summaryrefslogtreecommitdiffstats
path: root/mfbt/UniquePtr.h
blob: 97fb8d27aedf825a9780b3fa437ab40dbb6efcbd (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* Smart pointer managing sole ownership of a resource. */

#ifndef mozilla_UniquePtr_h
#define mozilla_UniquePtr_h

#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Compiler.h"
#include "mozilla/Move.h"
#include "mozilla/Pair.h"
#include "mozilla/TypeTraits.h"

namespace mozilla {

template<typename T> class DefaultDelete;
template<typename T, class D = DefaultDelete<T>> class UniquePtr;

} // namespace mozilla

namespace mozilla {

namespace detail {

struct HasPointerTypeHelper
{
  template <class U> static double Test(...);
  template <class U> static char Test(typename U::pointer* = 0);
};

template <class T>
class HasPointerType : public IntegralConstant<bool, sizeof(HasPointerTypeHelper::Test<T>(0)) == 1>
{
};

template <class T, class D, bool = HasPointerType<D>::value>
struct PointerTypeImpl
{
  typedef typename D::pointer Type;
};

template <class T, class D>
struct PointerTypeImpl<T, D, false>
{
  typedef T* Type;
};

template <class T, class D>
struct PointerType
{
  typedef typename PointerTypeImpl<T, typename RemoveReference<D>::Type>::Type Type;
};

} // namespace detail

/**
 * UniquePtr is a smart pointer that wholly owns a resource.  Ownership may be
 * transferred out of a UniquePtr through explicit action, but otherwise the
 * resource is destroyed when the UniquePtr is destroyed.
 *
 * UniquePtr is similar to C++98's std::auto_ptr, but it improves upon auto_ptr
 * in one crucial way: it's impossible to copy a UniquePtr.  Copying an auto_ptr
 * obviously *can't* copy ownership of its singly-owned resource.  So what
 * happens if you try to copy one?  Bizarrely, ownership is implicitly
 * *transferred*, preserving single ownership but breaking code that assumes a
 * copy of an object is identical to the original.  (This is why auto_ptr is
 * prohibited in STL containers.)
 *
 * UniquePtr solves this problem by being *movable* rather than copyable.
 * Instead of passing a |UniquePtr u| directly to the constructor or assignment
 * operator, you pass |Move(u)|.  In doing so you indicate that you're *moving*
 * ownership out of |u|, into the target of the construction/assignment.  After
 * the transfer completes, |u| contains |nullptr| and may be safely destroyed.
 * This preserves single ownership but also allows UniquePtr to be moved by
 * algorithms that have been made move-safe.  (Note: if |u| is instead a
 * temporary expression, don't use |Move()|: just pass the expression, because
 * it's already move-ready.  For more information see Move.h.)
 *
 * UniquePtr is also better than std::auto_ptr in that the deletion operation is
 * customizable.  An optional second template parameter specifies a class that
 * (through its operator()(T*)) implements the desired deletion policy.  If no
 * policy is specified, mozilla::DefaultDelete<T> is used -- which will either
 * |delete| or |delete[]| the resource, depending whether the resource is an
 * array.  Custom deletion policies ideally should be empty classes (no member
 * fields, no member fields in base classes, no virtual methods/inheritance),
 * because then UniquePtr can be just as efficient as a raw pointer.
 *
 * Use of UniquePtr proceeds like so:
 *
 *   UniquePtr<int> g1; // initializes to nullptr
 *   g1.reset(new int); // switch resources using reset()
 *   g1 = nullptr; // clears g1, deletes the int
 *
 *   UniquePtr<int> g2(new int); // owns that int
 *   int* p = g2.release(); // g2 leaks its int -- still requires deletion
 *   delete p; // now freed
 *
 *   struct S { int x; S(int x) : x(x) {} };
 *   UniquePtr<S> g3, g4(new S(5));
 *   g3 = Move(g4); // g3 owns the S, g4 cleared
 *   S* p = g3.get(); // g3 still owns |p|
 *   assert(g3->x == 5); // operator-> works (if .get() != nullptr)
 *   assert((*g3).x == 5); // also operator* (again, if not cleared)
 *   Swap(g3, g4); // g4 now owns the S, g3 cleared
 *   g3.swap(g4);  // g3 now owns the S, g4 cleared
 *   UniquePtr<S> g5(Move(g3)); // g5 owns the S, g3 cleared
 *   g5.reset(); // deletes the S, g5 cleared
 *
 *   struct FreePolicy { void operator()(void* p) { free(p); } };
 *   UniquePtr<int, FreePolicy> g6(static_cast<int*>(malloc(sizeof(int))));
 *   int* ptr = g6.get();
 *   g6 = nullptr; // calls free(ptr)
 *
 * Now, carefully note a few things you *can't* do:
 *
 *   UniquePtr<int> b1;
 *   b1 = new int; // BAD: can only assign another UniquePtr
 *   int* ptr = b1; // BAD: no auto-conversion to pointer, use get()
 *
 *   UniquePtr<int> b2(b1); // BAD: can't copy a UniquePtr
 *   UniquePtr<int> b3 = b1; // BAD: can't copy-assign a UniquePtr
 *
 * (Note that changing a UniquePtr to store a direct |new| expression is
 * permitted, but usually you should use MakeUnique, defined at the end of this
 * header.)
 *
 * A few miscellaneous notes:
 *
 * UniquePtr, when not instantiated for an array type, can be move-constructed
 * and move-assigned, not only from itself but from "derived" UniquePtr<U, E>
 * instantiations where U converts to T and E converts to D.  If you want to use
 * this, you're going to have to specify a deletion policy for both UniquePtr
 * instantations, and T pretty much has to have a virtual destructor.  In other
 * words, this doesn't work:
 *
 *   struct Base { virtual ~Base() {} };
 *   struct Derived : Base {};
 *
 *   UniquePtr<Base> b1;
 *   // BAD: DefaultDelete<Base> and DefaultDelete<Derived> don't interconvert
 *   UniquePtr<Derived> d1(Move(b));
 *
 *   UniquePtr<Base> b2;
 *   UniquePtr<Derived, DefaultDelete<Base>> d2(Move(b2)); // okay
 *
 * UniquePtr is specialized for array types.  Specializing with an array type
 * creates a smart-pointer version of that array -- not a pointer to such an
 * array.
 *
 *   UniquePtr<int[]> arr(new int[5]);
 *   arr[0] = 4;
 *
 * What else is different?  Deletion of course uses |delete[]|.  An operator[]
 * is provided.  Functionality that doesn't make sense for arrays is removed.
 * The constructors and mutating methods only accept array pointers (not T*, U*
 * that converts to T*, or UniquePtr<U[]> or UniquePtr<U>) or |nullptr|.
 *
 * It's perfectly okay for a function to return a UniquePtr. This transfers
 * the UniquePtr's sole ownership of the data, to the fresh UniquePtr created
 * in the calling function, that will then solely own that data. Such functions
 * can return a local variable UniquePtr, |nullptr|, |UniquePtr(ptr)| where
 * |ptr| is a |T*|, or a UniquePtr |Move()|'d from elsewhere.
 *
 * UniquePtr will commonly be a member of a class, with lifetime equivalent to
 * that of that class.  If you want to expose the related resource, you could
 * expose a raw pointer via |get()|, but ownership of a raw pointer is
 * inherently unclear.  So it's better to expose a |const UniquePtr&| instead.
 * This prohibits mutation but still allows use of |get()| when needed (but
 * operator-> is preferred).  Of course, you can only use this smart pointer as
 * long as the enclosing class instance remains live -- no different than if you
 * exposed the |get()| raw pointer.
 *
 * To pass a UniquePtr-managed resource as a pointer, use a |const UniquePtr&|
 * argument.  To specify an inout parameter (where the method may or may not
 * take ownership of the resource, or reset it), or to specify an out parameter
 * (where simply returning a |UniquePtr| isn't possible), use a |UniquePtr&|
 * argument.  To unconditionally transfer ownership of a UniquePtr
 * into a method, use a |UniquePtr| argument.  To conditionally transfer
 * ownership of a resource into a method, should the method want it, use a
 * |UniquePtr&&| argument.
 */
template<typename T, class D>
class UniquePtr
{
public:
  typedef T ElementType;
  typedef D DeleterType;
  typedef typename detail::PointerType<T, DeleterType>::Type Pointer;

private:
  Pair<Pointer, DeleterType> mTuple;

  Pointer& ptr() { return mTuple.first(); }
  const Pointer& ptr() const { return mTuple.first(); }

  DeleterType& del() { return mTuple.second(); }
  const DeleterType& del() const { return mTuple.second(); }

public:
  /**
   * Construct a UniquePtr containing |nullptr|.
   */
  constexpr UniquePtr()
    : mTuple(static_cast<Pointer>(nullptr), DeleterType())
  {
    static_assert(!IsPointer<D>::value, "must provide a deleter instance");
    static_assert(!IsReference<D>::value, "must provide a deleter instance");
  }

  /**
   * Construct a UniquePtr containing |aPtr|.
   */
  explicit UniquePtr(Pointer aPtr)
    : mTuple(aPtr, DeleterType())
  {
    static_assert(!IsPointer<D>::value, "must provide a deleter instance");
    static_assert(!IsReference<D>::value, "must provide a deleter instance");
  }

  UniquePtr(Pointer aPtr,
            typename Conditional<IsReference<D>::value,
                                 D,
                                 const D&>::Type aD1)
    : mTuple(aPtr, aD1)
  {}

  // If you encounter an error with MSVC10 about RemoveReference below, along
  // the lines that "more than one partial specialization matches the template
  // argument list": don't use UniquePtr<T, reference to function>!  Ideally
  // you should make deletion use the same function every time, using a
  // deleter policy:
  //
  //   // BAD, won't compile with MSVC10, deleter doesn't need to be a
  //   // variable at all
  //   typedef void (&FreeSignature)(void*);
  //   UniquePtr<int, FreeSignature> ptr((int*) malloc(sizeof(int)), free);
  //
  //   // GOOD, compiles with MSVC10, deletion behavior statically known and
  //   // optimizable
  //   struct DeleteByFreeing
  //   {
  //     void operator()(void* aPtr) { free(aPtr); }
  //   };
  //
  // If deletion really, truly, must be a variable: you might be able to work
  // around this with a deleter class that contains the function reference.
  // But this workaround is untried and untested, because variable deletion
  // behavior really isn't something you should use.
  UniquePtr(Pointer aPtr,
            typename RemoveReference<D>::Type&& aD2)
    : mTuple(aPtr, Move(aD2))
  {
    static_assert(!IsReference<D>::value,
                  "rvalue deleter can't be stored by reference");
  }

  UniquePtr(UniquePtr&& aOther)
    : mTuple(aOther.release(), Forward<DeleterType>(aOther.get_deleter()))
  {}

  MOZ_IMPLICIT
  UniquePtr(decltype(nullptr))
    : mTuple(nullptr, DeleterType())
  {
    static_assert(!IsPointer<D>::value, "must provide a deleter instance");
    static_assert(!IsReference<D>::value, "must provide a deleter instance");
  }

  template<typename U, class E>
  MOZ_IMPLICIT
  UniquePtr(UniquePtr<U, E>&& aOther,
            typename EnableIf<IsConvertible<typename UniquePtr<U, E>::Pointer,
                                            Pointer>::value &&
                              !IsArray<U>::value &&
                              (IsReference<D>::value
                               ? IsSame<D, E>::value
                               : IsConvertible<E, D>::value),
                              int>::Type aDummy = 0)
    : mTuple(aOther.release(), Forward<E>(aOther.get_deleter()))
  {
  }

  ~UniquePtr() { reset(nullptr); }

  UniquePtr& operator=(UniquePtr&& aOther)
  {
    reset(aOther.release());
    get_deleter() = Forward<DeleterType>(aOther.get_deleter());
    return *this;
  }

  template<typename U, typename E>
  UniquePtr& operator=(UniquePtr<U, E>&& aOther)
  {
    static_assert(IsConvertible<typename UniquePtr<U, E>::Pointer,
                                Pointer>::value,
                  "incompatible UniquePtr pointees");
    static_assert(!IsArray<U>::value,
                  "can't assign from UniquePtr holding an array");

    reset(aOther.release());
    get_deleter() = Forward<E>(aOther.get_deleter());
    return *this;
  }

  UniquePtr& operator=(decltype(nullptr))
  {
    reset(nullptr);
    return *this;
  }

  typename AddLvalueReference<T>::Type operator*() const { return *get(); }
  Pointer operator->() const
  {
    MOZ_ASSERT(get(), "dereferencing a UniquePtr containing nullptr");
    return get();
  }

  explicit operator bool() const { return get() != nullptr; }

  Pointer get() const { return ptr(); }

  DeleterType& get_deleter() { return del(); }
  const DeleterType& get_deleter() const { return del(); }

  MOZ_MUST_USE Pointer release()
  {
    Pointer p = ptr();
    ptr() = nullptr;
    return p;
  }

  void reset(Pointer aPtr = Pointer())
  {
    Pointer old = ptr();
    ptr() = aPtr;
    if (old != nullptr) {
      get_deleter()(old);
    }
  }

  void swap(UniquePtr& aOther)
  {
    mTuple.swap(aOther.mTuple);
  }

  UniquePtr(const UniquePtr& aOther) = delete; // construct using Move()!
  void operator=(const UniquePtr& aOther) = delete; // assign using Move()!
};

// In case you didn't read the comment by the main definition (you should!): the
// UniquePtr<T[]> specialization exists to manage array pointers.  It deletes
// such pointers using delete[], it will reject construction and modification
// attempts using U* or U[].  Otherwise it works like the normal UniquePtr.
template<typename T, class D>
class UniquePtr<T[], D>
{
public:
  typedef T* Pointer;
  typedef T ElementType;
  typedef D DeleterType;

private:
  Pair<Pointer, DeleterType> mTuple;

public:
  /**
   * Construct a UniquePtr containing nullptr.
   */
  constexpr UniquePtr()
    : mTuple(static_cast<Pointer>(nullptr), DeleterType())
  {
    static_assert(!IsPointer<D>::value, "must provide a deleter instance");
    static_assert(!IsReference<D>::value, "must provide a deleter instance");
  }

  /**
   * Construct a UniquePtr containing |aPtr|.
   */
  explicit UniquePtr(Pointer aPtr)
    : mTuple(aPtr, DeleterType())
  {
    static_assert(!IsPointer<D>::value, "must provide a deleter instance");
    static_assert(!IsReference<D>::value, "must provide a deleter instance");
  }

  // delete[] knows how to handle *only* an array of a single class type.  For
  // delete[] to work correctly, it must know the size of each element, the
  // fields and base classes of each element requiring destruction, and so on.
  // So forbid all overloads which would end up invoking delete[] on a pointer
  // of the wrong type.
  template<typename U>
  UniquePtr(U&& aU,
            typename EnableIf<IsPointer<U>::value &&
                              IsConvertible<U, Pointer>::value,
                              int>::Type aDummy = 0)
  = delete;

  UniquePtr(Pointer aPtr,
            typename Conditional<IsReference<D>::value,
                                 D,
                                 const D&>::Type aD1)
    : mTuple(aPtr, aD1)
  {}

  // If you encounter an error with MSVC10 about RemoveReference below, along
  // the lines that "more than one partial specialization matches the template
  // argument list": don't use UniquePtr<T[], reference to function>!  See the
  // comment by this constructor in the non-T[] specialization above.
  UniquePtr(Pointer aPtr,
            typename RemoveReference<D>::Type&& aD2)
    : mTuple(aPtr, Move(aD2))
  {
    static_assert(!IsReference<D>::value,
                  "rvalue deleter can't be stored by reference");
  }

  // Forbidden for the same reasons as stated above.
  template<typename U, typename V>
  UniquePtr(U&& aU, V&& aV,
            typename EnableIf<IsPointer<U>::value &&
                              IsConvertible<U, Pointer>::value,
                              int>::Type aDummy = 0)
  = delete;

  UniquePtr(UniquePtr&& aOther)
    : mTuple(aOther.release(), Forward<DeleterType>(aOther.get_deleter()))
  {}

  MOZ_IMPLICIT
  UniquePtr(decltype(nullptr))
    : mTuple(nullptr, DeleterType())
  {
    static_assert(!IsPointer<D>::value, "must provide a deleter instance");
    static_assert(!IsReference<D>::value, "must provide a deleter instance");
  }

  ~UniquePtr() { reset(nullptr); }

  UniquePtr& operator=(UniquePtr&& aOther)
  {
    reset(aOther.release());
    get_deleter() = Forward<DeleterType>(aOther.get_deleter());
    return *this;
  }

  UniquePtr& operator=(decltype(nullptr))
  {
    reset();
    return *this;
  }

  explicit operator bool() const { return get() != nullptr; }

  T& operator[](decltype(sizeof(int)) aIndex) const { return get()[aIndex]; }
  Pointer get() const { return mTuple.first(); }

  DeleterType& get_deleter() { return mTuple.second(); }
  const DeleterType& get_deleter() const { return mTuple.second(); }

  MOZ_MUST_USE Pointer release()
  {
    Pointer p = mTuple.first();
    mTuple.first() = nullptr;
    return p;
  }

  void reset(Pointer aPtr = Pointer())
  {
    Pointer old = mTuple.first();
    mTuple.first() = aPtr;
    if (old != nullptr) {
      mTuple.second()(old);
    }
  }

  void reset(decltype(nullptr))
  {
    Pointer old = mTuple.first();
    mTuple.first() = nullptr;
    if (old != nullptr) {
      mTuple.second()(old);
    }
  }

  template<typename U>
  void reset(U) = delete;

  void swap(UniquePtr& aOther) { mTuple.swap(aOther.mTuple); }

  UniquePtr(const UniquePtr& aOther) = delete; // construct using Move()!
  void operator=(const UniquePtr& aOther) = delete; // assign using Move()!
};

/**
 * A default deletion policy using plain old operator delete.
 *
 * Note that this type can be specialized, but authors should beware of the risk
 * that the specialization may at some point cease to match (either because it
 * gets moved to a different compilation unit or the signature changes). If the
 * non-specialized (|delete|-based) version compiles for that type but does the
 * wrong thing, bad things could happen.
 *
 * This is a non-issue for types which are always incomplete (i.e. opaque handle
 * types), since |delete|-ing such a type will always trigger a compilation
 * error.
 */
template<typename T>
class DefaultDelete
{
public:
  constexpr DefaultDelete() {}

  template<typename U>
  MOZ_IMPLICIT DefaultDelete(const DefaultDelete<U>& aOther,
                             typename EnableIf<mozilla::IsConvertible<U*, T*>::value,
                                               int>::Type aDummy = 0)
  {}

  void operator()(T* aPtr) const
  {
    static_assert(sizeof(T) > 0, "T must be complete");
    delete aPtr;
  }
};

/** A default deletion policy using operator delete[]. */
template<typename T>
class DefaultDelete<T[]>
{
public:
  constexpr DefaultDelete() {}

  void operator()(T* aPtr) const
  {
    static_assert(sizeof(T) > 0, "T must be complete");
    delete[] aPtr;
  }

  template<typename U>
  void operator()(U* aPtr) const = delete;
};

template<typename T, class D>
void
Swap(UniquePtr<T, D>& aX, UniquePtr<T, D>& aY)
{
  aX.swap(aY);
}

template<typename T, class D, typename U, class E>
bool
operator==(const UniquePtr<T, D>& aX, const UniquePtr<U, E>& aY)
{
  return aX.get() == aY.get();
}

template<typename T, class D, typename U, class E>
bool
operator!=(const UniquePtr<T, D>& aX, const UniquePtr<U, E>& aY)
{
  return aX.get() != aY.get();
}

template<typename T, class D>
bool
operator==(const UniquePtr<T, D>& aX, decltype(nullptr))
{
  return !aX;
}

template<typename T, class D>
bool
operator==(decltype(nullptr), const UniquePtr<T, D>& aX)
{
  return !aX;
}

template<typename T, class D>
bool
operator!=(const UniquePtr<T, D>& aX, decltype(nullptr))
{
  return bool(aX);
}

template<typename T, class D>
bool
operator!=(decltype(nullptr), const UniquePtr<T, D>& aX)
{
  return bool(aX);
}

// No operator<, operator>, operator<=, operator>= for now because simplicity.

namespace detail {

template<typename T>
struct UniqueSelector
{
  typedef UniquePtr<T> SingleObject;
};

template<typename T>
struct UniqueSelector<T[]>
{
  typedef UniquePtr<T[]> UnknownBound;
};

template<typename T, decltype(sizeof(int)) N>
struct UniqueSelector<T[N]>
{
  typedef UniquePtr<T[N]> KnownBound;
};

} // namespace detail

/**
 * MakeUnique is a helper function for allocating new'd objects and arrays,
 * returning a UniquePtr containing the resulting pointer.  The semantics of
 * MakeUnique<Type>(...) are as follows.
 *
 *   If Type is an array T[n]:
 *     Disallowed, deleted, no overload for you!
 *   If Type is an array T[]:
 *     MakeUnique<T[]>(size_t) is the only valid overload.  The pointer returned
 *     is as if by |new T[n]()|, which value-initializes each element.  (If T
 *     isn't a class type, this will zero each element.  If T is a class type,
 *     then roughly speaking, each element will be constructed using its default
 *     constructor.  See C++11 [dcl.init]p7 for the full gory details.)
 *   If Type is non-array T:
 *     The arguments passed to MakeUnique<T>(...) are forwarded into a
 *     |new T(...)| call, initializing the T as would happen if executing
 *     |T(...)|.
 *
 * There are various benefits to using MakeUnique instead of |new| expressions.
 *
 * First, MakeUnique eliminates use of |new| from code entirely.  If objects are
 * only created through UniquePtr, then (assuming all explicit release() calls
 * are safe, including transitively, and no type-safety casting funniness)
 * correctly maintained ownership of the UniquePtr guarantees no leaks are
 * possible.  (This pays off best if a class is only ever created through a
 * factory method on the class, using a private constructor.)
 *
 * Second, initializing a UniquePtr using a |new| expression requires repeating
 * the name of the new'd type, whereas MakeUnique in concert with the |auto|
 * keyword names it only once:
 *
 *   UniquePtr<char> ptr1(new char()); // repetitive
 *   auto ptr2 = MakeUnique<char>();   // shorter
 *
 * Of course this assumes the reader understands the operation MakeUnique
 * performs.  In the long run this is probably a reasonable assumption.  In the
 * short run you'll have to use your judgment about what readers can be expected
 * to know, or to quickly look up.
 *
 * Third, a call to MakeUnique can be assigned directly to a UniquePtr.  In
 * contrast you can't assign a pointer into a UniquePtr without using the
 * cumbersome reset().
 *
 *   UniquePtr<char> p;
 *   p = new char;           // ERROR
 *   p.reset(new char);      // works, but fugly
 *   p = MakeUnique<char>(); // preferred
 *
 * (And third, although not relevant to Mozilla: MakeUnique is exception-safe.
 * An exception thrown after |new T| succeeds will leak that memory, unless the
 * pointer is assigned to an object that will manage its ownership.  UniquePtr
 * ably serves this function.)
 */

template<typename T, typename... Args>
typename detail::UniqueSelector<T>::SingleObject
MakeUnique(Args&&... aArgs)
{
  return UniquePtr<T>(new T(Forward<Args>(aArgs)...));
}

template<typename T>
typename detail::UniqueSelector<T>::UnknownBound
MakeUnique(decltype(sizeof(int)) aN)
{
  typedef typename RemoveExtent<T>::Type ArrayType;
  return UniquePtr<T>(new ArrayType[aN]());
}

template<typename T, typename... Args>
typename detail::UniqueSelector<T>::KnownBound
MakeUnique(Args&&... aArgs) = delete;

} // namespace mozilla

#endif /* mozilla_UniquePtr_h */