1 // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
2 // Licensed under the MIT License:
3 //
4 // Permission is hereby granted, free of charge, to any person obtaining a copy
5 // of this software and associated documentation files (the "Software"), to deal
6 // in the Software without restriction, including without limitation the rights
7 // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
8 // copies of the Software, and to permit persons to whom the Software is
9 // furnished to do so, subject to the following conditions:
10 //
11 // The above copyright notice and this permission notice shall be included in
12 // all copies or substantial portions of the Software.
13 //
14 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
15 // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
16 // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
17 // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
18 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
19 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
20 // THE SOFTWARE.
21
22 #pragma once
23
24 #include "common.h"
25
26 KJ_BEGIN_HEADER
27
28 namespace kj {
29
30 template <typename T>
_kj_internal_isPolymorphic(T *)31 inline constexpr bool _kj_internal_isPolymorphic(T*) {
32 // If you get a compiler error here complaining that T is incomplete, it's because you are trying
33 // to use kj::Own<T> with a type that has only been forward-declared. Since KJ doesn't know if
34 // the type might be involved in inheritance (especially multiple inheritance), it doesn't know
35 // how to correctly call the disposer to destroy the type, since the object's true memory address
36 // may differ from the address used to point to a superclass.
37 //
38 // However, if you know for sure that T is NOT polymorphic (i.e. it doesn't have a vtable and
39 // isn't involved in inheritance), then you can use KJ_DECLARE_NON_POLYMORPHIC(T) to declare this
40 // to KJ without actually completing the type. Place this macro invocation either in the global
41 // scope, or in the same namespace as T is defined.
42 return __is_polymorphic(T);
43 }
44
45 #define KJ_DECLARE_NON_POLYMORPHIC(...) \
46 inline constexpr bool _kj_internal_isPolymorphic(__VA_ARGS__*) { \
47 return false; \
48 }
49 // If you want to use kj::Own<T> for an incomplete type T that you know is not polymorphic, then
50 // write `KJ_DECLARE_NON_POLYMORPHIC(T)` either at the global scope or in the same namespace as
51 // T is declared.
52 //
53 // This also works for templates, e.g.:
54 //
55 // template <typename X, typename Y>
56 // struct MyType;
57 // template <typename X, typename Y>
58 // KJ_DECLARE_NON_POLYMORPHIC(MyType<X, Y>)
59
60 namespace _ { // private
61
62 template <typename T> struct RefOrVoid_ { typedef T& Type; };
63 template <> struct RefOrVoid_<void> { typedef void Type; };
64 template <> struct RefOrVoid_<const void> { typedef void Type; };
65
66 template <typename T>
67 using RefOrVoid = typename RefOrVoid_<T>::Type;
68 // Evaluates to T&, unless T is `void`, in which case evaluates to `void`.
69 //
70 // This is a hack needed to avoid defining Own<void> as a totally separate class.
71
72 template <typename T, bool isPolymorphic = _kj_internal_isPolymorphic((T*)nullptr)>
73 struct CastToVoid_;
74
75 template <typename T>
76 struct CastToVoid_<T, false> {
77 static void* apply(T* ptr) {
78 return static_cast<void*>(ptr);
79 }
80 static const void* applyConst(T* ptr) {
81 const T* cptr = ptr;
82 return static_cast<const void*>(cptr);
83 }
84 };
85
86 template <typename T>
87 struct CastToVoid_<T, true> {
88 static void* apply(T* ptr) {
89 return dynamic_cast<void*>(ptr);
90 }
91 static const void* applyConst(T* ptr) {
92 const T* cptr = ptr;
93 return dynamic_cast<const void*>(cptr);
94 }
95 };
96
97 template <typename T>
98 void* castToVoid(T* ptr) {
99 return CastToVoid_<T>::apply(ptr);
100 }
101
102 template <typename T>
103 const void* castToConstVoid(T* ptr) {
104 return CastToVoid_<T>::applyConst(ptr);
105 }
106
107 } // namespace _ (private)
108
109 // =======================================================================================
110 // Disposer -- Implementation details.
111
112 class Disposer {
113 // Abstract interface for a thing that "disposes" of objects, where "disposing" usually means
114 // calling the destructor followed by freeing the underlying memory. `Own<T>` encapsulates an
115 // object pointer with corresponding Disposer.
116 //
117 // Few developers will ever touch this interface. It is primarily useful for those implementing
118 // custom memory allocators.
119
120 protected:
121 // Do not declare a destructor, as doing so will force a global initializer for each HeapDisposer
122 // instance. Eww!
123
124 virtual void disposeImpl(void* pointer) const = 0;
125 // Disposes of the object, given a pointer to the beginning of the object. If the object is
126 // polymorphic, this pointer is determined by dynamic_cast<void*>(). For non-polymorphic types,
127 // Own<T> does not allow any casting, so the pointer exactly matches the original one given to
128 // Own<T>.
129
130 public:
131
132 template <typename T>
133 void dispose(T* object) const;
134 // Helper wrapper around disposeImpl().
135 //
136 // If T is polymorphic, calls `disposeImpl(dynamic_cast<void*>(object))`, otherwise calls
137 // `disposeImpl(implicitCast<void*>(object))`.
138 //
139 // Callers must not call dispose() on the same pointer twice, even if the first call throws
140 // an exception.
141
142 private:
143 template <typename T, bool polymorphic = _kj_internal_isPolymorphic((T*)nullptr)>
144 struct Dispose_;
145 };
146
147 template <typename T>
148 class DestructorOnlyDisposer: public Disposer {
149 // A disposer that merely calls the type's destructor and nothing else.
150
151 public:
152 static const DestructorOnlyDisposer instance;
153
154 void disposeImpl(void* pointer) const override {
155 reinterpret_cast<T*>(pointer)->~T();
156 }
157 };
158
159 template <typename T>
160 const DestructorOnlyDisposer<T> DestructorOnlyDisposer<T>::instance = DestructorOnlyDisposer<T>();
161
162 class NullDisposer: public Disposer {
163 // A disposer that does nothing.
164
165 public:
166 static const NullDisposer instance;
167
168 void disposeImpl(void* pointer) const override {}
169 };
170
171 // =======================================================================================
172 // Own<T> -- An owned pointer.
173
174 template <typename T>
175 class Own {
176 // A transferrable title to a T. When an Own<T> goes out of scope, the object's Disposer is
177 // called to dispose of it. An Own<T> can be efficiently passed by move, without relocating the
178 // underlying object; this transfers ownership.
179 //
180 // This is much like std::unique_ptr, except:
181 // - You cannot release(). An owned object is not necessarily allocated with new (see next
182 // point), so it would be hard to use release() correctly.
183 // - The deleter is made polymorphic by virtual call rather than by template. This is much
184 // more powerful -- it allows the use of custom allocators, freelists, etc. This could
185 // _almost_ be accomplished with unique_ptr by forcing everyone to use something like
186 // std::unique_ptr<T, kj::Deleter>, except that things get hairy in the presence of multiple
187 // inheritance and upcasting, and anyway if you force everyone to use a custom deleter
188 // then you've lost any benefit to interoperating with the "standard" unique_ptr.
189
190 public:
191 KJ_DISALLOW_COPY(Own);
192 inline Own(): disposer(nullptr), ptr(nullptr) {}
193 inline Own(Own&& other) noexcept
194 : disposer(other.disposer), ptr(other.ptr) { other.ptr = nullptr; }
195 inline Own(Own<RemoveConstOrDisable<T>>&& other) noexcept
196 : disposer(other.disposer), ptr(other.ptr) { other.ptr = nullptr; }
197 template <typename U, typename = EnableIf<canConvert<U*, T*>()>>
198 inline Own(Own<U>&& other) noexcept
199 : disposer(other.disposer), ptr(cast(other.ptr)) {
200 other.ptr = nullptr;
201 }
202 inline Own(T* ptr, const Disposer& disposer) noexcept: disposer(&disposer), ptr(ptr) {}
203
204 ~Own() noexcept(false) { dispose(); }
205
206 inline Own& operator=(Own&& other) {
207 // Move-assignnment operator.
208
209 // Careful, this might own `other`. Therefore we have to transfer the pointers first, then
210 // dispose.
211 const Disposer* disposerCopy = disposer;
212 T* ptrCopy = ptr;
213 disposer = other.disposer;
214 ptr = other.ptr;
215 other.ptr = nullptr;
216 if (ptrCopy != nullptr) {
217 disposerCopy->dispose(const_cast<RemoveConst<T>*>(ptrCopy));
218 }
219 return *this;
220 }
221
222 inline Own& operator=(decltype(nullptr)) {
223 dispose();
224 return *this;
225 }
226
227 template <typename... Attachments>
228 Own<T> attach(Attachments&&... attachments) KJ_WARN_UNUSED_RESULT;
229 // Returns an Own<T> which points to the same object but which also ensures that all values
230 // passed to `attachments` remain alive until after this object is destroyed. Normally
231 // `attachments` are other Own<?>s pointing to objects that this one depends on.
232 //
233 // Note that attachments will eventually be destroyed in the order they are listed. Hence,
234 // foo.attach(bar, baz) is equivalent to (but more efficient than) foo.attach(bar).attach(baz).
235
236 template <typename U>
237 Own<U> downcast() {
238 // Downcast the pointer to Own<U>, destroying the original pointer. If this pointer does not
239 // actually point at an instance of U, the results are undefined (throws an exception in debug
240 // mode if RTTI is enabled, otherwise you're on your own).
241
242 Own<U> result;
243 if (ptr != nullptr) {
244 result.ptr = &kj::downcast<U>(*ptr);
245 result.disposer = disposer;
246 ptr = nullptr;
247 }
248 return result;
249 }
250
251 #define NULLCHECK KJ_IREQUIRE(ptr != nullptr, "null Own<> dereference")
252 inline T* operator->() { NULLCHECK; return ptr; }
253 inline const T* operator->() const { NULLCHECK; return ptr; }
254 inline _::RefOrVoid<T> operator*() { NULLCHECK; return *ptr; }
255 inline _::RefOrVoid<const T> operator*() const { NULLCHECK; return *ptr; }
256 #undef NULLCHECK
257 inline T* get() { return ptr; }
258 inline const T* get() const { return ptr; }
259 inline operator T*() { return ptr; }
260 inline operator const T*() const { return ptr; }
261
262 private:
263 const Disposer* disposer; // Only valid if ptr != nullptr.
264 T* ptr;
265
266 inline explicit Own(decltype(nullptr)): disposer(nullptr), ptr(nullptr) {}
267
268 inline bool operator==(decltype(nullptr)) { return ptr == nullptr; }
269 inline bool operator!=(decltype(nullptr)) { return ptr != nullptr; }
270 // Only called by Maybe<Own<T>>.
271
272 inline void dispose() {
273 // Make sure that if an exception is thrown, we are left with a null ptr, so we won't possibly
274 // dispose again.
275 T* ptrCopy = ptr;
276 if (ptrCopy != nullptr) {
277 ptr = nullptr;
278 disposer->dispose(const_cast<RemoveConst<T>*>(ptrCopy));
279 }
280 }
281
282 template <typename U>
283 static inline T* cast(U* ptr) {
284 static_assert(_kj_internal_isPolymorphic((T*)nullptr),
285 "Casting owned pointers requires that the target type is polymorphic.");
286 return ptr;
287 }
288
289 template <typename U>
290 friend class Own;
291 friend class Maybe<Own<T>>;
292 };
293
294 template <>
295 template <typename U>
296 inline void* Own<void>::cast(U* ptr) {
297 return _::castToVoid(ptr);
298 }
299
300 template <>
301 template <typename U>
302 inline const void* Own<const void>::cast(U* ptr) {
303 return _::castToConstVoid(ptr);
304 }
305
306 namespace _ { // private
307
308 template <typename T>
309 class OwnOwn {
310 public:
311 inline OwnOwn(Own<T>&& value) noexcept: value(kj::mv(value)) {}
312
313 inline Own<T>& operator*() & { return value; }
314 inline const Own<T>& operator*() const & { return value; }
315 inline Own<T>&& operator*() && { return kj::mv(value); }
316 inline const Own<T>&& operator*() const && { return kj::mv(value); }
317 inline Own<T>* operator->() { return &value; }
318 inline const Own<T>* operator->() const { return &value; }
319 inline operator Own<T>*() { return value ? &value : nullptr; }
320 inline operator const Own<T>*() const { return value ? &value : nullptr; }
321
322 private:
323 Own<T> value;
324 };
325
326 template <typename T>
327 OwnOwn<T> readMaybe(Maybe<Own<T>>&& maybe) { return OwnOwn<T>(kj::mv(maybe.ptr)); }
328 template <typename T>
329 Own<T>* readMaybe(Maybe<Own<T>>& maybe) { return maybe.ptr ? &maybe.ptr : nullptr; }
330 template <typename T>
331 const Own<T>* readMaybe(const Maybe<Own<T>>& maybe) { return maybe.ptr ? &maybe.ptr : nullptr; }
332
333 } // namespace _ (private)
334
335 template <typename T>
336 class Maybe<Own<T>> {
337 public:
338 inline Maybe(): ptr(nullptr) {}
339 inline Maybe(Own<T>&& t) noexcept: ptr(kj::mv(t)) {}
340 inline Maybe(Maybe&& other) noexcept: ptr(kj::mv(other.ptr)) {}
341
342 template <typename U>
343 inline Maybe(Maybe<Own<U>>&& other): ptr(mv(other.ptr)) {}
344 template <typename U>
345 inline Maybe(Own<U>&& other): ptr(mv(other)) {}
346
347 inline Maybe(decltype(nullptr)) noexcept: ptr(nullptr) {}
348
349 inline Own<T>& emplace(Own<T> value) {
350 // Assign the Maybe to the given value and return the content. This avoids the need to do a
351 // KJ_ASSERT_NONNULL() immediately after setting the Maybe just to read it back again.
352 ptr = kj::mv(value);
353 return ptr;
354 }
355
356 inline operator Maybe<T&>() { return ptr.get(); }
357 inline operator Maybe<const T&>() const { return ptr.get(); }
358
359 inline Maybe& operator=(Maybe&& other) { ptr = kj::mv(other.ptr); return *this; }
360
361 inline bool operator==(decltype(nullptr)) const { return ptr == nullptr; }
362 inline bool operator!=(decltype(nullptr)) const { return ptr != nullptr; }
363
364 Own<T>& orDefault(Own<T>& defaultValue) {
365 if (ptr == nullptr) {
366 return defaultValue;
367 } else {
368 return ptr;
369 }
370 }
371 const Own<T>& orDefault(const Own<T>& defaultValue) const {
372 if (ptr == nullptr) {
373 return defaultValue;
374 } else {
375 return ptr;
376 }
377 }
378
379 template <typename Func>
380 auto map(Func&& f) & -> Maybe<decltype(f(instance<Own<T>&>()))> {
381 if (ptr == nullptr) {
382 return nullptr;
383 } else {
384 return f(ptr);
385 }
386 }
387
388 template <typename Func>
389 auto map(Func&& f) const & -> Maybe<decltype(f(instance<const Own<T>&>()))> {
390 if (ptr == nullptr) {
391 return nullptr;
392 } else {
393 return f(ptr);
394 }
395 }
396
397 template <typename Func>
398 auto map(Func&& f) && -> Maybe<decltype(f(instance<Own<T>&&>()))> {
399 if (ptr == nullptr) {
400 return nullptr;
401 } else {
402 return f(kj::mv(ptr));
403 }
404 }
405
406 template <typename Func>
407 auto map(Func&& f) const && -> Maybe<decltype(f(instance<const Own<T>&&>()))> {
408 if (ptr == nullptr) {
409 return nullptr;
410 } else {
411 return f(kj::mv(ptr));
412 }
413 }
414
415 private:
416 Own<T> ptr;
417
418 template <typename U>
419 friend class Maybe;
420 template <typename U>
421 friend _::OwnOwn<U> _::readMaybe(Maybe<Own<U>>&& maybe);
422 template <typename U>
423 friend Own<U>* _::readMaybe(Maybe<Own<U>>& maybe);
424 template <typename U>
425 friend const Own<U>* _::readMaybe(const Maybe<Own<U>>& maybe);
426 };
427
428 namespace _ { // private
429
430 template <typename T>
431 class HeapDisposer final: public Disposer {
432 public:
433 virtual void disposeImpl(void* pointer) const override { delete reinterpret_cast<T*>(pointer); }
434
435 static const HeapDisposer instance;
436 };
437
438 #if _MSC_VER && _MSC_VER < 1920 && !defined(__clang__)
439 template <typename T>
440 __declspec(selectany) const HeapDisposer<T> HeapDisposer<T>::instance = HeapDisposer<T>();
441 // On MSVC 2017 we suddenly started seeing a linker error on one specific specialization of
442 // `HeapDisposer::instance` when seemingly-unrelated code was modified. Explicitly specifying
443 // `__declspec(selectany)` seems to fix it. But why? Shouldn't template members have `selectany`
444 // behavior by default? We don't know. It works and we're moving on.
445 #else
446 template <typename T>
447 const HeapDisposer<T> HeapDisposer<T>::instance = HeapDisposer<T>();
448 #endif
449
450 } // namespace _ (private)
451
452 template <typename T, typename... Params>
453 Own<T> heap(Params&&... params) {
454 // heap<T>(...) allocates a T on the heap, forwarding the parameters to its constructor. The
455 // exact heap implementation is unspecified -- for now it is operator new, but you should not
456 // assume this. (Since we know the object size at delete time, we could actually implement an
457 // allocator that is more efficient than operator new.)
458
459 return Own<T>(new T(kj::fwd<Params>(params)...), _::HeapDisposer<T>::instance);
460 }
461
462 template <typename T>
463 Own<Decay<T>> heap(T&& orig) {
464 // Allocate a copy (or move) of the argument on the heap.
465 //
466 // The purpose of this overload is to allow you to omit the template parameter as there is only
467 // one argument and the purpose is to copy it.
468
469 typedef Decay<T> T2;
470 return Own<T2>(new T2(kj::fwd<T>(orig)), _::HeapDisposer<T2>::instance);
471 }
472
473 template <typename T, typename... Attachments>
474 Own<Decay<T>> attachVal(T&& value, Attachments&&... attachments);
475 // Returns an Own<T> that takes ownership of `value` and `attachments`, and points to `value`.
476 //
477 // This is equivalent to heap(value).attach(attachments), but only does one allocation rather than
478 // two.
479
480 template <typename T, typename... Attachments>
481 Own<T> attachRef(T& value, Attachments&&... attachments);
482 // Like attach() but `value` is not moved; the resulting Own<T> points to its existing location.
483 // This is preferred if `value` is already owned by one of `attachments`.
484 //
485 // This is equivalent to Own<T>(&value, kj::NullDisposer::instance).attach(attachments), but
486 // is easier to write and allocates slightly less memory.
487
488 // =======================================================================================
489 // SpaceFor<T> -- assists in manual allocation
490
491 template <typename T>
492 class SpaceFor {
493 // A class which has the same size and alignment as T but does not call its constructor or
494 // destructor automatically. Instead, call construct() to construct a T in the space, which
495 // returns an Own<T> which will take care of calling T's destructor later.
496
497 public:
498 inline SpaceFor() {}
499 inline ~SpaceFor() {}
500
501 template <typename... Params>
502 Own<T> construct(Params&&... params) {
503 ctor(value, kj::fwd<Params>(params)...);
504 return Own<T>(&value, DestructorOnlyDisposer<T>::instance);
505 }
506
507 private:
508 union {
509 T value;
510 };
511 };
512
513 // =======================================================================================
514 // Inline implementation details
515
516 template <typename T>
517 struct Disposer::Dispose_<T, true> {
518 static void dispose(T* object, const Disposer& disposer) {
519 // Note that dynamic_cast<void*> does not require RTTI to be enabled, because the offset to
520 // the top of the object is in the vtable -- as it obviously needs to be to correctly implement
521 // operator delete.
522 disposer.disposeImpl(dynamic_cast<void*>(object));
523 }
524 };
525 template <typename T>
526 struct Disposer::Dispose_<T, false> {
527 static void dispose(T* object, const Disposer& disposer) {
528 disposer.disposeImpl(static_cast<void*>(object));
529 }
530 };
531
532 template <typename T>
533 void Disposer::dispose(T* object) const {
534 Dispose_<T>::dispose(object, *this);
535 }
536
537 namespace _ { // private
538
539 template <typename... T>
540 struct OwnedBundle;
541
542 template <>
543 struct OwnedBundle<> {};
544
545 template <typename First, typename... Rest>
546 struct OwnedBundle<First, Rest...>: public OwnedBundle<Rest...> {
547 OwnedBundle(First&& first, Rest&&... rest)
548 : OwnedBundle<Rest...>(kj::fwd<Rest>(rest)...), first(kj::fwd<First>(first)) {}
549
550 // Note that it's intentional that `first` is destroyed before `rest`. This way, doing
551 // ptr.attach(foo, bar, baz) is equivalent to ptr.attach(foo).attach(bar).attach(baz) in terms
552 // of destruction order (although the former does fewer allocations).
553 Decay<First> first;
554 };
555
556 template <typename... T>
557 struct DisposableOwnedBundle final: public Disposer, public OwnedBundle<T...> {
558 DisposableOwnedBundle(T&&... values): OwnedBundle<T...>(kj::fwd<T>(values)...) {}
559 void disposeImpl(void* pointer) const override { delete this; }
560 };
561
562 } // namespace _ (private)
563
564 template <typename T>
565 template <typename... Attachments>
566 Own<T> Own<T>::attach(Attachments&&... attachments) {
567 T* ptrCopy = ptr;
568
569 KJ_IREQUIRE(ptrCopy != nullptr, "cannot attach to null pointer");
570
571 // HACK: If someone accidentally calls .attach() on a null pointer in opt mode, try our best to
572 // accomplish reasonable behavior: We turn the pointer non-null but still invalid, so that the
573 // disposer will still be called when the pointer goes out of scope.
574 if (ptrCopy == nullptr) ptrCopy = reinterpret_cast<T*>(1);
575
576 auto bundle = new _::DisposableOwnedBundle<Own<T>, Attachments...>(
577 kj::mv(*this), kj::fwd<Attachments>(attachments)...);
578 return Own<T>(ptrCopy, *bundle);
579 }
580
581 template <typename T, typename... Attachments>
582 Own<T> attachRef(T& value, Attachments&&... attachments) {
583 auto bundle = new _::DisposableOwnedBundle<Attachments...>(kj::fwd<Attachments>(attachments)...);
584 return Own<T>(&value, *bundle);
585 }
586
587 template <typename T, typename... Attachments>
588 Own<Decay<T>> attachVal(T&& value, Attachments&&... attachments) {
589 auto bundle = new _::DisposableOwnedBundle<T, Attachments...>(
590 kj::fwd<T>(value), kj::fwd<Attachments>(attachments)...);
591 return Own<Decay<T>>(&bundle->first, *bundle);
592 }
593
594 } // namespace kj
595
596 KJ_END_HEADER
597