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>
231 template <typename FunctionType, bool Const = false>
234 template <typename ReturnType, typename... Args>
235 Function<ReturnType(Args...), true> constCastFunction(
236 Function<ReturnType(Args...), false>&&) noexcept;
242 enum class Op { MOVE, NUKE, FULL, HEAP };
246 typename std::aligned_storage<6 * sizeof(void*)>::type small;
249 template <bool If, typename T>
250 using ConstIf = typename std::conditional<If, const T, T>::type;
252 template <typename Fun, typename FunT = typename std::decay<Fun>::type>
253 using IsSmall = std::integral_constant<
255 (sizeof(FunT) <= sizeof(Data::small) &&
256 // Same as is_nothrow_move_constructible, but w/ no template instantiation.
257 noexcept(FunT(std::declval<FunT&&>()))
259 using SmallTag = std::true_type;
260 using HeapTag = std::false_type;
262 template <typename T>
263 bool isNullPtrFn(T* p) {
266 template <typename T>
267 std::false_type isNullPtrFn(T&&) {
271 template <typename ReturnType, typename... Args>
272 ReturnType uninitCall(Data&, Args&&...) {
273 throw std::bad_function_call();
275 inline bool uninitNoop(Op, Data*, Data*) {
279 } // namespace function
280 } // namespace detail
284 template <typename ReturnType, typename... Args, bool Const>
285 class Function<ReturnType(Args...), Const> final {
286 // These utility types are defined outside of the template to reduce
287 // the number of instantiations, and then imported in the class
288 // namespace for convenience.
289 using Data = detail::function::Data;
290 using Op = detail::function::Op;
291 using SmallTag = detail::function::SmallTag;
292 using HeapTag = detail::function::HeapTag;
293 using Call = ReturnType (*)(Data&, Args&&...);
294 using Exec = bool (*)(Op, Data*, Data*);
296 template <typename T>
297 using ConstIf = detail::function::ConstIf<Const, T>;
298 template <typename Fun>
299 using IsSmall = detail::function::IsSmall<Fun>;
305 friend Function<ReturnType(Args...), true> constCastFunction<>(
306 Function<ReturnType(Args...), false>&&) noexcept;
307 friend class Function<ReturnType(Args...), !Const>;
309 template <typename Fun>
311 using FunT = typename std::decay<Fun>::type;
312 static ReturnType call(Data& p, Args&&... args) {
313 return static_cast<ReturnType>((*static_cast<ConstIf<FunT>*>(
314 (void*)&p.small))(static_cast<Args&&>(args)...));
316 static bool exec(Op o, Data* src, Data* dst) {
319 ::new ((void*)&dst->small)
320 FunT(std::move(*static_cast<FunT*>((void*)&src->small)));
323 static_cast<FunT*>((void*)&src->small)->~FunT();
334 template <typename Fun>
335 Function(Fun&& fun, SmallTag) noexcept : Function() {
336 using Ops = OpsSmall<Fun>;
337 if (!detail::function::isNullPtrFn(fun)) {
338 ::new (&data_.small) typename Ops::FunT(static_cast<Fun&&>(fun));
344 template <typename Fun>
346 using FunT = typename std::decay<Fun>::type;
347 static ReturnType call(Data& p, Args&&... args) {
348 return static_cast<ReturnType>(
349 (*static_cast<ConstIf<FunT>*>(p.big))(static_cast<Args&&>(args)...));
351 static bool exec(Op o, Data* src, Data* dst) {
358 delete static_cast<FunT*>(src->big);
368 template <typename Fun>
369 Function(Fun&& fun, HeapTag) : Function() {
370 using Ops = OpsHeap<Fun>;
371 data_.big = new typename Ops::FunT(static_cast<Fun&&>(fun));
376 template <typename F, typename G = typename std::decay<F>::type>
377 using ResultOf = decltype(static_cast<ReturnType>(
378 std::declval<ConstIf<G>&>()(std::declval<Args>()...)));
382 * Default constructor. Constructs an empty Function.
385 : call_(&detail::function::uninitCall<ReturnType, Args...>),
386 exec_(&detail::function::uninitNoop) {}
389 // NOTE: Deleting the non-const copy constructor is unusual but necessary to
390 // prevent copies from non-const `Function` object from selecting the
391 // perfect forwarding implicit converting constructor below
392 // (i.e., `template <typename Fun> Function(Fun&&)`).
393 Function(Function&) = delete;
394 Function(const Function&) = delete;
399 Function(Function&& that) noexcept : Function() {
400 that.exec_(Op::MOVE, &that.data_, &data_);
401 std::swap(call_, that.call_);
402 std::swap(exec_, that.exec_);
406 * Constructs an empty `Function`.
408 /* implicit */ Function(std::nullptr_t) noexcept : Function() {}
411 * Constructs a new `Function` from any callable object. This
412 * handles function pointers, pointers to static member functions,
413 * `std::reference_wrapper` objects, `std::function` objects, and arbitrary
414 * objects that implement `operator()` if the parameter signature
415 * matches (i.e. it returns R when called with Args...).
416 * For a `Function` with a const function type, the object must be
417 * callable from a const-reference, i.e. implement `operator() const`.
418 * For a `Function` with a non-const function type, the object will
419 * be called from a non-const reference, which means that it will execute
420 * a non-const `operator()` if it is defined, and falls back to
421 * `operator() const` otherwise.
423 * \note `typename = ResultOf<Fun>` prevents this overload from being
424 * selected by overload resolution when `fun` is not a compatible function.
426 template <class Fun, typename = ResultOf<Fun>>
427 /* implicit */ Function(Fun&& fun) noexcept(IsSmall<Fun>::value)
428 : Function(static_cast<Fun&&>(fun), IsSmall<Fun>{}) {}
431 * For moving a `Function<X(Ys..) const>` into a `Function<X(Ys...)>`.
435 typename std::enable_if<!Const && OtherConst, int>::type = 0>
436 Function(Function<ReturnType(Args...), OtherConst>&& that) noexcept
438 that.exec_(Op::MOVE, &that.data_, &data_);
439 std::swap(call_, that.call_);
440 std::swap(exec_, that.exec_);
444 * If `ptr` is null, constructs an empty `Function`. Otherwise,
445 * this constructor is equivalent to `Function(std::mem_fn(ptr))`.
450 // Prevent this overload from being selected when `ptr` is not a
451 // compatible member function pointer.
452 typename = decltype(Function(std::mem_fn((Member Class::*)0)))>
453 /* implicit */ Function(Member Class::*ptr) noexcept : Function() {
455 *this = std::mem_fn(ptr);
460 exec_(Op::NUKE, &data_, nullptr);
463 Function& operator=(Function&) = delete;
464 Function& operator=(const Function&) = delete;
467 * Move assignment operator
469 Function& operator=(Function&& that) noexcept {
471 // Q: Why is is safe to destroy and reconstruct this object in place?
472 // A: Two reasons: First, `Function` is a final class, so in doing this
473 // we aren't slicing off any derived parts. And second, the move
474 // operation is guaranteed not to throw so we always leave the object
477 ::new (this) Function(std::move(that));
483 * Assigns a callable object to this `Function`. If the operation fails,
484 * `*this` is left unmodified.
486 * \note `typename = ResultOf<Fun>` prevents this overload from being
487 * selected by overload resolution when `fun` is not a compatible function.
489 template <class Fun, typename = ResultOf<Fun>>
490 Function& operator=(Fun&& fun) noexcept(
491 noexcept(/* implicit */ Function(std::declval<Fun>()))) {
492 // Doing this in place is more efficient when we can do so safely.
493 if (noexcept(/* implicit */ Function(std::declval<Fun>()))) {
494 // Q: Why is is safe to destroy and reconstruct this object in place?
495 // A: See the explanation in the move assignment operator.
497 ::new (this) Function(static_cast<Fun&&>(fun));
499 // Construct a temporary and (nothrow) swap.
500 Function(static_cast<Fun&&>(fun)).swap(*this);
506 * Clears this `Function`.
508 Function& operator=(std::nullptr_t) noexcept {
509 return (*this = Function());
513 * If `ptr` is null, clears this `Function`. Otherwise, this assignment
514 * operator is equivalent to `*this = std::mem_fn(ptr)`.
516 template <typename Member, typename Class>
517 auto operator=(Member Class::*ptr) noexcept
518 // Prevent this overload from being selected when `ptr` is not a
519 // compatible member function pointer.
520 -> decltype(operator=(std::mem_fn(ptr))) {
521 return ptr ? (*this = std::mem_fn(ptr)) : (*this = Function());
525 * Call the wrapped callable object with the specified arguments.
526 * If this `Function` object is a const `folly::Function` object,
527 * this overload shall not participate in overload resolution.
530 // `True` makes `operator()` a template so we can SFINAE on `Const`,
531 // which is non-deduced here.
533 typename std::enable_if<True && !Const, int>::type = 0>
534 ReturnType operator()(Args... args) {
535 return call_(data_, static_cast<Args&&>(args)...);
539 * Call the wrapped callable object with the specified arguments.
540 * If this `Function` object is not a const `folly::Function` object,
541 * this overload shall not participate in overload resolution.
544 // `True` makes `operator()` a template so we can SFINAE on `Const`,
545 // which is non-deduced here.
547 typename std::enable_if<True && Const, int>::type = 0>
548 ReturnType operator()(Args... args) const {
549 return call_(const_cast<Data&>(data_), static_cast<Args&&>(args)...);
553 * Exchanges the callable objects of `*this` and `that`.
555 void swap(Function& that) noexcept {
556 std::swap(*this, that);
560 * Returns `true` if this `Function` contains a callable, i.e. is
563 explicit operator bool() const noexcept {
564 return exec_(Op::FULL, nullptr, nullptr);
568 * Returns `true` if this `Function` stores the callable on the
569 * heap. If `false` is returned, there has been no additional memory
570 * allocation and the callable is stored inside the `Function`
573 bool hasAllocatedMemory() const noexcept {
574 return exec_(Op::HEAP, nullptr, nullptr);
578 * Construct a `std::function` by moving in the contents of this `Function`.
579 * Note that the returned `std::function` will share its state (i.e. captured
580 * data) across all copies you make of it, so be very careful when copying.
582 std::function<ReturnType(Args...)> asStdFunction() && {
584 std::shared_ptr<Function> sp_;
585 ReturnType operator()(Args&&... args) const {
586 return (*sp_)(static_cast<Args&&>(args)...);
589 return Impl{std::make_shared<Function>(std::move(*this))};
593 template <typename FunctionType, bool Const>
595 Function<FunctionType, Const>& lhs,
596 Function<FunctionType, Const>& rhs) noexcept {
600 template <typename FunctionType, bool Const>
601 bool operator==(const Function<FunctionType, Const>& fn, std::nullptr_t) {
605 template <typename FunctionType, bool Const>
606 bool operator==(std::nullptr_t, const Function<FunctionType, Const>& fn) {
610 template <typename FunctionType, bool Const>
611 bool operator!=(const Function<FunctionType, Const>& fn, std::nullptr_t) {
612 return !(fn == nullptr);
615 template <typename FunctionType, bool Const>
616 bool operator!=(std::nullptr_t, const Function<FunctionType, Const>& fn) {
617 return !(nullptr == fn);
620 template <typename ReturnType, typename... Args>
621 Function<ReturnType(Args...), true> constCastFunction(
622 Function<ReturnType(Args...), false>&& that) noexcept {
623 Function<ReturnType(Args...), true> fn{};
624 that.exec_(detail::function::Op::MOVE, &that.data_, &fn.data_);
625 std::swap(fn.call_, that.call_);
626 std::swap(fn.exec_, that.exec_);
630 template <typename FunctionType>
631 Function<FunctionType, true> constCastFunction(
632 Function<FunctionType, true>&& that) noexcept {
633 return std::move(that);
636 template <typename FunctionType>
637 struct MakeFunction {};
639 template <typename ReturnType, typename... Args>
640 struct MakeFunction<ReturnType(Args...)> {
641 using type = Function<ReturnType(Args...), false>;
644 template <typename ReturnType, typename... Args>
645 struct MakeFunction<ReturnType(Args...) const> {
646 using type = Function<ReturnType(Args...), true>;
650 /* using override */ using impl::constCastFunction;
652 template <typename FunctionType>
653 using Function = typename impl::MakeFunction<FunctionType>::type;