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&&>()))
255 using SmallTag = std::true_type;
256 using HeapTag = std::false_type;
260 template <typename T>
261 bool isNullPtrFn(T* p) {
264 template <typename T>
265 std::false_type isNullPtrFn(T&&) {
269 inline bool uninitNoop(Op, Data*, Data*) {
273 template <typename FunctionType>
274 struct FunctionTraits;
276 template <typename ReturnType, typename... Args>
277 struct FunctionTraits<ReturnType(Args...)> {
278 using Call = ReturnType (*)(Data&, Args&&...);
279 using IsConst = std::false_type;
280 using ConstSignature = ReturnType(Args...) const;
281 using NonConstSignature = ReturnType(Args...);
282 using OtherSignature = ConstSignature;
284 template <typename F, typename G = typename std::decay<F>::type>
285 using ResultOf = decltype(
286 static_cast<ReturnType>(std::declval<G&>()(std::declval<Args>()...)));
288 template <typename Fun>
289 static ReturnType callSmall(Data& p, Args&&... args) {
290 return static_cast<ReturnType>((*static_cast<Fun*>(
291 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
294 template <typename Fun>
295 static ReturnType callBig(Data& p, Args&&... args) {
296 return static_cast<ReturnType>(
297 (*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
300 static ReturnType uninitCall(Data&, Args&&...) {
301 throw std::bad_function_call();
304 ReturnType operator()(Args... args) {
305 auto& fn = *static_cast<Function<ReturnType(Args...)>*>(this);
306 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
309 struct SharedFunctionImpl {
310 std::shared_ptr<Function<ReturnType(Args...)>> sp_;
311 ReturnType operator()(Args&&... args) const {
312 return (*sp_)(static_cast<Args&&>(args)...);
317 template <typename ReturnType, typename... Args>
318 struct FunctionTraits<ReturnType(Args...) const> {
319 using Call = ReturnType (*)(Data&, Args&&...);
320 using IsConst = std::true_type;
321 using ConstSignature = ReturnType(Args...) const;
322 using NonConstSignature = ReturnType(Args...);
323 using OtherSignature = NonConstSignature;
325 template <typename F, typename G = typename std::decay<F>::type>
326 using ResultOf = decltype(static_cast<ReturnType>(
327 std::declval<const G&>()(std::declval<Args>()...)));
329 template <typename Fun>
330 static ReturnType callSmall(Data& p, Args&&... args) {
331 return static_cast<ReturnType>((*static_cast<const Fun*>(
332 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
335 template <typename Fun>
336 static ReturnType callBig(Data& p, Args&&... args) {
337 return static_cast<ReturnType>(
338 (*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
341 static ReturnType uninitCall(Data&, Args&&...) {
342 throw std::bad_function_call();
345 ReturnType operator()(Args... args) const {
346 auto& fn = *static_cast<const Function<ReturnType(Args...) const>*>(this);
347 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
350 struct SharedFunctionImpl {
351 std::shared_ptr<Function<ReturnType(Args...) const>> sp_;
352 ReturnType operator()(Args&&... args) const {
353 return (*sp_)(static_cast<Args&&>(args)...);
358 template <typename Fun>
359 bool execSmall(Op o, Data* src, Data* dst) {
362 ::new (static_cast<void*>(&dst->tiny))
363 Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->tiny))));
366 static_cast<Fun*>(static_cast<void*>(&src->tiny))->~Fun();
376 template <typename Fun>
377 bool execBig(Op o, Data* src, Data* dst) {
384 delete static_cast<Fun*>(src->big);
393 } // namespace function
394 } // namespace detail
397 FOLLY_MSVC_DISABLE_WARNING(4521) // Multiple copy constructors
398 FOLLY_MSVC_DISABLE_WARNING(4522) // Multiple assignment operators
399 template <typename FunctionType>
400 class Function final : private detail::function::FunctionTraits<FunctionType> {
401 // These utility types are defined outside of the template to reduce
402 // the number of instantiations, and then imported in the class
403 // namespace for convenience.
404 using Data = detail::function::Data;
405 using Op = detail::function::Op;
406 using SmallTag = detail::function::SmallTag;
407 using HeapTag = detail::function::HeapTag;
408 using CoerceTag = detail::function::CoerceTag;
410 using Traits = detail::function::FunctionTraits<FunctionType>;
411 using Call = typename Traits::Call;
412 using Exec = bool (*)(Op, Data*, Data*);
414 template <typename Fun>
415 using IsSmall = detail::function::IsSmall<Fun>;
417 using OtherSignature = typename Traits::OtherSignature;
419 // The `data_` member is mutable to allow `constCastFunction` to work without
420 // invoking undefined behavior. Const-correctness is only violated when
421 // `FunctionType` is a const function type (e.g., `int() const`) and `*this`
422 // is the result of calling `constCastFunction`.
424 Call call_{&Traits::uninitCall};
425 Exec exec_{&detail::function::uninitNoop};
428 friend Function<typename Traits::ConstSignature> folly::constCastFunction<>(
429 Function<typename Traits::NonConstSignature>&&) noexcept;
430 friend class Function<OtherSignature>;
432 template <typename Fun>
433 Function(Fun&& fun, SmallTag) noexcept {
434 using FunT = typename std::decay<Fun>::type;
435 if (!detail::function::isNullPtrFn(fun)) {
436 ::new (static_cast<void*>(&data_.tiny)) FunT(static_cast<Fun&&>(fun));
437 call_ = &Traits::template callSmall<FunT>;
438 exec_ = &detail::function::execSmall<FunT>;
442 template <typename Fun>
443 Function(Fun&& fun, HeapTag) {
444 using FunT = typename std::decay<Fun>::type;
445 data_.big = new FunT(static_cast<Fun&&>(fun));
446 call_ = &Traits::template callBig<FunT>;
447 exec_ = &detail::function::execBig<FunT>;
450 Function(Function<OtherSignature>&& that, CoerceTag) noexcept {
451 that.exec_(Op::MOVE, &that.data_, &data_);
452 std::swap(call_, that.call_);
453 std::swap(exec_, that.exec_);
458 * Default constructor. Constructs an empty Function.
460 Function() = default;
463 // NOTE: Deleting the non-const copy constructor is unusual but necessary to
464 // prevent copies from non-const `Function` object from selecting the
465 // perfect forwarding implicit converting constructor below
466 // (i.e., `template <typename Fun> Function(Fun&&)`).
467 Function(Function&) = delete;
468 Function(const Function&) = delete;
473 Function(Function&& that) noexcept {
474 that.exec_(Op::MOVE, &that.data_, &data_);
475 std::swap(call_, that.call_);
476 std::swap(exec_, that.exec_);
480 * Constructs an empty `Function`.
482 /* implicit */ Function(std::nullptr_t) noexcept {}
485 * Constructs a new `Function` from any callable object. This
486 * handles function pointers, pointers to static member functions,
487 * `std::reference_wrapper` objects, `std::function` objects, and arbitrary
488 * objects that implement `operator()` if the parameter signature
489 * matches (i.e. it returns R when called with Args...).
490 * For a `Function` with a const function type, the object must be
491 * callable from a const-reference, i.e. implement `operator() const`.
492 * For a `Function` with a non-const function type, the object will
493 * be called from a non-const reference, which means that it will execute
494 * a non-const `operator()` if it is defined, and falls back to
495 * `operator() const` otherwise.
497 * \note `typename = ResultOf<Fun>` prevents this overload from being
498 * selected by overload resolution when `fun` is not a compatible function.
500 template <class Fun, typename = typename Traits::template ResultOf<Fun>>
501 /* implicit */ Function(Fun&& fun) noexcept(IsSmall<Fun>::value)
502 : Function(static_cast<Fun&&>(fun), IsSmall<Fun>{}) {}
505 * For moving a `Function<X(Ys..) const>` into a `Function<X(Ys...)>`.
508 bool Const = Traits::IsConst::value,
509 typename std::enable_if<!Const, int>::type = 0>
510 Function(Function<OtherSignature>&& that) noexcept
511 : Function(std::move(that), CoerceTag{}) {}
514 * If `ptr` is null, constructs an empty `Function`. Otherwise,
515 * this constructor is equivalent to `Function(std::mem_fn(ptr))`.
520 // Prevent this overload from being selected when `ptr` is not a
521 // compatible member function pointer.
522 typename = decltype(Function(std::mem_fn((Member Class::*)0)))>
523 /* implicit */ Function(Member Class::*ptr) noexcept {
525 *this = std::mem_fn(ptr);
530 exec_(Op::NUKE, &data_, nullptr);
533 Function& operator=(Function&) = delete;
534 Function& operator=(const Function&) = delete;
537 * Move assignment operator
539 Function& operator=(Function&& that) noexcept {
541 // Q: Why is is safe to destroy and reconstruct this object in place?
542 // A: Two reasons: First, `Function` is a final class, so in doing this
543 // we aren't slicing off any derived parts. And second, the move
544 // operation is guaranteed not to throw so we always leave the object
547 ::new (this) Function(std::move(that));
553 * Assigns a callable object to this `Function`. If the operation fails,
554 * `*this` is left unmodified.
556 * \note `typename = ResultOf<Fun>` prevents this overload from being
557 * selected by overload resolution when `fun` is not a compatible function.
559 template <class Fun, typename = typename Traits::template ResultOf<Fun>>
560 Function& operator=(Fun&& fun) noexcept(
561 noexcept(/* implicit */ Function(std::declval<Fun>()))) {
562 // Doing this in place is more efficient when we can do so safely.
563 if (noexcept(/* implicit */ Function(std::declval<Fun>()))) {
564 // Q: Why is is safe to destroy and reconstruct this object in place?
565 // A: See the explanation in the move assignment operator.
567 ::new (this) Function(static_cast<Fun&&>(fun));
569 // Construct a temporary and (nothrow) swap.
570 Function(static_cast<Fun&&>(fun)).swap(*this);
576 * Clears this `Function`.
578 Function& operator=(std::nullptr_t) noexcept {
579 return (*this = Function());
583 * If `ptr` is null, clears this `Function`. Otherwise, this assignment
584 * operator is equivalent to `*this = std::mem_fn(ptr)`.
586 template <typename Member, typename Class>
587 auto operator=(Member Class::*ptr) noexcept
588 // Prevent this overload from being selected when `ptr` is not a
589 // compatible member function pointer.
590 -> decltype(operator=(std::mem_fn(ptr))) {
591 return ptr ? (*this = std::mem_fn(ptr)) : (*this = Function());
595 * Call the wrapped callable object with the specified arguments.
597 using Traits::operator();
600 * Exchanges the callable objects of `*this` and `that`.
602 void swap(Function& that) noexcept {
603 std::swap(*this, that);
607 * Returns `true` if this `Function` contains a callable, i.e. is
610 explicit operator bool() const noexcept {
611 return exec_(Op::FULL, nullptr, nullptr);
615 * Returns `true` if this `Function` stores the callable on the
616 * heap. If `false` is returned, there has been no additional memory
617 * allocation and the callable is stored inside the `Function`
620 bool hasAllocatedMemory() const noexcept {
621 return exec_(Op::HEAP, nullptr, nullptr);
625 * Construct a `std::function` by moving in the contents of this `Function`.
626 * Note that the returned `std::function` will share its state (i.e. captured
627 * data) across all copies you make of it, so be very careful when copying.
629 std::function<typename Traits::NonConstSignature> asStdFunction() && {
630 using Impl = typename Traits::SharedFunctionImpl;
631 return Impl{std::make_shared<Function>(std::move(*this))};
636 template <typename FunctionType>
637 void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {
641 template <typename FunctionType>
642 bool operator==(const Function<FunctionType>& fn, std::nullptr_t) {
646 template <typename FunctionType>
647 bool operator==(std::nullptr_t, const Function<FunctionType>& fn) {
651 template <typename FunctionType>
652 bool operator!=(const Function<FunctionType>& fn, std::nullptr_t) {
653 return !(fn == nullptr);
656 template <typename FunctionType>
657 bool operator!=(std::nullptr_t, const Function<FunctionType>& fn) {
658 return !(nullptr == fn);
662 * NOTE: See detailed note about `constCastFunction` at the top of the file.
663 * This is potentially dangerous and requires the equivalent of a `const_cast`.
665 template <typename ReturnType, typename... Args>
666 Function<ReturnType(Args...) const> constCastFunction(
667 Function<ReturnType(Args...)>&& that) noexcept {
668 return Function<ReturnType(Args...) const>{std::move(that),
669 detail::function::CoerceTag{}};
672 template <typename ReturnType, typename... Args>
673 Function<ReturnType(Args...) const> constCastFunction(
674 Function<ReturnType(Args...) const>&& that) noexcept {
675 return std::move(that);