly::Function` will not allow you to do that. You will have to decide
73 * whether you need to invoke your wrapped callable from a const reference
74 * (like in the example above), in which case it will only wrap a
75 * `operator() const`. If your functor does not implement that,
76 * compilation will fail. If you do not require to be able to invoke the
77 * wrapped function in a const context, you can wrap any functor that
78 * implements either or both of const and non-const `operator()`.
80 * The template parameter of `folly::Function`, the `FunctionType`, can be
81 * const-qualified. Be aware that the const is part of the function signature.
82 * It does not mean that the function type is a const type.
84 * using FunctionType = R(Args...);
85 * using ConstFunctionType = R(Args...) const;
87 * In this example, `FunctionType` and `ConstFunctionType` are different
88 * types. `ConstFunctionType` is not the same as `const FunctionType`.
89 * As a matter of fact, trying to use the latter should emit a compiler
90 * warning or error, because it has no defined meaning.
92 * // This will not compile:
93 * folly::Function<void(void) const> func = Foo();
94 * // because Foo does not have a member function of the form:
95 * // void operator()() const;
97 * // This will compile just fine:
98 * folly::Function<void(void)> func = Foo();
99 * // and it will wrap the existing member function:
100 * // void operator()();
102 * When should a const function type be used? As a matter of fact, you will
103 * probably not need to use const function types very often. See the following
107 * folly::Function<void()> func_;
108 * folly::Function<void() const> constFunc_;
110 * void someMethod() {
113 * // Can call constFunc_.
117 * void someConstMethod() const {
118 * // Can call constFunc_.
120 * // However, cannot call func_ because a non-const method cannot
121 * // be called from a const one.
125 * As you can see, whether the `folly::Function`'s function type should
126 * be declared const or not is identical to whether a corresponding method
127 * would be declared const or not.
129 * You only require a `folly::Function` to hold a const function type, if you
130 * intend to invoke it from within a const context. This is to ensure that
131 * you cannot mutate its inner state when calling in a const context.
133 * This is how the const/non-const choice relates to lambda functions:
135 * // Non-mutable lambdas: can be stored in a non-const...
136 * folly::Function<void(int)> print_number =
137 * [] (int number) { std::cout << number << std::endl; };
139 * // ...as well as in a const folly::Function
140 * folly::Function<void(int) const> print_number_const =
141 * [] (int number) { std::cout << number << std::endl; };
143 * // Mutable lambda: can only be stored in a non-const folly::Function:
145 * folly::Function<void()> print_number =
146 * [number] () mutable { std::cout << ++number << std::endl; };
147 * // Trying to store the above mutable lambda in a
148 * // `folly::Function<void() const>` would lead to a compiler error:
149 * // error: no viable conversion from '(lambda at ...)' to
150 * // 'folly::Function<void () const>'
152 * Casting between const and non-const `folly::Function`s:
153 * conversion from const to non-const signatures happens implicitly. Any
154 * function that takes a `folly::Function<R(Args...)>` can be passed
155 * a `folly::Function<R(Args...) const>` without explicit conversion.
156 * This is safe, because casting from const to non-const only entails giving
157 * up the ability to invoke the function from a const context.
158 * Casting from a non-const to a const signature is potentially dangerous,
159 * as it means that a function that may change its inner state when invoked
160 * is made possible to call from a const context. Therefore this cast does
161 * not happen implicitly. The function `folly::constCastFunction` can
162 * be used to perform the cast.
164 * // Mutable lambda: can only be stored in a non-const folly::Function:
166 * folly::Function<void()> print_number =
167 * [number] () mutable { std::cout << ++number << std::endl; };
169 * // const-cast to a const folly::Function:
170 * folly::Function<void() const> print_number_const =
171 * constCastFunction(std::move(print_number));
173 * When to use const function types?
174 * Generally, only when you need them. When you use a `folly::Function` as a
175 * member of a struct or class, only use a const function signature when you
176 * need to invoke the function from const context.
177 * When passing a `folly::Function` to a function, the function should accept
178 * a non-const `folly::Function` whenever possible, i.e. when it does not
179 * need to pass on or store a const `folly::Function`. This is the least
180 * possible constraint: you can always pass a const `folly::Function` when
181 * the function accepts a non-const one.
183 * How does the const behaviour compare to `std::function`?
184 * `std::function` can wrap object with non-const invokation behaviour but
185 * exposes them as const. The equivalent behaviour can be achieved with
186 * `folly::Function` like so:
188 * std::function<void(void)> stdfunc = someCallable;
190 * folly::Function<void(void) const> uniqfunc = constCastFunction(
191 * folly::Function<void(void)>(someCallable)
194 * You need to wrap the callable first in a non-const `folly::Function` to
195 * select a non-const invoke operator (or the const one if no non-const one is
196 * present), and then move it into a const `folly::Function` using
197 * `constCastFunction`.
198 * The name of `constCastFunction` should warn you that something
199 * potentially dangerous is happening. As a matter of fact, using
200 * `std::function` always involves this potentially dangerous aspect, which
201 * is why it is not considered fully const-safe or even const-correct.
202 * However, in most of the cases you will not need the dangerous aspect at all.
203 * Either you do not require invokation of the function from a const context,
204 * in which case you do not need to use `constCastFunction` and just
205 * use the inner `folly::Function` in the example above, i.e. just use a
206 * non-const `folly::Function`. Or, you may need invokation from const, but
207 * the callable you are wrapping does not mutate its state (e.g. it is a class
208 * object and implements `operator() const`, or it is a normal,
209 * non-mutable lambda), in which case you can wrap the callable in a const
210 * `folly::Function` directly, without using `constCastFunction`.
211 * Only if you require invokation from a const context of a callable that
212 * may mutate itself when invoked you have to go through the above procedure.
213 * However, in that case what you do is potentially dangerous and requires
214 * the equivalent of a `const_cast`, hence you need to call
215 * `constCastFunction`.
220 #include <functional>
223 #include <type_traits>
226 #include <folly/CppAttributes.h>
227 #include <folly/Portability.h>
228 #include <folly/Traits.h>
229 #include <folly/functional/Invoke.h>
233 template <typename FunctionType>
236 template <typename ReturnType, typename... Args>
237 Function<ReturnType(Args...) const> constCastFunction(
238 Function<ReturnType(Args...)>&&) noexcept;
240 #if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE
241 template <typename ReturnType, typename... Args>
242 Function<ReturnType(Args...) const noexcept> constCastFunction(
243 Function<ReturnType(Args...) noexcept>&&) noexcept;
249 enum class Op { MOVE, NUKE, FULL, HEAP };
253 std::aligned_storage<6 * sizeof(void*)>::type tiny;
256 template <typename Fun, typename = Fun*>
257 using IsSmall = Conjunction<
258 std::integral_constant<bool, (sizeof(Fun) <= sizeof(Data::tiny))>,
259 std::is_nothrow_move_constructible<Fun>>;
260 using SmallTag = std::true_type;
261 using HeapTag = std::false_type;
263 template <typename T>
264 struct NotFunction : std::true_type {};
265 template <typename T>
266 struct NotFunction<Function<T>> : std::false_type {};
268 template <typename T>
269 using EnableIfNotFunction =
270 typename std::enable_if<NotFunction<T>::value>::type;
274 template <typename T>
275 bool isNullPtrFn(T* p) {
278 template <typename T>
279 std::false_type isNullPtrFn(T&&) {
283 inline bool uninitNoop(Op, Data*, Data*) {
287 template <typename F, typename... Args>
288 using CallableResult = decltype(std::declval<F>()(std::declval<Args>()...));
293 typename = typename std::enable_if<
294 !std::is_reference<To>::value || std::is_reference<From>::value>::type>
295 using SafeResultOf = decltype(static_cast<To>(std::declval<From>()));
297 template <typename FunctionType>
298 struct FunctionTraits;
300 template <typename ReturnType, typename... Args>
301 struct FunctionTraits<ReturnType(Args...)> {
302 using Call = ReturnType (*)(Data&, Args&&...);
303 using IsConst = std::false_type;
304 using ConstSignature = ReturnType(Args...) const;
305 using NonConstSignature = ReturnType(Args...);
306 using OtherSignature = ConstSignature;
308 template <typename F>
310 SafeResultOf<CallableResult<_t<std::decay<F>>&, Args...>, ReturnType>;
312 template <typename Fun>
313 static ReturnType callSmall(Data& p, Args&&... args) {
314 return static_cast<ReturnType>((*static_cast<Fun*>(
315 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
318 template <typename Fun>
319 static ReturnType callBig(Data& p, Args&&... args) {
320 return static_cast<ReturnType>(
321 (*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
324 static ReturnType uninitCall(Data&, Args&&...) {
325 throw std::bad_function_call();
328 ReturnType operator()(Args... args) {
329 auto& fn = *static_cast<Function<NonConstSignature>*>(this);
330 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
334 std::shared_ptr<Function<NonConstSignature>> sp_;
337 explicit SharedProxy(Function<NonConstSignature>&& func)
338 : sp_(std::make_shared<Function<NonConstSignature>>(std::move(func))) {}
339 ReturnType operator()(Args&&... args) const {
340 return (*sp_)(static_cast<Args&&>(args)...);
345 template <typename ReturnType, typename... Args>
346 struct FunctionTraits<ReturnType(Args...) const> {
347 using Call = ReturnType (*)(Data&, Args&&...);
348 using IsConst = std::true_type;
349 using ConstSignature = ReturnType(Args...) const;
350 using NonConstSignature = ReturnType(Args...);
351 using OtherSignature = NonConstSignature;
353 template <typename F>
354 using ResultOf = SafeResultOf<
355 CallableResult<const _t<std::decay<F>>&, Args...>,
358 template <typename Fun>
359 static ReturnType callSmall(Data& p, Args&&... args) {
360 return static_cast<ReturnType>((*static_cast<const Fun*>(
361 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
364 template <typename Fun>
365 static ReturnType callBig(Data& p, Args&&... args) {
366 return static_cast<ReturnType>(
367 (*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
370 static ReturnType uninitCall(Data&, Args&&...) {
371 throw std::bad_function_call();
374 ReturnType operator()(Args... args) const {
375 auto& fn = *static_cast<const Function<ConstSignature>*>(this);
376 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
380 std::shared_ptr<Function<ConstSignature>> sp_;
383 explicit SharedProxy(Function<ConstSignature>&& func)
384 : sp_(std::make_shared<Function<ConstSignature>>(std::move(func))) {}
385 ReturnType operator()(Args&&... args) const {
386 return (*sp_)(static_cast<Args&&>(args)...);
391 #if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE
392 template <typename ReturnType, typename... Args>
393 struct FunctionTraits<ReturnType(Args...) noexcept> {
394 using Call = ReturnType (*)(Data&, Args&&...) noexcept;
395 using IsConst = std::false_type;
396 using ConstSignature = ReturnType(Args...) const noexcept;
397 using NonConstSignature = ReturnType(Args...) noexcept;
398 using OtherSignature = ConstSignature;
400 template <typename F>
402 SafeResultOf<CallableResult<_t<std::decay<F>>&, Args...>, ReturnType>;
404 template <typename Fun>
405 static ReturnType callSmall(Data& p, Args&&... args) noexcept {
406 return static_cast<ReturnType>((*static_cast<Fun*>(
407 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
410 template <typename Fun>
411 static ReturnType callBig(Data& p, Args&&... args) noexcept {
412 return static_cast<ReturnType>(
413 (*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
416 static ReturnType uninitCall(Data&, Args&&...) noexcept {
417 throw std::bad_function_call();
420 ReturnType operator()(Args... args) noexcept {
421 auto& fn = *static_cast<Function<NonConstSignature>*>(this);
422 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
426 std::shared_ptr<Function<NonConstSignature>> sp_;
429 explicit SharedProxy(Function<NonConstSignature>&& func)
430 : sp_(std::make_shared<Function<NonConstSignature>>(std::move(func))) {}
431 ReturnType operator()(Args&&... args) const {
432 return (*sp_)(static_cast<Args&&>(args)...);
437 template <typename ReturnType, typename... Args>
438 struct FunctionTraits<ReturnType(Args...) const noexcept> {
439 using Call = ReturnType (*)(Data&, Args&&...) noexcept;
440 using IsConst = std::true_type;
441 using ConstSignature = ReturnType(Args...) const noexcept;
442 using NonConstSignature = ReturnType(Args...) noexcept;
443 using OtherSignature = NonConstSignature;
445 template <typename F>
446 using ResultOf = SafeResultOf<
447 CallableResult<const _t<std::decay<F>>&, Args...>,
450 template <typename Fun>
451 static ReturnType callSmall(Data& p, Args&&... args) noexcept {
452 return static_cast<ReturnType>((*static_cast<const Fun*>(
453 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
456 template <typename Fun>
457 static ReturnType callBig(Data& p, Args&&... args) noexcept {
458 return static_cast<ReturnType>(
459 (*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
462 static ReturnType uninitCall(Data&, Args&&...) noexcept {
463 throw std::bad_function_call();
466 ReturnType operator()(Args... args) const noexcept {
467 auto& fn = *static_cast<const Function<ConstSignature>*>(this);
468 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
472 std::shared_ptr<Function<ConstSignature>> sp_;
475 explicit SharedProxy(Function<ConstSignature>&& func)
476 : sp_(std::make_shared<Function<ConstSignature>>(std::move(func))) {}
477 ReturnType operator()(Args&&... args) const {
478 return (*sp_)(static_cast<Args&&>(args)...);
484 template <typename Fun>
485 bool execSmall(Op o, Data* src, Data* dst) {
488 ::new (static_cast<void*>(&dst->tiny))
489 Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->tiny))));
492 static_cast<Fun*>(static_cast<void*>(&src->tiny))->~Fun();
502 template <typename Fun>
503 bool execBig(Op o, Data* src, Data* dst) {
510 delete static_cast<Fun*>(src->big);
519 } // namespace function
520 } // namespace detail
522 template <typename FunctionType>
523 class Function final : private detail::function::FunctionTraits<FunctionType> {
524 // These utility types are defined outside of the template to reduce
525 // the number of instantiations, and then imported in the class
526 // namespace for convenience.
527 using Data = detail::function::Data;
528 using Op = detail::function::Op;
529 using SmallTag = detail::function::SmallTag;
530 using HeapTag = detail::function::HeapTag;
531 using CoerceTag = detail::function::CoerceTag;
533 using Traits = detail::function::FunctionTraits<FunctionType>;
534 using Call = typename Traits::Call;
535 using Exec = bool (*)(Op, Data*, Data*);
537 template <typename Fun>
538 using IsSmall = detail::function::IsSmall<Fun>;
540 // The `data_` member is mutable to allow `constCastFunction` to work without
541 // invoking undefined behavior. Const-correctness is only violated when
542 // `FunctionType` is a const function type (e.g., `int() const`) and `*this`
543 // is the result of calling `constCastFunction`.
544 mutable Data data_{};
545 Call call_{&Traits::uninitCall};
546 Exec exec_{&detail::function::uninitNoop};
549 friend Function<typename Traits::ConstSignature> folly::constCastFunction<>(
550 Function<typename Traits::NonConstSignature>&&) noexcept;
551 friend class Function<typename Traits::OtherSignature>;
553 template <typename Fun>
554 Function(Fun&& fun, SmallTag) noexcept {
555 using FunT = typename std::decay<Fun>::type;
556 if (!detail::function::isNullPtrFn(fun)) {
557 ::new (static_cast<void*>(&data_.tiny)) FunT(static_cast<Fun&&>(fun));
558 call_ = &Traits::template callSmall<FunT>;
559 exec_ = &detail::function::execSmall<FunT>;
563 template <typename Fun>
564 Function(Fun&& fun, HeapTag) {
565 using FunT = typename std::decay<Fun>::type;
566 data_.big = new FunT(static_cast<Fun&&>(fun));
567 call_ = &Traits::template callBig<FunT>;
568 exec_ = &detail::function::execBig<FunT>;
571 template <typename Signature>
572 Function(Function<Signature>&& that, CoerceTag)
573 : Function(static_cast<Function<Signature>&&>(that), HeapTag{}) {}
576 Function<typename Traits::OtherSignature>&& that,
577 CoerceTag) noexcept {
578 that.exec_(Op::MOVE, &that.data_, &data_);
579 std::swap(call_, that.call_);
580 std::swap(exec_, that.exec_);
585 * Default constructor. Constructs an empty Function.
587 Function() = default;
590 Function(const Function&) = delete;
593 // Make sure Objective C blocks are copied
594 template <class ReturnType, class... Args>
595 /*implicit*/ Function(ReturnType (^objCBlock)(Args... args))
596 : Function([blockCopy = (ReturnType (^)(Args...))[objCBlock copy]](
597 Args... args) { return blockCopy(args...); }){};
603 Function(Function&& that) noexcept {
604 that.exec_(Op::MOVE, &that.data_, &data_);
605 std::swap(call_, that.call_);
606 std::swap(exec_, that.exec_);
610 * Constructs an empty `Function`.
612 /* implicit */ Function(std::nullptr_t) noexcept {}
615 * Constructs a new `Function` from any callable object that is _not_ a
616 * `folly::Function`. This handles function pointers, pointers to static
617 * member functions, `std::reference_wrapper` objects, `std::function`
618 * objects, and arbitrary objects that implement `operator()` if the parameter
619 * signature matches (i.e. it returns an object convertible to `R` when called
622 * \note `typename Traits::template ResultOf<Fun>` prevents this overload
623 * from being selected by overload resolution when `fun` is not a compatible
626 * \note The noexcept requires some explanation. `IsSmall` is true when the
627 * decayed type fits within the internal buffer and is noexcept-movable. But
628 * this ctor might copy, not move. What we need here, if this ctor does a
629 * copy, is that this ctor be noexcept when the copy is noexcept. That is not
630 * checked in `IsSmall`, and shouldn't be, because once the `Function` is
631 * constructed, the contained object is never copied. This check is for this
632 * ctor only, in the case that this ctor does a copy.
636 typename = detail::function::EnableIfNotFunction<Fun>,
637 typename = typename Traits::template ResultOf<Fun>>
638 /* implicit */ Function(Fun fun) noexcept(
639 IsSmall<Fun>::value && noexcept(Fun(std::declval<Fun>())))
640 : Function(std::move(fun), IsSmall<Fun>{}) {}
643 * For move-constructing from a `folly::Function<X(Ys...) [const?]>`.
644 * For a `Function` with a `const` function type, the object must be
645 * callable from a `const`-reference, i.e. implement `operator() const`.
646 * For a `Function` with a non-`const` function type, the object will
647 * be called from a non-const reference, which means that it will execute
648 * a non-const `operator()` if it is defined, and falls back to
649 * `operator() const` otherwise.
653 typename = typename Traits::template ResultOf<Function<Signature>>>
654 Function(Function<Signature>&& that) noexcept(
655 noexcept(Function(std::move(that), CoerceTag{})))
656 : Function(std::move(that), CoerceTag{}) {}
659 * If `ptr` is null, constructs an empty `Function`. Otherwise,
660 * this constructor is equivalent to `Function(std::mem_fn(ptr))`.
665 // Prevent this overload from being selected when `ptr` is not a
666 // compatible member function pointer.
667 typename = decltype(Function(std::mem_fn((Member Class::*)0)))>
668 /* implicit */ Function(Member Class::*ptr) noexcept {
670 *this = std::mem_fn(ptr);
675 exec_(Op::NUKE, &data_, nullptr);
678 Function& operator=(const Function&) = delete;
681 // Make sure Objective C blocks are copied
682 template <class ReturnType, class... Args>
683 /* implicit */ Function &operator=(ReturnType (^objCBlock)(Args... args)) {
684 (*this) = [blockCopy = (ReturnType (^)(Args...))[objCBlock copy]](
685 Args... args) { return blockCopy(args...); };
691 * Move assignment operator
693 * \note Leaves `that` in a valid but unspecified state. If `&that == this`
694 * then `*this` is left in a valid but unspecified state.
696 Function& operator=(Function&& that) noexcept {
697 // Q: Why is it safe to destroy and reconstruct this object in place?
698 // A: Two reasons: First, `Function` is a final class, so in doing this
699 // we aren't slicing off any derived parts. And second, the move
700 // operation is guaranteed not to throw so we always leave the object
702 // In the case of self-move (this == &that), this leaves the object in
703 // a default-constructed state. First the object is destroyed, then we
704 // pass the destroyed object to the move constructor. The first thing the
705 // move constructor does is default-construct the object. That object is
706 // "moved" into itself, which is a no-op for a default-constructed Function.
708 ::new (this) Function(std::move(that));
713 * Assigns a callable object to this `Function`. If the operation fails,
714 * `*this` is left unmodified.
716 * \note `typename = decltype(Function(std::declval<Fun>()))` prevents this
717 * overload from being selected by overload resolution when `fun` is not a
718 * compatible function.
720 template <typename Fun, typename = decltype(Function(std::declval<Fun>()))>
721 Function& operator=(Fun fun) noexcept(
722 noexcept(/* implicit */ Function(std::declval<Fun>()))) {
723 // Doing this in place is more efficient when we can do so safely.
724 if (noexcept(/* implicit */ Function(std::declval<Fun>()))) {
725 // Q: Why is is safe to destroy and reconstruct this object in place?
726 // A: See the explanation in the move assignment operator.
728 ::new (this) Function(std::move(fun));
730 // Construct a temporary and (nothrow) swap.
731 Function(std::move(fun)).swap(*this);
737 * For assigning from a `Function<X(Ys..) [const?]>`.
741 typename = typename Traits::template ResultOf<Function<Signature>>>
742 Function& operator=(Function<Signature>&& that) noexcept(
743 noexcept(Function(std::move(that)))) {
744 return (*this = Function(std::move(that)));
748 * Clears this `Function`.
750 Function& operator=(std::nullptr_t) noexcept {
751 return (*this = Function());
755 * If `ptr` is null, clears this `Function`. Otherwise, this assignment
756 * operator is equivalent to `*this = std::mem_fn(ptr)`.
758 template <typename Member, typename Class>
759 auto operator=(Member Class::*ptr) noexcept
760 // Prevent this overload from being selected when `ptr` is not a
761 // compatible member function pointer.
762 -> decltype(operator=(std::mem_fn(ptr))) {
763 return ptr ? (*this = std::mem_fn(ptr)) : (*this = Function());
767 * Call the wrapped callable object with the specified arguments.
769 using Traits::operator();
772 * Exchanges the callable objects of `*this` and `that`.
774 void swap(Function& that) noexcept {
775 std::swap(*this, that);
779 * Returns `true` if this `Function` contains a callable, i.e. is
782 explicit operator bool() const noexcept {
783 return exec_(Op::FULL, nullptr, nullptr);
787 * Returns `true` if this `Function` stores the callable on the
788 * heap. If `false` is returned, there has been no additional memory
789 * allocation and the callable is stored inside the `Function`
792 bool hasAllocatedMemory() const noexcept {
793 return exec_(Op::HEAP, nullptr, nullptr);
796 using typename Traits::SharedProxy;
799 * Move this `Function` into a copyable callable object, of which all copies
802 SharedProxy asSharedProxy() && {
803 return SharedProxy{std::move(*this)};
807 * Construct a `std::function` by moving in the contents of this `Function`.
808 * Note that the returned `std::function` will share its state (i.e. captured
809 * data) across all copies you make of it, so be very careful when copying.
811 std::function<typename Traits::NonConstSignature> asStdFunction() && {
812 return std::move(*this).asSharedProxy();
816 template <typename FunctionType>
817 void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {
821 template <typename FunctionType>
822 bool operator==(const Function<FunctionType>& fn, std::nullptr_t) {
826 template <typename FunctionType>
827 bool operator==(std::nullptr_t, const Function<FunctionType>& fn) {
831 template <typename FunctionType>
832 bool operator!=(const Function<FunctionType>& fn, std::nullptr_t) {
833 return !(fn == nullptr);
836 template <typename FunctionType>
837 bool operator!=(std::nullptr_t, const Function<FunctionType>& fn) {
838 return !(nullptr == fn);
842 * NOTE: See detailed note about `constCastFunction` at the top of the file.
843 * This is potentially dangerous and requires the equivalent of a `const_cast`.
845 template <typename ReturnType, typename... Args>
846 Function<ReturnType(Args...) const> constCastFunction(
847 Function<ReturnType(Args...)>&& that) noexcept {
848 return Function<ReturnType(Args...) const>{std::move(that),
849 detail::function::CoerceTag{}};
852 template <typename ReturnType, typename... Args>
853 Function<ReturnType(Args...) const> constCastFunction(
854 Function<ReturnType(Args...) const>&& that) noexcept {
855 return std::move(that);
858 #if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE
859 template <typename ReturnType, typename... Args>
860 Function<ReturnType(Args...) const noexcept> constCastFunction(
861 Function<ReturnType(Args...) noexcept>&& that) noexcept {
862 return Function<ReturnType(Args...) const noexcept>{
863 std::move(that), detail::function::CoerceTag{}};
866 template <typename ReturnType, typename... Args>
867 Function<ReturnType(Args...) const noexcept> constCastFunction(
868 Function<ReturnType(Args...) const noexcept>&& that) noexcept {
869 return std::move(that);
875 template <typename Fun, typename FunctionType, typename = void>
876 struct IsCallableAsImpl : std::false_type {};
878 template <typename Fun, typename ReturnType, typename... Args>
879 struct IsCallableAsImpl<
882 void_t<typename std::result_of<Fun && (Args && ...)>::type>>
883 : std::is_convertible<
884 typename std::result_of<Fun && (Args && ...)>::type,
887 template <typename Fun, typename FunctionType>
888 struct IsCallableAs : IsCallableAsImpl<Fun, FunctionType> {};
889 } // namespace function
890 } // namespace detail
895 * @brief A reference wrapper for callable objects
897 * FunctionRef is similar to std::reference_wrapper, but the template parameter
898 * is the function signature type rather than the type of the referenced object.
899 * A folly::FunctionRef is cheap to construct as it contains only a pointer to
900 * the referenced callable and a pointer to a function which invokes the
903 * The user of FunctionRef must be aware of the reference semantics: storing a
904 * copy of a FunctionRef is potentially dangerous and should be avoided unless
905 * the referenced object definitely outlives the FunctionRef object. Thus any
906 * function that accepts a FunctionRef parameter should only use it to invoke
907 * the referenced function and not store a copy of it. Knowing that FunctionRef
908 * itself has reference semantics, it is generally okay to use it to reference
909 * lambdas that capture by reference.
912 template <typename FunctionType>
915 template <typename ReturnType, typename... Args>
916 class FunctionRef<ReturnType(Args...)> final {
917 using Call = ReturnType (*)(void*, Args&&...);
919 static ReturnType uninitCall(void*, Args&&...) {
920 throw std::bad_function_call();
923 template <typename Fun>
924 static ReturnType call(void* object, Args&&... args) {
925 using Pointer = _t<std::add_pointer<Fun>>;
926 return static_cast<ReturnType>(invoke(
927 static_cast<Fun&&>(*static_cast<Pointer>(object)),
928 static_cast<Args&&>(args)...));
931 void* object_{nullptr};
932 Call call_{&FunctionRef::uninitCall};
936 * Default constructor. Constructs an empty FunctionRef.
938 * Invoking it will throw std::bad_function_call.
940 FunctionRef() = default;
943 * Construct a FunctionRef from a reference to a callable object.
947 typename std::enable_if<
949 Negation<std::is_same<FunctionRef, _t<std::decay<Fun>>>>,
950 detail::function::IsCallableAs<Fun, ReturnType(Args...)>>::value,
952 constexpr /* implicit */ FunctionRef(Fun&& fun) noexcept
953 // `Fun` may be a const type, in which case we have to do a const_cast
954 // to store the address in a `void*`. This is safe because the `void*`
955 // will be cast back to `Fun*` (which is a const pointer whenever `Fun`
956 // is a const type) inside `FunctionRef::call`
958 const_cast<void*>(static_cast<void const*>(std::addressof(fun)))),
959 call_(&FunctionRef::call<Fun>) {}
961 ReturnType operator()(Args... args) const {
962 return call_(object_, static_cast<Args&&>(args)...);
965 constexpr explicit operator bool() const {