2 * Copyright 2016 Facebook, Inc.
4 * @author Eric Niebler (eniebler@fb.com), Sven Over (over@fb.com)
6 * Licensed under the Apache License, Version 2.0 (the "License");
7 * you may not use this file except in compliance with the License.
8 * You may obtain a copy of the License at
10 * http://www.apache.org/licenses/LICENSE-2.0
12 * Unless required by applicable law or agreed to in writing, software
13 * distributed under the License is distributed on an "AS IS" BASIS,
14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15 * See the License for the specific language governing permissions and
16 * limitations under the License.
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>
231 template <typename FunctionType>
234 template <typename ReturnType, typename... Args>
235 Function<ReturnType(Args...) const> constCastFunction(
236 Function<ReturnType(Args...)>&&) noexcept;
241 enum class Op { MOVE, NUKE, FULL, HEAP };
245 std::aligned_storage<6 * sizeof(void*)>::type tiny;
248 template <typename Fun, typename FunT = typename std::decay<Fun>::type>
249 using IsSmall = std::integral_constant<
251 (sizeof(FunT) <= sizeof(Data::tiny) &&
252 // Same as is_nothrow_move_constructible, but w/ no template instantiation.
253 noexcept(FunT(std::declval<FunT&&>())))>;
254 using SmallTag = std::true_type;
255 using HeapTag = std::false_type;
259 template <typename T>
260 bool isNullPtrFn(T* p) {
263 template <typename T>
264 std::false_type isNullPtrFn(T&&) {
268 inline bool uninitNoop(Op, Data*, Data*) {
272 template <typename FunctionType>
273 struct FunctionTraits;
275 template <typename ReturnType, typename... Args>
276 struct FunctionTraits<ReturnType(Args...)> {
277 using Call = ReturnType (*)(Data&, Args&&...);
278 using IsConst = std::false_type;
279 using ConstSignature = ReturnType(Args...) const;
280 using NonConstSignature = ReturnType(Args...);
281 using OtherSignature = ConstSignature;
283 template <typename F, typename G = typename std::decay<F>::type>
284 using ResultOf = decltype(
285 static_cast<ReturnType>(std::declval<G&>()(std::declval<Args>()...)));
287 template <typename Fun>
288 static ReturnType callSmall(Data& p, Args&&... args) {
289 return static_cast<ReturnType>((*static_cast<Fun*>(
290 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
293 template <typename Fun>
294 static ReturnType callBig(Data& p, Args&&... args) {
295 return static_cast<ReturnType>(
296 (*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
299 static ReturnType uninitCall(Data&, Args&&...) {
300 throw std::bad_function_call();
303 ReturnType operator()(Args... args) {
304 auto& fn = *static_cast<Function<ReturnType(Args...)>*>(this);
305 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
308 struct SharedFunctionImpl {
309 std::shared_ptr<Function<ReturnType(Args...)>> sp_;
310 ReturnType operator()(Args&&... args) const {
311 return (*sp_)(static_cast<Args&&>(args)...);
316 template <typename ReturnType, typename... Args>
317 struct FunctionTraits<ReturnType(Args...) const> {
318 using Call = ReturnType (*)(Data&, Args&&...);
319 using IsConst = std::true_type;
320 using ConstSignature = ReturnType(Args...) const;
321 using NonConstSignature = ReturnType(Args...);
322 using OtherSignature = NonConstSignature;
324 template <typename F, typename G = typename std::decay<F>::type>
325 using ResultOf = decltype(static_cast<ReturnType>(
326 std::declval<const G&>()(std::declval<Args>()...)));
328 template <typename Fun>
329 static ReturnType callSmall(Data& p, Args&&... args) {
330 return static_cast<ReturnType>((*static_cast<const Fun*>(
331 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
334 template <typename Fun>
335 static ReturnType callBig(Data& p, Args&&... args) {
336 return static_cast<ReturnType>(
337 (*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
340 static ReturnType uninitCall(Data&, Args&&...) {
341 throw std::bad_function_call();
344 ReturnType operator()(Args... args) const {
345 auto& fn = *static_cast<const Function<ReturnType(Args...) const>*>(this);
346 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
349 struct SharedFunctionImpl {
350 std::shared_ptr<Function<ReturnType(Args...) const>> sp_;
351 ReturnType operator()(Args&&... args) const {
352 return (*sp_)(static_cast<Args&&>(args)...);
357 template <typename Fun>
358 bool execSmall(Op o, Data* src, Data* dst) {
361 ::new (static_cast<void*>(&dst->tiny))
362 Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->tiny))));
365 static_cast<Fun*>(static_cast<void*>(&src->tiny))->~Fun();
375 template <typename Fun>
376 bool execBig(Op o, Data* src, Data* dst) {
383 delete static_cast<Fun*>(src->big);
393 template <typename F, typename... Args>
394 inline auto invoke(F&& f, Args&&... args)
395 -> decltype(std::forward<F>(f)(std::forward<Args>(args)...)) {
396 return std::forward<F>(f)(std::forward<Args>(args)...);
399 template <typename M, typename C, typename... Args>
400 inline auto invoke(M(C::*d), Args&&... args)
401 -> decltype(std::mem_fn(d)(std::forward<Args>(args)...)) {
402 return std::mem_fn(d)(std::forward<Args>(args)...);
405 } // namespace function
406 } // namespace detail
409 FOLLY_MSVC_DISABLE_WARNING(4521) // Multiple copy constructors
410 FOLLY_MSVC_DISABLE_WARNING(4522) // Multiple assignment operators
411 template <typename FunctionType>
412 class Function final : private detail::function::FunctionTraits<FunctionType> {
413 // These utility types are defined outside of the template to reduce
414 // the number of instantiations, and then imported in the class
415 // namespace for convenience.
416 using Data = detail::function::Data;
417 using Op = detail::function::Op;
418 using SmallTag = detail::function::SmallTag;
419 using HeapTag = detail::function::HeapTag;
420 using CoerceTag = detail::function::CoerceTag;
422 using Traits = detail::function::FunctionTraits<FunctionType>;
423 using Call = typename Traits::Call;
424 using Exec = bool (*)(Op, Data*, Data*);
426 template <typename Fun>
427 using IsSmall = detail::function::IsSmall<Fun>;
429 using OtherSignature = typename Traits::OtherSignature;
431 // The `data_` member is mutable to allow `constCastFunction` to work without
432 // invoking undefined behavior. Const-correctness is only violated when
433 // `FunctionType` is a const function type (e.g., `int() const`) and `*this`
434 // is the result of calling `constCastFunction`.
436 Call call_{&Traits::uninitCall};
437 Exec exec_{&detail::function::uninitNoop};
440 friend Function<typename Traits::ConstSignature> folly::constCastFunction<>(
441 Function<typename Traits::NonConstSignature>&&) noexcept;
442 friend class Function<OtherSignature>;
444 template <typename Fun>
445 Function(Fun&& fun, SmallTag) noexcept {
446 using FunT = typename std::decay<Fun>::type;
447 if (!detail::function::isNullPtrFn(fun)) {
448 ::new (static_cast<void*>(&data_.tiny)) FunT(static_cast<Fun&&>(fun));
449 call_ = &Traits::template callSmall<FunT>;
450 exec_ = &detail::function::execSmall<FunT>;
454 template <typename Fun>
455 Function(Fun&& fun, HeapTag) {
456 using FunT = typename std::decay<Fun>::type;
457 data_.big = new FunT(static_cast<Fun&&>(fun));
458 call_ = &Traits::template callBig<FunT>;
459 exec_ = &detail::function::execBig<FunT>;
462 Function(Function<OtherSignature>&& that, CoerceTag) noexcept {
463 that.exec_(Op::MOVE, &that.data_, &data_);
464 std::swap(call_, that.call_);
465 std::swap(exec_, that.exec_);
470 * Default constructor. Constructs an empty Function.
472 Function() = default;
475 // NOTE: Deleting the non-const copy constructor is unusual but necessary to
476 // prevent copies from non-const `Function` object from selecting the
477 // perfect forwarding implicit converting constructor below
478 // (i.e., `template <typename Fun> Function(Fun&&)`).
479 Function(Function&) = delete;
480 Function(const Function&) = delete;
481 Function(const Function&&) = delete;
486 Function(Function&& that) noexcept {
487 that.exec_(Op::MOVE, &that.data_, &data_);
488 std::swap(call_, that.call_);
489 std::swap(exec_, that.exec_);
493 * Constructs an empty `Function`.
495 /* implicit */ Function(std::nullptr_t) noexcept {}
498 * Constructs a new `Function` from any callable object. This
499 * handles function pointers, pointers to static member functions,
500 * `std::reference_wrapper` objects, `std::function` objects, and arbitrary
501 * objects that implement `operator()` if the parameter signature
502 * matches (i.e. it returns R when called with Args...).
503 * For a `Function` with a const function type, the object must be
504 * callable from a const-reference, i.e. implement `operator() const`.
505 * For a `Function` with a non-const function type, the object will
506 * be called from a non-const reference, which means that it will execute
507 * a non-const `operator()` if it is defined, and falls back to
508 * `operator() const` otherwise.
510 * \note `typename = ResultOf<Fun>` prevents this overload from being
511 * selected by overload resolution when `fun` is not a compatible function.
513 template <class Fun, typename = typename Traits::template ResultOf<Fun>>
514 /* implicit */ Function(Fun&& fun) noexcept(IsSmall<Fun>::value)
515 : Function(static_cast<Fun&&>(fun), IsSmall<Fun>{}) {}
518 * For moving a `Function<X(Ys..) const>` into a `Function<X(Ys...)>`.
521 bool Const = Traits::IsConst::value,
522 typename std::enable_if<!Const, int>::type = 0>
523 Function(Function<OtherSignature>&& that) noexcept
524 : Function(std::move(that), CoerceTag{}) {}
527 * If `ptr` is null, constructs an empty `Function`. Otherwise,
528 * this constructor is equivalent to `Function(std::mem_fn(ptr))`.
533 // Prevent this overload from being selected when `ptr` is not a
534 // compatible member function pointer.
535 typename = decltype(Function(std::mem_fn((Member Class::*)0)))>
536 /* implicit */ Function(Member Class::*ptr) noexcept {
538 *this = std::mem_fn(ptr);
543 exec_(Op::NUKE, &data_, nullptr);
546 Function& operator=(Function&) = delete;
547 Function& operator=(const Function&) = delete;
550 * Move assignment operator
552 Function& operator=(Function&& that) noexcept {
554 // Q: Why is is safe to destroy and reconstruct this object in place?
555 // A: Two reasons: First, `Function` is a final class, so in doing this
556 // we aren't slicing off any derived parts. And second, the move
557 // operation is guaranteed not to throw so we always leave the object
560 ::new (this) Function(std::move(that));
566 * Assigns a callable object to this `Function`. If the operation fails,
567 * `*this` is left unmodified.
569 * \note `typename = ResultOf<Fun>` prevents this overload from being
570 * selected by overload resolution when `fun` is not a compatible function.
572 template <class Fun, typename = typename Traits::template ResultOf<Fun>>
573 Function& operator=(Fun&& fun) noexcept(
574 noexcept(/* implicit */ Function(std::declval<Fun>()))) {
575 // Doing this in place is more efficient when we can do so safely.
576 if (noexcept(/* implicit */ Function(std::declval<Fun>()))) {
577 // Q: Why is is safe to destroy and reconstruct this object in place?
578 // A: See the explanation in the move assignment operator.
580 ::new (this) Function(static_cast<Fun&&>(fun));
582 // Construct a temporary and (nothrow) swap.
583 Function(static_cast<Fun&&>(fun)).swap(*this);
589 * Clears this `Function`.
591 Function& operator=(std::nullptr_t) noexcept {
592 return (*this = Function());
596 * If `ptr` is null, clears this `Function`. Otherwise, this assignment
597 * operator is equivalent to `*this = std::mem_fn(ptr)`.
599 template <typename Member, typename Class>
600 auto operator=(Member Class::*ptr) noexcept
601 // Prevent this overload from being selected when `ptr` is not a
602 // compatible member function pointer.
603 -> decltype(operator=(std::mem_fn(ptr))) {
604 return ptr ? (*this = std::mem_fn(ptr)) : (*this = Function());
608 * Call the wrapped callable object with the specified arguments.
610 using Traits::operator();
613 * Exchanges the callable objects of `*this` and `that`.
615 void swap(Function& that) noexcept {
616 std::swap(*this, that);
620 * Returns `true` if this `Function` contains a callable, i.e. is
623 explicit operator bool() const noexcept {
624 return exec_(Op::FULL, nullptr, nullptr);
628 * Returns `true` if this `Function` stores the callable on the
629 * heap. If `false` is returned, there has been no additional memory
630 * allocation and the callable is stored inside the `Function`
633 bool hasAllocatedMemory() const noexcept {
634 return exec_(Op::HEAP, nullptr, nullptr);
638 * Construct a `std::function` by moving in the contents of this `Function`.
639 * Note that the returned `std::function` will share its state (i.e. captured
640 * data) across all copies you make of it, so be very careful when copying.
642 std::function<typename Traits::NonConstSignature> asStdFunction() && {
643 using Impl = typename Traits::SharedFunctionImpl;
644 return Impl{std::make_shared<Function>(std::move(*this))};
649 template <typename FunctionType>
650 void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {
654 template <typename FunctionType>
655 bool operator==(const Function<FunctionType>& fn, std::nullptr_t) {
659 template <typename FunctionType>
660 bool operator==(std::nullptr_t, const Function<FunctionType>& fn) {
664 template <typename FunctionType>
665 bool operator!=(const Function<FunctionType>& fn, std::nullptr_t) {
666 return !(fn == nullptr);
669 template <typename FunctionType>
670 bool operator!=(std::nullptr_t, const Function<FunctionType>& fn) {
671 return !(nullptr == fn);
675 * NOTE: See detailed note about `constCastFunction` at the top of the file.
676 * This is potentially dangerous and requires the equivalent of a `const_cast`.
678 template <typename ReturnType, typename... Args>
679 Function<ReturnType(Args...) const> constCastFunction(
680 Function<ReturnType(Args...)>&& that) noexcept {
681 return Function<ReturnType(Args...) const>{std::move(that),
682 detail::function::CoerceTag{}};
685 template <typename ReturnType, typename... Args>
686 Function<ReturnType(Args...) const> constCastFunction(
687 Function<ReturnType(Args...) const>&& that) noexcept {
688 return std::move(that);
694 * @brief A reference wrapper for callable objects
696 * FunctionRef is similar to std::reference_wrapper, but the template parameter
697 * is the function signature type rather than the type of the referenced object.
698 * A folly::FunctionRef is cheap to construct as it contains only a pointer to
699 * the referenced callable and a pointer to a function which invokes the
702 * The user of FunctionRef must be aware of the reference semantics: storing a
703 * copy of a FunctionRef is potentially dangerous and should be avoided unless
704 * the referenced object definitely outlives the FunctionRef object. Thus any
705 * function that accepts a FunctionRef parameter should only use it to invoke
706 * the referenced function and not store a copy of it. Knowing that FunctionRef
707 * itself has reference semantics, it is generally okay to use it to reference
708 * lambdas that capture by reference.
711 template <typename FunctionType>
714 template <typename ReturnType, typename... Args>
715 class FunctionRef<ReturnType(Args...)> final {
716 using Call = ReturnType (*)(void*, Args&&...);
718 void* object_{nullptr};
719 Call call_{&FunctionRef::uninitCall};
721 static ReturnType uninitCall(void*, Args&&...) {
722 throw std::bad_function_call();
725 template <typename Fun>
726 static ReturnType call(void* object, Args&&... args) {
727 return static_cast<ReturnType>(detail::function::invoke(
728 *static_cast<Fun*>(object), static_cast<Args&&>(args)...));
733 * Default constructor. Constructs an empty FunctionRef.
735 * Invoking it will throw std::bad_function_call.
737 FunctionRef() = default;
740 * Construct a FunctionRef from a reference to a callable object.
742 template <typename Fun>
743 /* implicit */ FunctionRef(Fun&& fun) noexcept {
744 using ReferencedType = typename std::remove_reference<Fun>::type;
748 typename std::result_of<ReferencedType&(Args && ...)>::type,
750 "FunctionRef cannot be constructed from object with "
751 "incompatible function signature");
753 // `Fun` may be a const type, in which case we have to do a const_cast
754 // to store the address in a `void*`. This is safe because the `void*`
755 // will be cast back to `Fun*` (which is a const pointer whenever `Fun`
756 // is a const type) inside `FunctionRef::call`
757 object_ = const_cast<void*>(static_cast<void const*>(std::addressof(fun)));
758 call_ = &FunctionRef::call<ReferencedType>;
761 ReturnType operator()(Args... args) const {
762 return call_(object_, static_cast<Args&&>(args)...);