1/*
2 * Copyright 2016-present Facebook, Inc.
3 *
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
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
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.
15 */
16/*
17 * @author Eric Niebler (eniebler@fb.com), Sven Over (over@fb.com)
18 * Acknowledgements: Giuseppe Ottaviano (ott@fb.com)
19 */
20
21/**
22 * @class Function
23 *
24 * @brief A polymorphic function wrapper that is not copyable and does not
25 * require the wrapped function to be copy constructible.
26 *
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
33 *
34 * int operator(int);
35 * int operator(int) const;
36 *
37 * If both are defined, the non-const one takes precedence.
38 *
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.
42 *
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
48 * const-reference.
49 *
50 * For example:
51 *
52 * class Foo {
53 * public:
54 * void operator()() {
55 * // mutates the Foo object
56 * }
57 * };
58 *
59 * class Bar {
60 * std::function<void(void)> foo_; // wraps a Foo object
61 * public:
62 * void mutateFoo() const
63 * {
64 * foo_();
65 * }
66 * };
67 *
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.
71 *
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()`.
79 *
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.
83 *
84 * using FunctionType = R(Args...);
85 * using ConstFunctionType = R(Args...) const;
86 *
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.
91 *
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;
96 *
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()();
101 *
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
104 * example:
105 *
106 * class Bar {
107 * folly::Function<void()> func_;
108 * folly::Function<void() const> constFunc_;
109 *
110 * void someMethod() {
111 * // Can call func_.
112 * func_();
113 * // Can call constFunc_.
114 * constFunc_();
115 * }
116 *
117 * void someConstMethod() const {
118 * // Can call constFunc_.
119 * constFunc_();
120 * // However, cannot call func_ because a non-const method cannot
121 * // be called from a const one.
122 * }
123 * };
124 *
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.
128 *
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.
132 *
133 * This is how the const/non-const choice relates to lambda functions:
134 *
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; };
138 *
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; };
142 *
143 * // Mutable lambda: can only be stored in a non-const folly::Function:
144 * int number = 0;
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>'
151 *
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.
163 *
164 * // Mutable lambda: can only be stored in a non-const folly::Function:
165 * int number = 0;
166 * folly::Function<void()> print_number =
167 * [number] () mutable { std::cout << ++number << std::endl; };
168 *
169 * // const-cast to a const folly::Function:
170 * folly::Function<void() const> print_number_const =
171 * constCastFunction(std::move(print_number));
172 *
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.
182 *
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:
187 *
188 * std::function<void(void)> stdfunc = someCallable;
189 *
190 * folly::Function<void(void) const> uniqfunc = constCastFunction(
191 * folly::Function<void(void)>(someCallable)
192 * );
193 *
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`.
216 */
217
218#pragma once
219
220#include <functional>
221#include <memory>
222#include <new>
223#include <type_traits>
224#include <utility>
225
226#include <folly/CppAttributes.h>
227#include <folly/Portability.h>
228#include <folly/Traits.h>
229#include <folly/functional/Invoke.h>
230#include <folly/lang/Exception.h>
231
232namespace folly {
233
234template <typename FunctionType>
235class Function;
236
237template <typename ReturnType, typename... Args>
238Function<ReturnType(Args...) const> constCastFunction(
239 Function<ReturnType(Args...)>&&) noexcept;
240
241#if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE
242template <typename ReturnType, typename... Args>
243Function<ReturnType(Args...) const noexcept> constCastFunction(
244 Function<ReturnType(Args...) noexcept>&&) noexcept;
245#endif
246
247namespace detail {
248namespace function {
249
250enum class Op { MOVE, NUKE, HEAP };
251
252union Data {
253 Data() {}
254 void* big;
255 std::aligned_storage<6 * sizeof(void*)>::type tiny;
256};
257
258template <typename Fun, typename = Fun*>
259using IsSmall = Conjunction<
260 bool_constant<(sizeof(Fun) <= sizeof(Data::tiny))>,
261 std::is_nothrow_move_constructible<Fun>>;
262using SmallTag = std::true_type;
263using HeapTag = std::false_type;
264
265template <typename T>
266struct NotFunction : std::true_type {};
267template <typename T>
268struct NotFunction<Function<T>> : std::false_type {};
269
270template <typename T>
271using EnableIfNotFunction =
272 typename std::enable_if<NotFunction<T>::value>::type;
273
274struct CoerceTag {};
275
276template <typename, typename T>
277struct IsFunctionNullptrTestable : std::false_type {};
278
279template <typename T>
280struct IsFunctionNullptrTestable<
281 void_t<decltype(
282 static_cast<bool>(static_cast<T const&>(T(nullptr)) == nullptr))>,
283 T> : std::true_type {};
284
285template <typename T>
286constexpr std::enable_if_t< //
287 !IsFunctionNullptrTestable<void, T>::value,
288 std::false_type>
289isEmptyFunction(T const&) {
290 return {};
291}
292template <typename T>
293constexpr std::enable_if_t<IsFunctionNullptrTestable<void, T>::value, bool>
294isEmptyFunction(T const& t) {
295 return static_cast<bool>(t == nullptr);
296}
297
298template <typename F, typename... Args>
299using CallableResult = decltype(std::declval<F>()(std::declval<Args>()...));
300
301template <
302 typename From,
303 typename To,
304 typename = typename std::enable_if<
305 !std::is_reference<To>::value || std::is_reference<From>::value>::type>
306using SafeResultOf = decltype(static_cast<To>(std::declval<From>()));
307
308template <typename F, typename R, typename... A>
309class FunctionTraitsSharedProxy {
310 std::shared_ptr<Function<F>> sp_;
311
312 public:
313 explicit FunctionTraitsSharedProxy(std::nullptr_t) noexcept {}
314 explicit FunctionTraitsSharedProxy(Function<F>&& func)
315 : sp_(std::make_shared<Function<F>>(std::move(func))) {}
316 R operator()(A&&... args) const {
317 return (*sp_)(static_cast<A&&>(args)...);
318 }
319
320 friend bool operator==(
321 FunctionTraitsSharedProxy<F, R, A...> const& proxy,
322 std::nullptr_t) noexcept {
323 return proxy.sp_ == nullptr;
324 }
325 friend bool operator!=(
326 FunctionTraitsSharedProxy<F, R, A...> const& proxy,
327 std::nullptr_t) noexcept {
328 return proxy.sp_ != nullptr;
329 }
330
331 friend bool operator==(
332 std::nullptr_t,
333 FunctionTraitsSharedProxy<F, R, A...> const& proxy) noexcept {
334 return proxy.sp_ == nullptr;
335 }
336 friend bool operator!=(
337 std::nullptr_t,
338 FunctionTraitsSharedProxy<F, R, A...> const& proxy) noexcept {
339 return proxy.sp_ != nullptr;
340 }
341};
342
343template <typename FunctionType>
344struct FunctionTraits;
345
346template <typename ReturnType, typename... Args>
347struct FunctionTraits<ReturnType(Args...)> {
348 using Call = ReturnType (*)(Data&, Args&&...);
349 using IsConst = std::false_type;
350 using ConstSignature = ReturnType(Args...) const;
351 using NonConstSignature = ReturnType(Args...);
352 using OtherSignature = ConstSignature;
353
354 template <typename F>
355 using ResultOf =
356 SafeResultOf<CallableResult<std::decay_t<F>&, Args...>, ReturnType>;
357
358 template <typename Fun>
359 static ReturnType callSmall(Data& p, Args&&... args) {
360 return static_cast<ReturnType>((*static_cast<Fun*>(
361 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
362 }
363
364 template <typename Fun>
365 static ReturnType callBig(Data& p, Args&&... args) {
366 return static_cast<ReturnType>(
367 (*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
368 }
369
370 static ReturnType uninitCall(Data&, Args&&...) {
371 throw_exception<std::bad_function_call>();
372 }
373
374 ReturnType operator()(Args... args) {
375 auto& fn = *static_cast<Function<NonConstSignature>*>(this);
376 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
377 }
378
379 using SharedProxy =
380 FunctionTraitsSharedProxy<NonConstSignature, ReturnType, Args...>;
381};
382
383template <typename ReturnType, typename... Args>
384struct FunctionTraits<ReturnType(Args...) const> {
385 using Call = ReturnType (*)(Data&, Args&&...);
386 using IsConst = std::true_type;
387 using ConstSignature = ReturnType(Args...) const;
388 using NonConstSignature = ReturnType(Args...);
389 using OtherSignature = NonConstSignature;
390
391 template <typename F>
392 using ResultOf =
393 SafeResultOf<CallableResult<const std::decay_t<F>&, Args...>, ReturnType>;
394
395 template <typename Fun>
396 static ReturnType callSmall(Data& p, Args&&... args) {
397 return static_cast<ReturnType>((*static_cast<const Fun*>(
398 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
399 }
400
401 template <typename Fun>
402 static ReturnType callBig(Data& p, Args&&... args) {
403 return static_cast<ReturnType>(
404 (*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
405 }
406
407 static ReturnType uninitCall(Data&, Args&&...) {
408 throw_exception<std::bad_function_call>();
409 }
410
411 ReturnType operator()(Args... args) const {
412 auto& fn = *static_cast<const Function<ConstSignature>*>(this);
413 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
414 }
415
416 using SharedProxy =
417 FunctionTraitsSharedProxy<ConstSignature, ReturnType, Args...>;
418};
419
420#if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE
421template <typename ReturnType, typename... Args>
422struct FunctionTraits<ReturnType(Args...) noexcept> {
423 using Call = ReturnType (*)(Data&, Args&&...) noexcept;
424 using IsConst = std::false_type;
425 using ConstSignature = ReturnType(Args...) const noexcept;
426 using NonConstSignature = ReturnType(Args...) noexcept;
427 using OtherSignature = ConstSignature;
428
429 template <typename F>
430 using ResultOf =
431 SafeResultOf<CallableResult<std::decay_t<F>&, Args...>, ReturnType>;
432
433 template <typename Fun>
434 static ReturnType callSmall(Data& p, Args&&... args) noexcept {
435 return static_cast<ReturnType>((*static_cast<Fun*>(
436 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
437 }
438
439 template <typename Fun>
440 static ReturnType callBig(Data& p, Args&&... args) noexcept {
441 return static_cast<ReturnType>(
442 (*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
443 }
444
445 static ReturnType uninitCall(Data&, Args&&...) noexcept {
446 terminate_with<std::bad_function_call>();
447 }
448
449 ReturnType operator()(Args... args) noexcept {
450 auto& fn = *static_cast<Function<NonConstSignature>*>(this);
451 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
452 }
453
454 using SharedProxy =
455 FunctionTraitsSharedProxy<NonConstSignature, ReturnType, Args...>;
456};
457
458template <typename ReturnType, typename... Args>
459struct FunctionTraits<ReturnType(Args...) const noexcept> {
460 using Call = ReturnType (*)(Data&, Args&&...) noexcept;
461 using IsConst = std::true_type;
462 using ConstSignature = ReturnType(Args...) const noexcept;
463 using NonConstSignature = ReturnType(Args...) noexcept;
464 using OtherSignature = NonConstSignature;
465
466 template <typename F>
467 using ResultOf =
468 SafeResultOf<CallableResult<const std::decay_t<F>&, Args...>, ReturnType>;
469
470 template <typename Fun>
471 static ReturnType callSmall(Data& p, Args&&... args) noexcept {
472 return static_cast<ReturnType>((*static_cast<const Fun*>(
473 static_cast<void*>(&p.tiny)))(static_cast<Args&&>(args)...));
474 }
475
476 template <typename Fun>
477 static ReturnType callBig(Data& p, Args&&... args) noexcept {
478 return static_cast<ReturnType>(
479 (*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
480 }
481
482 static ReturnType uninitCall(Data&, Args&&...) noexcept {
483 throw_exception<std::bad_function_call>();
484 }
485
486 ReturnType operator()(Args... args) const noexcept {
487 auto& fn = *static_cast<const Function<ConstSignature>*>(this);
488 return fn.call_(fn.data_, static_cast<Args&&>(args)...);
489 }
490
491 using SharedProxy =
492 FunctionTraitsSharedProxy<ConstSignature, ReturnType, Args...>;
493};
494#endif
495
496template <typename Fun>
497bool execSmall(Op o, Data* src, Data* dst) {
498 switch (o) {
499 case Op::MOVE:
500 ::new (static_cast<void*>(&dst->tiny))
501 Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->tiny))));
502 FOLLY_FALLTHROUGH;
503 case Op::NUKE:
504 static_cast<Fun*>(static_cast<void*>(&src->tiny))->~Fun();
505 break;
506 case Op::HEAP:
507 break;
508 }
509 return false;
510}
511
512template <typename Fun>
513bool execBig(Op o, Data* src, Data* dst) {
514 switch (o) {
515 case Op::MOVE:
516 dst->big = src->big;
517 src->big = nullptr;
518 break;
519 case Op::NUKE:
520 delete static_cast<Fun*>(src->big);
521 break;
522 case Op::HEAP:
523 break;
524 }
525 return true;
526}
527
528} // namespace function
529} // namespace detail
530
531template <typename FunctionType>
532class Function final : private detail::function::FunctionTraits<FunctionType> {
533 // These utility types are defined outside of the template to reduce
534 // the number of instantiations, and then imported in the class
535 // namespace for convenience.
536 using Data = detail::function::Data;
537 using Op = detail::function::Op;
538 using SmallTag = detail::function::SmallTag;
539 using HeapTag = detail::function::HeapTag;
540 using CoerceTag = detail::function::CoerceTag;
541
542 using Traits = detail::function::FunctionTraits<FunctionType>;
543 using Call = typename Traits::Call;
544 using Exec = bool (*)(Op, Data*, Data*);
545
546 template <typename Fun>
547 using IsSmall = detail::function::IsSmall<Fun>;
548
549 // The `data_` member is mutable to allow `constCastFunction` to work without
550 // invoking undefined behavior. Const-correctness is only violated when
551 // `FunctionType` is a const function type (e.g., `int() const`) and `*this`
552 // is the result of calling `constCastFunction`.
553 mutable Data data_{};
554 Call call_{&Traits::uninitCall};
555 Exec exec_{nullptr};
556
557 bool exec(Op o, Data* src, Data* dst) const {
558 return exec_ && exec_(o, src, dst);
559 }
560
561 friend Traits;
562 friend Function<typename Traits::ConstSignature> folly::constCastFunction<>(
563 Function<typename Traits::NonConstSignature>&&) noexcept;
564 friend class Function<typename Traits::OtherSignature>;
565
566 template <typename Fun>
567 Function(Fun&& fun, SmallTag) noexcept {
568 using FunT = typename std::decay<Fun>::type;
569 if (!detail::function::isEmptyFunction(fun)) {
570 ::new (static_cast<void*>(&data_.tiny)) FunT(static_cast<Fun&&>(fun));
571 call_ = &Traits::template callSmall<FunT>;
572 exec_ = &detail::function::execSmall<FunT>;
573 }
574 }
575
576 template <typename Fun>
577 Function(Fun&& fun, HeapTag) {
578 using FunT = typename std::decay<Fun>::type;
579 if (!detail::function::isEmptyFunction(fun)) {
580 data_.big = new FunT(static_cast<Fun&&>(fun));
581 call_ = &Traits::template callBig<FunT>;
582 exec_ = &detail::function::execBig<FunT>;
583 }
584 }
585
586 template <typename Signature>
587 Function(Function<Signature>&& that, CoerceTag)
588 : Function(static_cast<Function<Signature>&&>(that), HeapTag{}) {}
589
590 Function(Function<typename Traits::OtherSignature>&& that, CoerceTag) noexcept
591 : call_(that.call_), exec_(that.exec_) {
592 that.call_ = &Traits::uninitCall;
593 that.exec_ = nullptr;
594 exec(Op::MOVE, &that.data_, &data_);
595 }
596
597 public:
598 /**
599 * Default constructor. Constructs an empty Function.
600 */
601 Function() = default;
602
603 // not copyable
604 Function(const Function&) = delete;
605
606#if __OBJC__
607 // Make sure Objective C blocks are copied
608 template <class ReturnType, class... Args>
609 /*implicit*/ Function(ReturnType (^objCBlock)(Args... args))
610 : Function([blockCopy = (ReturnType(^)(Args...))[objCBlock copy]](
611 Args... args) { return blockCopy(args...); }){};
612#endif
613
614 /**
615 * Move constructor
616 */
617 Function(Function&& that) noexcept : call_(that.call_), exec_(that.exec_) {
618 // that must be uninitialized before exec() call in the case of self move
619 that.call_ = &Traits::uninitCall;
620 that.exec_ = nullptr;
621 exec(Op::MOVE, &that.data_, &data_);
622 }
623
624 /**
625 * Constructs an empty `Function`.
626 */
627 /* implicit */ Function(std::nullptr_t) noexcept {}
628
629 /**
630 * Constructs a new `Function` from any callable object that is _not_ a
631 * `folly::Function`. This handles function pointers, pointers to static
632 * member functions, `std::reference_wrapper` objects, `std::function`
633 * objects, and arbitrary objects that implement `operator()` if the parameter
634 * signature matches (i.e. it returns an object convertible to `R` when called
635 * with `Args...`).
636 *
637 * \note `typename Traits::template ResultOf<Fun>` prevents this overload
638 * from being selected by overload resolution when `fun` is not a compatible
639 * function.
640 *
641 * \note The noexcept requires some explanation. `IsSmall` is true when the
642 * decayed type fits within the internal buffer and is noexcept-movable. But
643 * this ctor might copy, not move. What we need here, if this ctor does a
644 * copy, is that this ctor be noexcept when the copy is noexcept. That is not
645 * checked in `IsSmall`, and shouldn't be, because once the `Function` is
646 * constructed, the contained object is never copied. This check is for this
647 * ctor only, in the case that this ctor does a copy.
648 */
649 template <
650 typename Fun,
651 typename = detail::function::EnableIfNotFunction<Fun>,
652 typename = typename Traits::template ResultOf<Fun>>
653 /* implicit */ Function(Fun fun) noexcept(
654 IsSmall<Fun>::value&& noexcept(Fun(std::declval<Fun>())))
655 : Function(std::move(fun), IsSmall<Fun>{}) {}
656
657 /**
658 * For move-constructing from a `folly::Function<X(Ys...) [const?]>`.
659 * For a `Function` with a `const` function type, the object must be
660 * callable from a `const`-reference, i.e. implement `operator() const`.
661 * For a `Function` with a non-`const` function type, the object will
662 * be called from a non-const reference, which means that it will execute
663 * a non-const `operator()` if it is defined, and falls back to
664 * `operator() const` otherwise.
665 */
666 template <
667 typename Signature,
668 typename = typename Traits::template ResultOf<Function<Signature>>>
669 Function(Function<Signature>&& that) noexcept(
670 noexcept(Function(std::move(that), CoerceTag{})))
671 : Function(std::move(that), CoerceTag{}) {}
672
673 /**
674 * If `ptr` is null, constructs an empty `Function`. Otherwise,
675 * this constructor is equivalent to `Function(std::mem_fn(ptr))`.
676 */
677 template <
678 typename Member,
679 typename Class,
680 // Prevent this overload from being selected when `ptr` is not a
681 // compatible member function pointer.
682 typename = decltype(Function(std::mem_fn((Member Class::*)0)))>
683 /* implicit */ Function(Member Class::*ptr) noexcept {
684 if (ptr) {
685 *this = std::mem_fn(ptr);
686 }
687 }
688
689 ~Function() {
690 exec(Op::NUKE, &data_, nullptr);
691 }
692
693 Function& operator=(const Function&) = delete;
694
695#if __OBJC__
696 // Make sure Objective C blocks are copied
697 template <class ReturnType, class... Args>
698 /* implicit */ Function& operator=(ReturnType (^objCBlock)(Args... args)) {
699 (*this) = [blockCopy = (ReturnType(^)(Args...))[objCBlock copy]](
700 Args... args) { return blockCopy(args...); };
701 return *this;
702 }
703#endif
704
705 /**
706 * Move assignment operator
707 *
708 * \note Leaves `that` in a valid but unspecified state. If `&that == this`
709 * then `*this` is left in a valid but unspecified state.
710 */
711 Function& operator=(Function&& that) noexcept {
712 // Q: Why is it safe to destroy and reconstruct this object in place?
713 // A: Two reasons: First, `Function` is a final class, so in doing this
714 // we aren't slicing off any derived parts. And second, the move
715 // operation is guaranteed not to throw so we always leave the object
716 // in a valid state.
717 // In the case of self-move (this == &that), this leaves the object in
718 // a default-constructed state. First the object is destroyed, then we
719 // pass the destroyed object to the move constructor. The first thing the
720 // move constructor does is default-construct the object. That object is
721 // "moved" into itself, which is a no-op for a default-constructed Function.
722 this->~Function();
723 ::new (this) Function(std::move(that));
724 return *this;
725 }
726
727 /**
728 * Assigns a callable object to this `Function`. If the operation fails,
729 * `*this` is left unmodified.
730 *
731 * \note `typename = decltype(Function(std::declval<Fun>()))` prevents this
732 * overload from being selected by overload resolution when `fun` is not a
733 * compatible function.
734 */
735 template <typename Fun, typename = decltype(Function(std::declval<Fun>()))>
736 Function& operator=(Fun fun) noexcept(
737 noexcept(/* implicit */ Function(std::declval<Fun>()))) {
738 // Doing this in place is more efficient when we can do so safely.
739 if (noexcept(/* implicit */ Function(std::declval<Fun>()))) {
740 // Q: Why is is safe to destroy and reconstruct this object in place?
741 // A: See the explanation in the move assignment operator.
742 this->~Function();
743 ::new (this) Function(std::move(fun));
744 } else {
745 // Construct a temporary and (nothrow) swap.
746 Function(std::move(fun)).swap(*this);
747 }
748 return *this;
749 }
750
751 /**
752 * For assigning from a `Function<X(Ys..) [const?]>`.
753 */
754 template <
755 typename Signature,
756 typename = typename Traits::template ResultOf<Function<Signature>>>
757 Function& operator=(Function<Signature>&& that) noexcept(
758 noexcept(Function(std::move(that)))) {
759 return (*this = Function(std::move(that)));
760 }
761
762 /**
763 * Clears this `Function`.
764 */
765 Function& operator=(std::nullptr_t) noexcept {
766 return (*this = Function());
767 }
768
769 /**
770 * If `ptr` is null, clears this `Function`. Otherwise, this assignment
771 * operator is equivalent to `*this = std::mem_fn(ptr)`.
772 */
773 template <typename Member, typename Class>
774 auto operator=(Member Class::*ptr) noexcept
775 // Prevent this overload from being selected when `ptr` is not a
776 // compatible member function pointer.
777 -> decltype(operator=(std::mem_fn(ptr))) {
778 return ptr ? (*this = std::mem_fn(ptr)) : (*this = Function());
779 }
780
781 /**
782 * Call the wrapped callable object with the specified arguments.
783 */
784 using Traits::operator();
785
786 /**
787 * Exchanges the callable objects of `*this` and `that`.
788 */
789 void swap(Function& that) noexcept {
790 std::swap(*this, that);
791 }
792
793 /**
794 * Returns `true` if this `Function` contains a callable, i.e. is
795 * non-empty.
796 */
797 explicit operator bool() const noexcept {
798 return exec_ != nullptr;
799 }
800
801 /**
802 * Returns `true` if this `Function` stores the callable on the
803 * heap. If `false` is returned, there has been no additional memory
804 * allocation and the callable is stored inside the `Function`
805 * object itself.
806 */
807 bool hasAllocatedMemory() const noexcept {
808 return exec(Op::HEAP, nullptr, nullptr);
809 }
810
811 using typename Traits::SharedProxy;
812
813 /**
814 * Move this `Function` into a copyable callable object, of which all copies
815 * share the state.
816 */
817 SharedProxy asSharedProxy() && {
818 return SharedProxy{std::move(*this)};
819 }
820
821 /**
822 * Construct a `std::function` by moving in the contents of this `Function`.
823 * Note that the returned `std::function` will share its state (i.e. captured
824 * data) across all copies you make of it, so be very careful when copying.
825 */
826 std::function<typename Traits::NonConstSignature> asStdFunction() && {
827 return std::move(*this).asSharedProxy();
828 }
829};
830
831template <typename FunctionType>
832void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {
833 lhs.swap(rhs);
834}
835
836template <typename FunctionType>
837bool operator==(const Function<FunctionType>& fn, std::nullptr_t) {
838 return !fn;
839}
840
841template <typename FunctionType>
842bool operator==(std::nullptr_t, const Function<FunctionType>& fn) {
843 return !fn;
844}
845
846template <typename FunctionType>
847bool operator!=(const Function<FunctionType>& fn, std::nullptr_t) {
848 return !(fn == nullptr);
849}
850
851template <typename FunctionType>
852bool operator!=(std::nullptr_t, const Function<FunctionType>& fn) {
853 return !(nullptr == fn);
854}
855
856/**
857 * NOTE: See detailed note about `constCastFunction` at the top of the file.
858 * This is potentially dangerous and requires the equivalent of a `const_cast`.
859 */
860template <typename ReturnType, typename... Args>
861Function<ReturnType(Args...) const> constCastFunction(
862 Function<ReturnType(Args...)>&& that) noexcept {
863 return Function<ReturnType(Args...) const>{std::move(that),
864 detail::function::CoerceTag{}};
865}
866
867template <typename ReturnType, typename... Args>
868Function<ReturnType(Args...) const> constCastFunction(
869 Function<ReturnType(Args...) const>&& that) noexcept {
870 return std::move(that);
871}
872
873#if FOLLY_HAVE_NOEXCEPT_FUNCTION_TYPE
874template <typename ReturnType, typename... Args>
875Function<ReturnType(Args...) const noexcept> constCastFunction(
876 Function<ReturnType(Args...) noexcept>&& that) noexcept {
877 return Function<ReturnType(Args...) const noexcept>{
878 std::move(that), detail::function::CoerceTag{}};
879}
880
881template <typename ReturnType, typename... Args>
882Function<ReturnType(Args...) const noexcept> constCastFunction(
883 Function<ReturnType(Args...) const noexcept>&& that) noexcept {
884 return std::move(that);
885}
886#endif
887
888/**
889 * @class FunctionRef
890 *
891 * @brief A reference wrapper for callable objects
892 *
893 * FunctionRef is similar to std::reference_wrapper, but the template parameter
894 * is the function signature type rather than the type of the referenced object.
895 * A folly::FunctionRef is cheap to construct as it contains only a pointer to
896 * the referenced callable and a pointer to a function which invokes the
897 * callable.
898 *
899 * The user of FunctionRef must be aware of the reference semantics: storing a
900 * copy of a FunctionRef is potentially dangerous and should be avoided unless
901 * the referenced object definitely outlives the FunctionRef object. Thus any
902 * function that accepts a FunctionRef parameter should only use it to invoke
903 * the referenced function and not store a copy of it. Knowing that FunctionRef
904 * itself has reference semantics, it is generally okay to use it to reference
905 * lambdas that capture by reference.
906 */
907
908template <typename FunctionType>
909class FunctionRef;
910
911template <typename ReturnType, typename... Args>
912class FunctionRef<ReturnType(Args...)> final {
913 using Call = ReturnType (*)(void*, Args&&...);
914
915 static ReturnType uninitCall(void*, Args&&...) {
916 throw_exception<std::bad_function_call>();
917 }
918
919 template <typename Fun>
920 static ReturnType call(void* object, Args&&... args) {
921 using Pointer = std::add_pointer_t<Fun>;
922 return static_cast<ReturnType>(invoke(
923 static_cast<Fun&&>(*static_cast<Pointer>(object)),
924 static_cast<Args&&>(args)...));
925 }
926
927 void* object_{nullptr};
928 Call call_{&FunctionRef::uninitCall};
929
930 public:
931 /**
932 * Default constructor. Constructs an empty FunctionRef.
933 *
934 * Invoking it will throw std::bad_function_call.
935 */
936 constexpr FunctionRef() = default;
937
938 /**
939 * Like default constructor. Constructs an empty FunctionRef.
940 *
941 * Invoking it will throw std::bad_function_call.
942 */
943 constexpr explicit FunctionRef(std::nullptr_t) noexcept {}
944
945 /**
946 * Construct a FunctionRef from a reference to a callable object.
947 */
948 template <
949 typename Fun,
950 typename std::enable_if<
951 Conjunction<
952 Negation<std::is_same<FunctionRef, std::decay_t<Fun>>>,
953 is_invocable_r<ReturnType, Fun&&, Args&&...>>::value,
954 int>::type = 0>
955 constexpr /* implicit */ FunctionRef(Fun&& fun) noexcept
956 // `Fun` may be a const type, in which case we have to do a const_cast
957 // to store the address in a `void*`. This is safe because the `void*`
958 // will be cast back to `Fun*` (which is a const pointer whenever `Fun`
959 // is a const type) inside `FunctionRef::call`
960 : object_(
961 const_cast<void*>(static_cast<void const*>(std::addressof(fun)))),
962 call_(&FunctionRef::call<Fun>) {}
963
964 ReturnType operator()(Args... args) const {
965 return call_(object_, static_cast<Args&&>(args)...);
966 }
967
968 constexpr explicit operator bool() const noexcept {
969 return object_;
970 }
971
972 constexpr friend bool operator==(
973 FunctionRef<ReturnType(Args...)> ref,
974 std::nullptr_t) noexcept {
975 return ref.object_ == nullptr;
976 }
977 constexpr friend bool operator!=(
978 FunctionRef<ReturnType(Args...)> ref,
979 std::nullptr_t) noexcept {
980 return ref.object_ != nullptr;
981 }
982
983 constexpr friend bool operator==(
984 std::nullptr_t,
985 FunctionRef<ReturnType(Args...)> ref) noexcept {
986 return ref.object_ == nullptr;
987 }
988 constexpr friend bool operator!=(
989 std::nullptr_t,
990 FunctionRef<ReturnType(Args...)> ref) noexcept {
991 return ref.object_ != nullptr;
992 }
993};
994
995} // namespace folly
996