1 | /* |
2 | * Copyright 2017-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 | // TODO: [x] "cast" from Poly<C&> to Poly<C&&> |
18 | // TODO: [ ] copy/move from Poly<C&>/Poly<C&&> to Poly<C> |
19 | // TODO: [ ] copy-on-write? |
20 | // TODO: [ ] down- and cross-casting? (Possible?) |
21 | // TODO: [ ] shared ownership? (Dubious.) |
22 | // TODO: [ ] can games be played with making the VTable a member of a struct |
23 | // with strange alignment such that the address of the VTable can |
24 | // be used to tell whether the object is stored in-situ or not? |
25 | |
26 | #pragma once |
27 | |
28 | #if defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5 |
29 | #error Folly.Poly requires gcc-5 or greater |
30 | #endif |
31 | |
32 | #include <cassert> |
33 | #include <new> |
34 | #include <type_traits> |
35 | #include <typeinfo> |
36 | #include <utility> |
37 | |
38 | #include <folly/CPortability.h> |
39 | #include <folly/CppAttributes.h> |
40 | #include <folly/Traits.h> |
41 | #include <folly/detail/TypeList.h> |
42 | #include <folly/lang/Assume.h> |
43 | |
44 | #if !defined(__cpp_inline_variables) |
45 | #define FOLLY_INLINE_CONSTEXPR constexpr |
46 | #else |
47 | #define FOLLY_INLINE_CONSTEXPR inline constexpr |
48 | #endif |
49 | |
50 | #include <folly/PolyException.h> |
51 | #include <folly/detail/PolyDetail.h> |
52 | |
53 | namespace folly { |
54 | template <class I> |
55 | struct Poly; |
56 | |
57 | /** |
58 | * Within the definition of interface `I`, `PolySelf<Base>` is an alias for |
59 | * the instance of `Poly` that is currently being instantiated. It is |
60 | * one of: `Poly<J>`, `Poly<J&&>`, `Poly<J&>`, or `Poly<J const&>`; where |
61 | * `J` is either `I` or some interface that extends `I`. |
62 | * |
63 | * It can be used within interface definitions to declare members that accept |
64 | * other `Poly` objects of the same type as `*this`. |
65 | * |
66 | * The first parameter may optionally be cv- and/or reference-qualified, in |
67 | * which case, the qualification is applies to the type of the interface in the |
68 | * resulting `Poly<>` instance. The second template parameter controls whether |
69 | * or not the interface is decayed before the cv-ref qualifiers of the first |
70 | * argument are applied. For example, given the following: |
71 | * |
72 | * struct Foo { |
73 | * template <class Base> |
74 | * struct Interface : Base { |
75 | * using A = PolySelf<Base>; |
76 | * using B = PolySelf<Base &>; |
77 | * using C = PolySelf<Base const &>; |
78 | * using X = PolySelf<Base, PolyDecay>; |
79 | * using Y = PolySelf<Base &, PolyDecay>; |
80 | * using Z = PolySelf<Base const &, PolyDecay>; |
81 | * }; |
82 | * // ... |
83 | * }; |
84 | * struct Bar : PolyExtends<Foo> { |
85 | * // ... |
86 | * }; |
87 | * |
88 | * Then for `Poly<Bar>`, the typedefs are aliases for the following types: |
89 | * - `A` is `Poly<Bar>` |
90 | * - `B` is `Poly<Bar &>` |
91 | * - `C` is `Poly<Bar const &>` |
92 | * - `X` is `Poly<Bar>` |
93 | * - `Y` is `Poly<Bar &>` |
94 | * - `Z` is `Poly<Bar const &>` |
95 | * |
96 | * And for `Poly<Bar &>`, the typedefs are aliases for the following types: |
97 | * - `A` is `Poly<Bar &>` |
98 | * - `B` is `Poly<Bar &>` |
99 | * - `C` is `Poly<Bar &>` |
100 | * - `X` is `Poly<Bar>` |
101 | * - `Y` is `Poly<Bar &>` |
102 | * - `Z` is `Poly<Bar const &>` |
103 | */ |
104 | template < |
105 | class Node, |
106 | class Tfx = detail::MetaIdentity, |
107 | class Access = detail::PolyAccess> |
108 | using PolySelf = decltype(Access::template self_<Node, Tfx>()); |
109 | |
110 | /** |
111 | * When used in conjunction with `PolySelf`, controls how to construct `Poly` |
112 | * types related to the one currently being instantiated. |
113 | * |
114 | * \sa PolySelf |
115 | */ |
116 | using PolyDecay = detail::MetaQuote<std::decay_t>; |
117 | |
118 | #if !defined(__cpp_template_auto) |
119 | |
120 | /** |
121 | * Use `FOLLY_POLY_MEMBERS(MEMS...)` on pre-C++17 compilers to specify a |
122 | * comma-separated list of member function bindings. |
123 | * |
124 | * For example: |
125 | * |
126 | * struct IFooBar { |
127 | * template <class Base> |
128 | * struct Interface : Base { |
129 | * int foo() const { return folly::poly_call<0>(*this); } |
130 | * void bar() { folly::poly_call<1>(*this); } |
131 | * }; |
132 | * template <class T> |
133 | * using Members = FOLLY_POLY_MEMBERS(&T::foo, &T::bar); |
134 | * }; |
135 | */ |
136 | #define FOLLY_POLY_MEMBERS(...) \ |
137 | typename decltype(::folly::detail::deduceMembers( \ |
138 | __VA_ARGS__))::template Members<__VA_ARGS__> |
139 | |
140 | /** |
141 | * Use `FOLLY_POLY_MEMBER(SIG, MEM)` on pre-C++17 compilers to specify a member |
142 | * function binding that needs to be disambiguated because of overloads. `SIG` |
143 | * should the (possibly const-qualified) signature of the `MEM` member function |
144 | * pointer. |
145 | * |
146 | * For example: |
147 | * |
148 | * struct IFoo { |
149 | * template <class Base> struct Interface : Base { |
150 | * int foo() const { return folly::poly_call<0>(*this); } |
151 | * }; |
152 | * template <class T> using Members = FOLLY_POLY_MEMBERS( |
153 | * // This works even if T::foo is overloaded: |
154 | * FOLLY_POLY_MEMBER(int()const, &T::foo) |
155 | * ); |
156 | * }; |
157 | */ |
158 | #define FOLLY_POLY_MEMBER(SIG, MEM) \ |
159 | ::folly::detail::MemberDef< \ |
160 | ::folly::detail::Member<decltype(::folly::sig<SIG>(MEM)), MEM>>::value |
161 | |
162 | /** |
163 | * A list of member function bindings. |
164 | */ |
165 | template <class... Ts> |
166 | using PolyMembers = detail::TypeList<Ts...>; |
167 | |
168 | #else |
169 | #define FOLLY_POLY_MEMBER(SIG, MEM) ::folly::sig<SIG>(MEM) |
170 | #define FOLLY_POLY_MEMBERS(...) ::folly::PolyMembers<__VA_ARGS__> |
171 | |
172 | template <auto... Ps> |
173 | struct PolyMembers {}; |
174 | |
175 | #endif |
176 | |
177 | /** |
178 | * Used in the definition of a `Poly` interface to say that the current |
179 | * interface is an extension of a set of zero or more interfaces. |
180 | * |
181 | * Example: |
182 | * |
183 | * struct IFoo { |
184 | * template <class Base> struct Interface : Base { |
185 | * void foo() { folly::poly_call<0>(*this); } |
186 | * }; |
187 | * template <class T> using Members = FOLLY_POLY_MEMBERS(&T::foo); |
188 | * } |
189 | * struct IBar : PolyExtends<IFoo> { |
190 | * template <class Base> struct Interface : Base { |
191 | * void bar(int i) { folly::poly_call<0>(*this, i); } |
192 | * }; |
193 | * template <class T> using Members = FOLLY_POLY_MEMBERS(&T::bar); |
194 | * } |
195 | */ |
196 | template <class... I> |
197 | struct PolyExtends : virtual I... { |
198 | using Subsumptions = detail::TypeList<I...>; |
199 | |
200 | template <class Base> |
201 | struct Interface : Base { |
202 | Interface() = default; |
203 | using Base::Base; |
204 | }; |
205 | |
206 | template <class...> |
207 | using Members = PolyMembers<>; |
208 | }; |
209 | |
210 | //////////////////////////////////////////////////////////////////////////////// |
211 | /** |
212 | * Call the N-th member of the currently-being-defined interface. When the |
213 | * first parameter is an object of type `PolySelf<Base>` (as opposed to `*this`) |
214 | * you must explicitly specify which interface through which to dispatch. |
215 | * For instance: |
216 | * |
217 | * struct IAddable { |
218 | * template <class Base> |
219 | * struct Interface : Base { |
220 | * friend PolySelf<Base, Decay> |
221 | * operator+(PolySelf<Base> const& a, PolySelf<Base> const& b) { |
222 | * return folly::poly_call<0, IAddable>(a, b); |
223 | * } |
224 | * }; |
225 | * template <class T> |
226 | * static auto plus_(T const& a, T const& b) -> decltype(a + b) { |
227 | * return a + b; |
228 | * } |
229 | * template <class T> |
230 | * using Members = FOLLY_POLY_MEMBERS(&plus_<std::decay_t<T>>); |
231 | * }; |
232 | * |
233 | * \sa PolySelf |
234 | */ |
235 | template <std::size_t N, typename This, typename... As> |
236 | auto poly_call(This&& _this, As&&... as) |
237 | -> decltype(detail::PolyAccess::call<N>( |
238 | static_cast<This&&>(_this), |
239 | static_cast<As&&>(as)...)) { |
240 | return detail::PolyAccess::call<N>( |
241 | static_cast<This&&>(_this), static_cast<As&&>(as)...); |
242 | } |
243 | |
244 | /// \overload |
245 | template <std::size_t N, class I, class Tail, typename... As> |
246 | decltype(auto) poly_call(detail::PolyNode<I, Tail>&& _this, As&&... as) { |
247 | using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>; |
248 | return detail::PolyAccess::call<N>( |
249 | static_cast<This&&>(_this), static_cast<As&&>(as)...); |
250 | } |
251 | |
252 | /// \overload |
253 | template <std::size_t N, class I, class Tail, typename... As> |
254 | decltype(auto) poly_call(detail::PolyNode<I, Tail>& _this, As&&... as) { |
255 | using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>; |
256 | return detail::PolyAccess::call<N>( |
257 | static_cast<This&>(_this), static_cast<As&&>(as)...); |
258 | } |
259 | |
260 | /// \overload |
261 | template <std::size_t N, class I, class Tail, typename... As> |
262 | decltype(auto) poly_call(detail::PolyNode<I, Tail> const& _this, As&&... as) { |
263 | using This = detail::InterfaceOf<I, detail::PolyNode<I, Tail>>; |
264 | return detail::PolyAccess::call<N>( |
265 | static_cast<This const&>(_this), static_cast<As&&>(as)...); |
266 | } |
267 | |
268 | /// \overload |
269 | template < |
270 | std::size_t N, |
271 | class I, |
272 | class Poly, |
273 | typename... As, |
274 | std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0> |
275 | auto poly_call(Poly&& _this, As&&... as) -> decltype(poly_call<N, I>( |
276 | static_cast<Poly&&>(_this).get(), |
277 | static_cast<As&&>(as)...)) { |
278 | return poly_call<N, I>( |
279 | static_cast<Poly&&>(_this).get(), static_cast<As&&>(as)...); |
280 | } |
281 | |
282 | /// \cond |
283 | /// \overload |
284 | template <std::size_t N, class I, typename... As> |
285 | [[noreturn]] detail::Bottom poly_call(detail::ArchetypeBase const&, As&&...) { |
286 | assume_unreachable(); |
287 | } |
288 | /// \endcond |
289 | |
290 | //////////////////////////////////////////////////////////////////////////////// |
291 | /** |
292 | * Try to cast the `Poly` object to the requested type. If the `Poly` stores an |
293 | * object of that type, return a reference to the object; otherwise, throw an |
294 | * exception. |
295 | * \tparam T The (unqualified) type to which to cast the `Poly` object. |
296 | * \tparam Poly The type of the `Poly` object. |
297 | * \param that The `Poly` object to be cast. |
298 | * \return A reference to the `T` object stored in or refered to by `that`. |
299 | * \throw BadPolyAccess if `that` is empty. |
300 | * \throw BadPolyCast if `that` does not store or refer to an object of type |
301 | * `T`. |
302 | */ |
303 | template <class T, class I> |
304 | detail::AddCvrefOf<T, I>&& poly_cast(detail::PolyRoot<I>&& that) { |
305 | return detail::PolyAccess::cast<T>(std::move(that)); |
306 | } |
307 | |
308 | /// \overload |
309 | template <class T, class I> |
310 | detail::AddCvrefOf<T, I>& poly_cast(detail::PolyRoot<I>& that) { |
311 | return detail::PolyAccess::cast<T>(that); |
312 | } |
313 | |
314 | /// \overload |
315 | template <class T, class I> |
316 | detail::AddCvrefOf<T, I> const& poly_cast(detail::PolyRoot<I> const& that) { |
317 | return detail::PolyAccess::cast<T>(that); |
318 | } |
319 | |
320 | /// \cond |
321 | /// \overload |
322 | template <class T, class I> |
323 | [[noreturn]] detail::AddCvrefOf<T, I>&& poly_cast(detail::ArchetypeRoot<I>&&) { |
324 | assume_unreachable(); |
325 | } |
326 | |
327 | /// \overload |
328 | template <class T, class I> |
329 | [[noreturn]] detail::AddCvrefOf<T, I>& poly_cast(detail::ArchetypeRoot<I>&) { |
330 | assume_unreachable(); |
331 | } |
332 | |
333 | /// \overload |
334 | template <class T, class I> |
335 | [[noreturn]] detail::AddCvrefOf<T, I> const& poly_cast( |
336 | detail::ArchetypeRoot<I> const&) { |
337 | assume_unreachable(); |
338 | } |
339 | /// \endcond |
340 | |
341 | /// \overload |
342 | template < |
343 | class T, |
344 | class Poly, |
345 | std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0> |
346 | constexpr auto poly_cast(Poly&& that) |
347 | -> decltype(poly_cast<T>(std::declval<Poly>().get())) { |
348 | return poly_cast<T>(static_cast<Poly&&>(that).get()); |
349 | } |
350 | |
351 | //////////////////////////////////////////////////////////////////////////////// |
352 | /** |
353 | * Returns a reference to the `std::type_info` object corresponding to the |
354 | * object currently stored in `that`. If `that` is empty, returns |
355 | * `typeid(void)`. |
356 | */ |
357 | template <class I> |
358 | std::type_info const& poly_type(detail::PolyRoot<I> const& that) noexcept { |
359 | return detail::PolyAccess::type(that); |
360 | } |
361 | |
362 | /// \cond |
363 | /// \overload |
364 | [[noreturn]] inline std::type_info const& poly_type( |
365 | detail::ArchetypeBase const&) noexcept { |
366 | assume_unreachable(); |
367 | } |
368 | /// \endcond |
369 | |
370 | /// \overload |
371 | template <class Poly, std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0> |
372 | constexpr auto poly_type(Poly const& that) noexcept |
373 | -> decltype(poly_type(that.get())) { |
374 | return poly_type(that.get()); |
375 | } |
376 | |
377 | //////////////////////////////////////////////////////////////////////////////// |
378 | /** |
379 | * Returns `true` if `that` is not currently storing an object; `false`, |
380 | * otherwise. |
381 | */ |
382 | template <class I> |
383 | bool poly_empty(detail::PolyRoot<I> const& that) noexcept { |
384 | return detail::State::eEmpty == detail::PolyAccess::vtable(that)->state_; |
385 | } |
386 | |
387 | /// \overload |
388 | template <class I> |
389 | constexpr bool poly_empty(detail::PolyRoot<I&&> const&) noexcept { |
390 | return false; |
391 | } |
392 | |
393 | /// \overload |
394 | template <class I> |
395 | constexpr bool poly_empty(detail::PolyRoot<I&> const&) noexcept { |
396 | return false; |
397 | } |
398 | |
399 | /// \overload |
400 | template <class I> |
401 | constexpr bool poly_empty(Poly<I&&> const&) noexcept { |
402 | return false; |
403 | } |
404 | |
405 | /// \overload |
406 | template <class I> |
407 | constexpr bool poly_empty(Poly<I&> const&) noexcept { |
408 | return false; |
409 | } |
410 | |
411 | /// \cond |
412 | [[noreturn]] inline bool poly_empty(detail::ArchetypeBase const&) noexcept { |
413 | assume_unreachable(); |
414 | } |
415 | /// \endcond |
416 | |
417 | //////////////////////////////////////////////////////////////////////////////// |
418 | /** |
419 | * Given a `Poly<I&>`, return a `Poly<I&&>`. Otherwise, when `I` is not a |
420 | * reference type, returns a `Poly<I>&&` when given a `Poly<I>&`, like |
421 | * `std::move`. |
422 | */ |
423 | template < |
424 | class I, |
425 | std::enable_if_t<detail::Not<std::is_reference<I>>::value, int> = 0> |
426 | constexpr Poly<I>&& poly_move(detail::PolyRoot<I>& that) noexcept { |
427 | return static_cast<Poly<I>&&>(static_cast<Poly<I>&>(that)); |
428 | } |
429 | |
430 | /// \overload |
431 | template < |
432 | class I, |
433 | std::enable_if_t<detail::Not<std::is_const<I>>::value, int> = 0> |
434 | Poly<I&&> poly_move(detail::PolyRoot<I&> const& that) noexcept { |
435 | return detail::PolyAccess::move(that); |
436 | } |
437 | |
438 | /// \overload |
439 | template <class I> |
440 | Poly<I const&> poly_move(detail::PolyRoot<I const&> const& that) noexcept { |
441 | return detail::PolyAccess::move(that); |
442 | } |
443 | |
444 | /// \cond |
445 | /// \overload |
446 | [[noreturn]] inline detail::ArchetypeBase poly_move( |
447 | detail::ArchetypeBase const&) noexcept { |
448 | assume_unreachable(); |
449 | } |
450 | /// \endcond |
451 | |
452 | /// \overload |
453 | template <class Poly, std::enable_if_t<detail::IsPoly<Poly>::value, int> = 0> |
454 | constexpr auto poly_move(Poly& that) noexcept |
455 | -> decltype(poly_move(that.get())) { |
456 | return poly_move(that.get()); |
457 | } |
458 | |
459 | /// \cond |
460 | namespace detail { |
461 | /** |
462 | * The implementation for `Poly` for when the interface type is not |
463 | * reference-like qualified, as in `Poly<SemiRegular>`. |
464 | */ |
465 | template <class I> |
466 | struct PolyVal : PolyImpl<I> { |
467 | private: |
468 | friend PolyAccess; |
469 | |
470 | struct NoneSuch {}; |
471 | using Copyable = std::is_copy_constructible<PolyImpl<I>>; |
472 | using PolyOrNonesuch = If<Copyable::value, PolyVal, NoneSuch>; |
473 | |
474 | using PolyRoot<I>::vptr_; |
475 | |
476 | PolyRoot<I>& _polyRoot_() noexcept { |
477 | return *this; |
478 | } |
479 | PolyRoot<I> const& _polyRoot_() const noexcept { |
480 | return *this; |
481 | } |
482 | |
483 | Data* _data_() noexcept { |
484 | return PolyAccess::data(*this); |
485 | } |
486 | Data const* _data_() const noexcept { |
487 | return PolyAccess::data(*this); |
488 | } |
489 | |
490 | public: |
491 | /** |
492 | * Default constructor. |
493 | * \post `poly_empty(*this) == true` |
494 | */ |
495 | PolyVal() = default; |
496 | /** |
497 | * Move constructor. |
498 | * \post `poly_empty(that) == true` |
499 | */ |
500 | PolyVal(PolyVal&& that) noexcept; |
501 | /** |
502 | * A copy constructor if `I` is copyable; otherwise, a useless constructor |
503 | * from a private, incomplete type. |
504 | */ |
505 | /* implicit */ PolyVal(PolyOrNonesuch const& that); |
506 | |
507 | ~PolyVal(); |
508 | |
509 | /** |
510 | * Inherit any constructors defined by any of the interfaces. |
511 | */ |
512 | using PolyImpl<I>::PolyImpl; |
513 | |
514 | /** |
515 | * Copy assignment, destroys the object currently held (if any) and makes |
516 | * `*this` equal to `that` by stealing its guts. |
517 | */ |
518 | Poly<I>& operator=(PolyVal that) noexcept; |
519 | |
520 | /** |
521 | * Construct a Poly<I> from a concrete type that satisfies the I concept |
522 | */ |
523 | template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0> |
524 | /* implicit */ PolyVal(T&& t); |
525 | |
526 | /** |
527 | * Construct a `Poly` from a compatible `Poly`. "Compatible" here means: the |
528 | * other interface extends this one either directly or indirectly. |
529 | */ |
530 | template <class I2, std::enable_if_t<ValueCompatible<I, I2>::value, int> = 0> |
531 | /* implicit */ PolyVal(Poly<I2> that); |
532 | |
533 | /** |
534 | * Assign to this `Poly<I>` from a concrete type that satisfies the `I` |
535 | * concept. |
536 | */ |
537 | template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0> |
538 | Poly<I>& operator=(T&& t); |
539 | |
540 | /** |
541 | * Assign a compatible `Poly` to `*this`. "Compatible" here means: the |
542 | * other interface extends this one either directly or indirectly. |
543 | */ |
544 | template <class I2, std::enable_if_t<ValueCompatible<I, I2>::value, int> = 0> |
545 | Poly<I>& operator=(Poly<I2> that); |
546 | |
547 | /** |
548 | * Swaps the values of two `Poly` objects. |
549 | */ |
550 | void swap(Poly<I>& that) noexcept; |
551 | }; |
552 | |
553 | //////////////////////////////////////////////////////////////////////////////// |
554 | /** |
555 | * The implementation of `Poly` for when the interface type is |
556 | * reference-quelified, like `Poly<SemuRegular &>`. |
557 | */ |
558 | template <class I> |
559 | struct PolyRef : private PolyImpl<I> { |
560 | private: |
561 | friend PolyAccess; |
562 | |
563 | AddCvrefOf<PolyRoot<I>, I>& _polyRoot_() const noexcept; |
564 | |
565 | Data* _data_() noexcept { |
566 | return PolyAccess::data(*this); |
567 | } |
568 | Data const* _data_() const noexcept { |
569 | return PolyAccess::data(*this); |
570 | } |
571 | |
572 | static constexpr RefType refType() noexcept; |
573 | |
574 | protected: |
575 | template <class That, class I2> |
576 | PolyRef(That&& that, Type<I2>); |
577 | |
578 | public: |
579 | /** |
580 | * Copy constructor |
581 | * \post `&poly_cast<T>(*this) == &poly_cast<T>(that)`, where `T` is the |
582 | * type of the object held by `that`. |
583 | */ |
584 | PolyRef(PolyRef const& that) noexcept; |
585 | |
586 | /** |
587 | * Copy assignment |
588 | * \post `&poly_cast<T>(*this) == &poly_cast<T>(that)`, where `T` is the |
589 | * type of the object held by `that`. |
590 | */ |
591 | Poly<I>& operator=(PolyRef const& that) noexcept; |
592 | |
593 | /** |
594 | * Construct a `Poly<I>` from a concrete type that satisfies concept `I`. |
595 | * \post `!poly_empty(*this)` |
596 | */ |
597 | template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0> |
598 | /* implicit */ PolyRef(T&& t) noexcept; |
599 | |
600 | /** |
601 | * Construct a `Poly<I>` from a compatible `Poly<I2>`. |
602 | */ |
603 | template < |
604 | class I2, |
605 | std::enable_if_t<ReferenceCompatible<I, I2, I2&&>::value, int> = 0> |
606 | /* implicit */ PolyRef(Poly<I2>&& that) noexcept( |
607 | std::is_reference<I2>::value); |
608 | |
609 | template < |
610 | class I2, |
611 | std::enable_if_t<ReferenceCompatible<I, I2, I2&>::value, int> = 0> |
612 | /* implicit */ PolyRef(Poly<I2>& that) noexcept(std::is_reference<I2>::value) |
613 | : PolyRef{that, Type<I2>{}} {} |
614 | |
615 | template < |
616 | class I2, |
617 | std::enable_if_t<ReferenceCompatible<I, I2, I2 const&>::value, int> = 0> |
618 | /* implicit */ PolyRef(Poly<I2> const& that) noexcept( |
619 | std::is_reference<I2>::value) |
620 | : PolyRef{that, Type<I2>{}} {} |
621 | |
622 | /** |
623 | * Assign to a `Poly<I>` from a concrete type that satisfies concept `I`. |
624 | * \post `!poly_empty(*this)` |
625 | */ |
626 | template <class T, std::enable_if_t<ModelsInterface<T, I>::value, int> = 0> |
627 | Poly<I>& operator=(T&& t) noexcept; |
628 | |
629 | /** |
630 | * Assign to `*this` from another compatible `Poly`. |
631 | */ |
632 | template < |
633 | class I2, |
634 | std::enable_if_t<ReferenceCompatible<I, I2, I2&&>::value, int> = 0> |
635 | Poly<I>& operator=(Poly<I2>&& that) noexcept(std::is_reference<I2>::value); |
636 | |
637 | /** |
638 | * \overload |
639 | */ |
640 | template < |
641 | class I2, |
642 | std::enable_if_t<ReferenceCompatible<I, I2, I2&>::value, int> = 0> |
643 | Poly<I>& operator=(Poly<I2>& that) noexcept(std::is_reference<I2>::value); |
644 | |
645 | /** |
646 | * \overload |
647 | */ |
648 | template < |
649 | class I2, |
650 | std::enable_if_t<ReferenceCompatible<I, I2, I2 const&>::value, int> = 0> |
651 | Poly<I>& operator=(Poly<I2> const& that) noexcept( |
652 | std::is_reference<I2>::value); |
653 | |
654 | /** |
655 | * Swap which object this `Poly` references ("shallow" swap). |
656 | */ |
657 | void swap(Poly<I>& that) noexcept; |
658 | |
659 | /** |
660 | * Get a reference to the interface, with correct `const`-ness applied. |
661 | */ |
662 | AddCvrefOf<PolyImpl<I>, I>& get() const noexcept; |
663 | |
664 | /** |
665 | * Get a reference to the interface, with correct `const`-ness applied. |
666 | */ |
667 | AddCvrefOf<PolyImpl<I>, I>& operator*() const noexcept { |
668 | return get(); |
669 | } |
670 | |
671 | /** |
672 | * Get a pointer to the interface, with correct `const`-ness applied. |
673 | */ |
674 | auto operator-> () const noexcept { |
675 | return &get(); |
676 | } |
677 | }; |
678 | |
679 | template <class I> |
680 | using PolyValOrRef = If<std::is_reference<I>::value, PolyRef<I>, PolyVal<I>>; |
681 | } // namespace detail |
682 | /// \endcond |
683 | |
684 | /** |
685 | * `Poly` is a class template that makes it relatively easy to define a |
686 | * type-erasing polymorphic object wrapper. |
687 | * |
688 | * \par Type-erasure |
689 | * |
690 | * \par |
691 | * `std::function` is one example of a type-erasing polymorphic object wrapper; |
692 | * `folly::exception_wrapper` is another. Type-erasure is often used as an |
693 | * alternative to dynamic polymorphism via inheritance-based virtual dispatch. |
694 | * The distinguishing characteristic of type-erasing wrappers are: |
695 | * \li **Duck typing:** Types do not need to inherit from an abstract base |
696 | * class in order to be assignable to a type-erasing wrapper; they merely |
697 | * need to satisfy a particular interface. |
698 | * \li **Value semantics:** Type-erasing wrappers are objects that can be |
699 | * passed around _by value_. This is in contrast to abstract base classes |
700 | * which must be passed by reference or by pointer or else suffer from |
701 | * _slicing_, which causes them to lose their polymorphic behaviors. |
702 | * Reference semantics make it difficult to reason locally about code. |
703 | * \li **Automatic memory management:** When dealing with inheritance-based |
704 | * dynamic polymorphism, it is often necessary to allocate and manage |
705 | * objects on the heap. This leads to a proliferation of `shared_ptr`s and |
706 | * `unique_ptr`s in APIs, complicating their point-of-use. APIs that take |
707 | * type-erasing wrappers, on the other hand, can often store small objects |
708 | * in-situ, with no dynamic allocation. The memory management, if any, is |
709 | * handled for you, and leads to cleaner APIs: consumers of your API don't |
710 | * need to pass `shared_ptr<AbstractBase>`; they can simply pass any object |
711 | * that satisfies the interface you require. (`std::function` is a |
712 | * particularly compelling example of this benefit. Far worse would be an |
713 | * inheritance-based callable solution like |
714 | * `shared_ptr<ICallable<void(int)>>`. ) |
715 | * |
716 | * \par Example: Defining a type-erasing function wrapper with `folly::Poly` |
717 | * |
718 | * \par |
719 | * Defining a polymorphic wrapper with `Poly` is a matter of defining two |
720 | * things: |
721 | * \li An *interface*, consisting of public member functions, and |
722 | * \li A *mapping* from a concrete type to a set of member function bindings. |
723 | * |
724 | * Below is a (heavily commented) example of a simple implementation of a |
725 | * `std::function`-like polymorphic wrapper. Its interface has only a simgle |
726 | * member function: `operator()` |
727 | * |
728 | * // An interface for a callable object of a particular signature, Fun |
729 | * // (most interfaces don't need to be templates, FWIW). |
730 | * template <class Fun> |
731 | * struct IFunction; |
732 | * |
733 | * template <class R, class... As> |
734 | * struct IFunction<R(As...)> { |
735 | * // An interface is defined as a nested class template called |
736 | * // Interface that takes a single template parameter, Base, from |
737 | * // which it inherits. |
738 | * template <class Base> |
739 | * struct Interface : Base { |
740 | * // The Interface has public member functions. These become the |
741 | * // public interface of the resulting Poly instantiation. |
742 | * // (Implementation note: Poly<IFunction<Sig>> will publicly |
743 | * // inherit from this struct, which is what gives it the right |
744 | * // member functions.) |
745 | * R operator()(As... as) const { |
746 | * // The definition of each member function in your interface will |
747 | * // always consist of a single line dispatching to |
748 | * // folly::poly_call<N>. The "N" corresponds to the N-th member |
749 | * // function in the list of member function bindings, Members, |
750 | * // defined below. The first argument will always be *this, and the |
751 | * // rest of the arguments should simply forward (if necessary) the |
752 | * // member function's arguments. |
753 | * return static_cast<R>( |
754 | * folly::poly_call<0>(*this, std::forward<As>(as)...)); |
755 | * } |
756 | * }; |
757 | * |
758 | * // The "Members" alias template is a comma-separated list of bound |
759 | * // member functions for a given concrete type "T". The |
760 | * // "FOLLY_POLY_MEMBERS" macro accepts a comma-separated list, and the |
761 | * // (optional) "FOLLY_POLY_MEMBER" macro lets you disambiguate overloads |
762 | * // by explicitly specifying the function signature the target member |
763 | * // function should have. In this case, we require "T" to have a |
764 | * // function call operator with the signature `R(As...) const`. |
765 | * // |
766 | * // If you are using a C++17-compatible compiler, you can do away with |
767 | * // the macros and write this as: |
768 | * // |
769 | * // template <class T> |
770 | * // using Members = folly::PolyMembers< |
771 | * // folly::sig<R(As...) const>(&T::operator())>; |
772 | * // |
773 | * // And since `folly::sig` is only needed for disambiguation in case of |
774 | * // overloads, if you are not concerned about objects with overloaded |
775 | * // function call operators, it could be further simplified to: |
776 | * // |
777 | * // template <class T> |
778 | * // using Members = folly::PolyMembers<&T::operator()>; |
779 | * // |
780 | * template <class T> |
781 | * using Members = FOLLY_POLY_MEMBERS( |
782 | * FOLLY_POLY_MEMBER(R(As...) const, &T::operator())); |
783 | * }; |
784 | * |
785 | * // Now that we have defined the interface, we can pass it to Poly to |
786 | * // create our type-erasing wrapper: |
787 | * template <class Fun> |
788 | * using Function = Poly<IFunction<Fun>>; |
789 | * |
790 | * \par |
791 | * Given the above definition of `Function`, users can now initialize instances |
792 | * of (say) `Function<int(int, int)>` with function objects like |
793 | * `std::plus<int>` and `std::multiplies<int>`, as below: |
794 | * |
795 | * Function<int(int, int)> fun = std::plus<int>{}; |
796 | * assert(5 == fun(2, 3)); |
797 | * fun = std::multiplies<int>{}; |
798 | * assert(6 = fun(2, 3)); |
799 | * |
800 | * \par Defining an interface with C++17 |
801 | * |
802 | * \par |
803 | * With C++17, defining an interface to be used with `Poly` is fairly |
804 | * straightforward. As in the `Function` example above, there is a struct with |
805 | * a nested `Interface` class template and a nested `Members` alias template. |
806 | * No macros are needed with C++17. |
807 | * \par |
808 | * Imagine we were defining something like a Java-style iterator. If we are |
809 | * using a C++17 compiler, our interface would look something like this: |
810 | * |
811 | * template <class Value> |
812 | * struct IJavaIterator { |
813 | * template <class Base> |
814 | * struct Interface : Base { |
815 | * bool Done() const { return folly::poly_call<0>(*this); } |
816 | * Value Current() const { return folly::poly_call<1>(*this); } |
817 | * void Next() { folly::poly_call<2>(*this); } |
818 | * }; |
819 | * // NOTE: This works in C++17 only: |
820 | * template <class T> |
821 | * using Members = folly::PolyMembers<&T::Done, &T::Current, &T::Next>; |
822 | * }; |
823 | * |
824 | * template <class Value> |
825 | * using JavaIterator = Poly<IJavaIterator>; |
826 | * |
827 | * \par |
828 | * Given the above definition, `JavaIterator<int>` can be used to hold instances |
829 | * of any type that has `Done`, `Current`, and `Next` member functions with the |
830 | * correct (or compatible) signatures. |
831 | * |
832 | * \par |
833 | * The presence of overloaded member functions complicates this picture. Often, |
834 | * property members are faked in C++ with `const` and non-`const` member |
835 | * function overloads, like in the interface specified below: |
836 | * |
837 | * struct IIntProperty { |
838 | * template <class Base> |
839 | * struct Interface : Base { |
840 | * int Value() const { return folly::poly_call<0>(*this); } |
841 | * void Value(int i) { folly::poly_call<1>(*this, i); } |
842 | * }; |
843 | * // NOTE: This works in C++17 only: |
844 | * template <class T> |
845 | * using Members = folly::PolyMembers< |
846 | * folly::sig<int() const>(&T::Value), |
847 | * folly::sig<void(int)>(&T::Value)>; |
848 | * }; |
849 | * |
850 | * using IntProperty = Poly<IIntProperty>; |
851 | * |
852 | * \par |
853 | * Now, any object that has `Value` members of compatible signatures can be |
854 | * assigned to instances of `IntProperty` object. Note how `folly::sig` is used |
855 | * to disambiguate the overloads of `&T::Value`. |
856 | * |
857 | * \par Defining an interface with C++14 |
858 | * |
859 | * \par |
860 | * In C++14, the nice syntax above doesn't work, so we have to resort to macros. |
861 | * The two examples above would look like this: |
862 | * |
863 | * template <class Value> |
864 | * struct IJavaIterator { |
865 | * template <class Base> |
866 | * struct Interface : Base { |
867 | * bool Done() const { return folly::poly_call<0>(*this); } |
868 | * Value Current() const { return folly::poly_call<1>(*this); } |
869 | * void Next() { folly::poly_call<2>(*this); } |
870 | * }; |
871 | * // NOTE: This works in C++14 and C++17: |
872 | * template <class T> |
873 | * using Members = FOLLY_POLY_MEMBERS(&T::Done, &T::Current, &T::Next); |
874 | * }; |
875 | * |
876 | * template <class Value> |
877 | * using JavaIterator = Poly<IJavaIterator>; |
878 | * |
879 | * \par |
880 | * and |
881 | * |
882 | * struct IIntProperty { |
883 | * template <class Base> |
884 | * struct Interface : Base { |
885 | * int Value() const { return folly::poly_call<0>(*this); } |
886 | * void Value(int i) { return folly::poly_call<1>(*this, i); } |
887 | * }; |
888 | * // NOTE: This works in C++14 and C++17: |
889 | * template <class T> |
890 | * using Members = FOLLY_POLY_MEMBERS( |
891 | * FOLLY_POLY_MEMBER(int() const, &T::Value), |
892 | * FOLLY_POLY_MEMBER(void(int), &T::Value)); |
893 | * }; |
894 | * |
895 | * using IntProperty = Poly<IIntProperty>; |
896 | * |
897 | * \par Extending interfaces |
898 | * |
899 | * \par |
900 | * One typical advantage of inheritance-based solutions to runtime polymorphism |
901 | * is that one polymorphic interface could extend another through inheritance. |
902 | * The same can be accomplished with type-erasing polymorphic wrappers. In |
903 | * the `Poly` library, you can use `folly::PolyExtends` to say that one |
904 | * interface extends another. |
905 | * |
906 | * struct IFoo { |
907 | * template <class Base> |
908 | * struct Interface : Base { |
909 | * void Foo() const { return folly::poly_call<0>(*this); } |
910 | * }; |
911 | * template <class T> |
912 | * using Members = FOLLY_POLY_MEMBERS(&T::Foo); |
913 | * }; |
914 | * |
915 | * // The IFooBar interface extends the IFoo interface |
916 | * struct IFooBar : PolyExtends<IFoo> { |
917 | * template <class Base> |
918 | * struct Interface : Base { |
919 | * void Bar() const { return folly::poly_call<0>(*this); } |
920 | * }; |
921 | * template <class T> |
922 | * using Members = FOLLY_POLY_MEMBERS(&T::Bar); |
923 | * }; |
924 | * |
925 | * using FooBar = Poly<IFooBar>; |
926 | * |
927 | * \par |
928 | * Given the above defintion, instances of type `FooBar` have both `Foo()` and |
929 | * `Bar()` member functions. |
930 | * |
931 | * \par |
932 | * The sensible conversions exist between a wrapped derived type and a wrapped |
933 | * base type. For instance, assuming `IDerived` extends `IBase` with |
934 | * `PolyExtends`: |
935 | * |
936 | * Poly<IDerived> derived = ...; |
937 | * Poly<IBase> base = derived; // This conversion is OK. |
938 | * |
939 | * \par |
940 | * As you would expect, there is no conversion in the other direction, and at |
941 | * present there is no `Poly` equivalent to `dynamic_cast`. |
942 | * |
943 | * \par Type-erasing polymorphic reference wrappers |
944 | * |
945 | * \par |
946 | * Sometimes you don't need to own a copy of an object; a reference will do. For |
947 | * that you can use `Poly` to capture a _reference_ to an object satisfying an |
948 | * interface rather than the whole object itself. The syntax is intuitive. |
949 | * |
950 | * int i = 42; |
951 | * // Capture a mutable reference to an object of any IRegular type: |
952 | * Poly<IRegular &> intRef = i; |
953 | * assert(42 == folly::poly_cast<int>(intRef)); |
954 | * // Assert that we captured the address of "i": |
955 | * assert(&i == &folly::poly_cast<int>(intRef)); |
956 | * |
957 | * \par |
958 | * A reference-like `Poly` has a different interface than a value-like `Poly`. |
959 | * Rather than calling member functions with the `obj.fun()` syntax, you would |
960 | * use the `obj->fun()` syntax. This is for the sake of `const`-correctness. |
961 | * For example, consider the code below: |
962 | * |
963 | * struct IFoo { |
964 | * template <class Base> |
965 | * struct Interface { |
966 | * void Foo() { folly::poly_call<0>(*this); } |
967 | * }; |
968 | * template <class T> |
969 | * using Members = folly::PolyMembers<&T::Foo>; |
970 | * }; |
971 | * |
972 | * struct SomeFoo { |
973 | * void Foo() { std::printf("SomeFoo::Foo\n"); } |
974 | * }; |
975 | * |
976 | * SomeFoo foo; |
977 | * Poly<IFoo &> const anyFoo = foo; |
978 | * anyFoo->Foo(); // prints "SomeFoo::Foo" |
979 | * |
980 | * \par |
981 | * Notice in the above code that the `Foo` member function is non-`const`. |
982 | * Notice also that the `anyFoo` object is `const`. However, since it has |
983 | * captured a non-`const` reference to the `foo` object, it should still be |
984 | * possible to dispatch to the non-`const` `Foo` member function. When |
985 | * instantiated with a reference type, `Poly` has an overloaded `operator->` |
986 | * member that returns a pointer to the `IFoo` interface with the correct |
987 | * `const`-ness, which makes this work. |
988 | * |
989 | * \par |
990 | * The same mechanism also prevents users from calling non-`const` member |
991 | * functions on `Poly` objects that have captured `const` references, which |
992 | * would violate `const`-correctness. |
993 | * |
994 | * \par |
995 | * Sensible conversions exist between non-reference and reference `Poly`s. For |
996 | * instance: |
997 | * |
998 | * Poly<IRegular> value = 42; |
999 | * Poly<IRegular &> mutable_ref = value; |
1000 | * Poly<IRegular const &> const_ref = mutable_ref; |
1001 | * |
1002 | * assert(&poly_cast<int>(value) == &poly_cast<int>(mutable_ref)); |
1003 | * assert(&poly_cast<int>(value) == &poly_cast<int>(const_ref)); |
1004 | * |
1005 | * \par Non-member functions (C++17) |
1006 | * |
1007 | * \par |
1008 | * If you wanted to write the interface `ILogicallyNegatable`, which captures |
1009 | * all types that can be negated with unary `operator!`, you could do it |
1010 | * as we've shown above, by binding `&T::operator!` in the nested `Members` |
1011 | * alias template, but that has the problem that it won't work for types that |
1012 | * have defined unary `operator!` as a free function. To handle this case, |
1013 | * the `Poly` library lets you use a free function instead of a member function |
1014 | * when creating a binding. |
1015 | * |
1016 | * \par |
1017 | * With C++17 you may use a lambda to create a binding, as shown in the example |
1018 | * below: |
1019 | * |
1020 | * struct ILogicallyNegatable { |
1021 | * template <class Base> |
1022 | * struct Interface : Base { |
1023 | * bool operator!() const { return folly::poly_call<0>(*this); } |
1024 | * }; |
1025 | * template <class T> |
1026 | * using Members = folly::PolyMembers< |
1027 | * +[](T const& t) -> decltype(!t) { return !t; }>; |
1028 | * }; |
1029 | * |
1030 | * \par |
1031 | * This requires some explanation. The unary `operator+` in front of the lambda |
1032 | * is necessary! It causes the lambda to decay to a C-style function pointer, |
1033 | * which is one of the types that `folly::PolyMembers` accepts. The `decltype` |
1034 | * in the lambda return type is also necessary. Through the magic of SFINAE, it |
1035 | * will cause `Poly<ILogicallyNegatable>` to reject any types that don't support |
1036 | * unary `operator!`. |
1037 | * |
1038 | * \par |
1039 | * If you are using a free function to create a binding, the first parameter is |
1040 | * implicitly the `this` parameter. It will receive the type-erased object. |
1041 | * |
1042 | * \par Non-member functions (C++14) |
1043 | * |
1044 | * \par |
1045 | * If you are using a C++14 compiler, the defintion of `ILogicallyNegatable` |
1046 | * above will fail because lambdas are not `constexpr`. We can get the same |
1047 | * effect by writing the lambda as a named free function, as show below: |
1048 | * |
1049 | * struct ILogicallyNegatable { |
1050 | * template <class Base> |
1051 | * struct Interface : Base { |
1052 | * bool operator!() const { return folly::poly_call<0>(*this); } |
1053 | * }; |
1054 | * |
1055 | * template <class T> |
1056 | * static auto negate(T const& t) -> decltype(!t) { return !t; } |
1057 | * |
1058 | * template <class T> |
1059 | * using Members = FOLLY_POLY_MEMBERS(&negate<T>); |
1060 | * }; |
1061 | * |
1062 | * \par |
1063 | * As with the example that uses the lambda in the preceding section, the first |
1064 | * parameter is implicitly the `this` parameter. It will receive the type-erased |
1065 | * object. |
1066 | * |
1067 | * \par Multi-dispatch |
1068 | * |
1069 | * \par |
1070 | * What if you want to create an `IAddable` interface for things that can be |
1071 | * added? Adding requires _two_ objects, both of which are type-erased. This |
1072 | * interface requires dispatching on both objects, doing the addition only |
1073 | * if the types are the same. For this we make use of the `PolySelf` template |
1074 | * alias to define an interface that takes more than one object of the the |
1075 | * erased type. |
1076 | * |
1077 | * struct IAddable { |
1078 | * template <class Base> |
1079 | * struct Interface : Base { |
1080 | * friend PolySelf<Base, Decay> |
1081 | * operator+(PolySelf<Base> const& a, PolySelf<Base> const& b) { |
1082 | * return folly::poly_call<0, IAddable>(a, b); |
1083 | * } |
1084 | * }; |
1085 | * |
1086 | * template <class T> |
1087 | * using Members = folly::PolyMembers< |
1088 | * +[](T const& a, T const& b) -> decltype(a + b) { return a + b; }>; |
1089 | * }; |
1090 | * |
1091 | * \par |
1092 | * Given the above defintion of `IAddable` we would be able to do the following: |
1093 | * |
1094 | * Poly<IAddable> a = 2, b = 3; |
1095 | * Poly<IAddable> c = a + b; |
1096 | * assert(poly_cast<int>(c) == 5); |
1097 | * |
1098 | * \par |
1099 | * If `a` and `b` stored objects of different types, a `BadPolyCast` exception |
1100 | * would be thrown. |
1101 | * |
1102 | * \par Move-only types |
1103 | * |
1104 | * \par |
1105 | * If you want to store move-only types, then your interface should extend the |
1106 | * `IMoveOnly` interface. |
1107 | * |
1108 | * \par Implementation notes |
1109 | * \par |
1110 | * `Poly` will store "small" objects in an internal buffer, avoiding the cost of |
1111 | * of dynamic allocations. At present, this size is not configurable; it is |
1112 | * pegged at the size of two `double`s. |
1113 | * |
1114 | * \par |
1115 | * `Poly` objects are always nothrow movable. If you store an object in one that |
1116 | * has a potentially throwing move contructor, the object will be stored on the |
1117 | * heap, even if it could fit in the internal storage of the `Poly` object. |
1118 | * (So be sure to give your objects nothrow move constructors!) |
1119 | * |
1120 | * \par |
1121 | * `Poly` implements type-erasure in a manner very similar to how the compiler |
1122 | * accomplishes virtual dispatch. Every `Poly` object contains a pointer to a |
1123 | * table of function pointers. Member function calls involve a double- |
1124 | * indirection: once through the v-pointer, and other indirect function call |
1125 | * through the function pointer. |
1126 | */ |
1127 | template <class I> |
1128 | struct Poly final : detail::PolyValOrRef<I> { |
1129 | friend detail::PolyAccess; |
1130 | Poly() = default; |
1131 | using detail::PolyValOrRef<I>::PolyValOrRef; |
1132 | using detail::PolyValOrRef<I>::operator=; |
1133 | }; |
1134 | |
1135 | /** |
1136 | * Swap two `Poly<I>` instances. |
1137 | */ |
1138 | template <class I> |
1139 | void swap(Poly<I>& left, Poly<I>& right) noexcept { |
1140 | left.swap(right); |
1141 | } |
1142 | |
1143 | /** |
1144 | * Pseudo-function template handy for disambiguating function overloads. |
1145 | * |
1146 | * For example, given: |
1147 | * struct S { |
1148 | * int property() const; |
1149 | * void property(int); |
1150 | * }; |
1151 | * |
1152 | * You can get a member function pointer to the first overload with: |
1153 | * folly::sig<int()const>(&S::property); |
1154 | * |
1155 | * This is arguably a nicer syntax that using the built-in `static_cast`: |
1156 | * static_cast<int (S::*)() const>(&S::property); |
1157 | * |
1158 | * `sig` is also more permissive than `static_cast` about `const`. For instance, |
1159 | * the following also works: |
1160 | * folly::sig<int()>(&S::property); |
1161 | * |
1162 | * The above is permitted |
1163 | */ |
1164 | template <class Sig> |
1165 | FOLLY_INLINE_CONSTEXPR detail::Sig<Sig> const sig = {}; |
1166 | |
1167 | } // namespace folly |
1168 | |
1169 | #include <folly/Poly-inl.h> |
1170 | |
1171 | #undef FOLLY_INLINE_CONSTEXPR |
1172 | |