mfbt/UniquePtr.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* Smart pointer managing sole ownership of a resource. */
   8
   9 #ifndef mozilla_UniquePtr_h
  10 #define mozilla_UniquePtr_h
  11
  12 #include <type_traits>
  13 #include <utility>
  14
  15 #include "mozilla/Assertions.h"
  16 #include "mozilla/Attributes.h"
  17 #include "mozilla/CompactPair.h"
  18 #include "mozilla/Compiler.h"
  19
  20 namespace mozilla {
  21
  22 template <typename T>
  23 class DefaultDelete;
  24 template <typename T, class D = DefaultDelete<T>>
  25 class UniquePtr;
  26
  27 }  // namespace mozilla
  28
  29 namespace mozilla {
  30
  31 namespace detail {
  32
  33 struct HasPointerTypeHelper {
  34   template <class U>
  35   static double Test(...);
  36   template <class U>
  37   static char Test(typename U::pointer* = 0);
  38 };
  39
  40 template <class T>
  41 class HasPointerType
  42     : public std::integral_constant<bool, sizeof(HasPointerTypeHelper::Test<T>(
  43                                               0)) == 1> {};
  44
  45 template <class T, class D, bool = HasPointerType<D>::value>
  46 struct PointerTypeImpl {
  47   typedef typename D::pointer Type;
  48 };
  49
  50 template <class T, class D>
  51 struct PointerTypeImpl<T, D, false> {
  52   typedef T* Type;
  53 };
  54
  55 template <class T, class D>
  56 struct PointerType {
  57   typedef typename PointerTypeImpl<T, std::remove_reference_t<D>>::Type Type;
  58 };
  59
  60 }  // namespace detail
  61
  62 /**
  63  * UniquePtr is a smart pointer that wholly owns a resource.  Ownership may be
  64  * transferred out of a UniquePtr through explicit action, but otherwise the
  65  * resource is destroyed when the UniquePtr is destroyed.
  66  *
  67  * UniquePtr is similar to C++98's std::auto_ptr, but it improves upon auto_ptr
  68  * in one crucial way: it's impossible to copy a UniquePtr.  Copying an auto_ptr
  69  * obviously *can't* copy ownership of its singly-owned resource.  So what
  70  * happens if you try to copy one?  Bizarrely, ownership is implicitly
  71  * *transferred*, preserving single ownership but breaking code that assumes a
  72  * copy of an object is identical to the original.  (This is why auto_ptr is
  73  * prohibited in STL containers.)
  74  *
  75  * UniquePtr solves this problem by being *movable* rather than copyable.
  76  * Instead of passing a |UniquePtr u| directly to the constructor or assignment
  77  * operator, you pass |Move(u)|.  In doing so you indicate that you're *moving*
  78  * ownership out of |u|, into the target of the construction/assignment.  After
  79  * the transfer completes, |u| contains |nullptr| and may be safely destroyed.
  80  * This preserves single ownership but also allows UniquePtr to be moved by
  81  * algorithms that have been made move-safe.  (Note: if |u| is instead a
  82  * temporary expression, don't use |Move()|: just pass the expression, because
  83  * it's already move-ready.  For more information see Move.h.)
  84  *
  85  * UniquePtr is also better than std::auto_ptr in that the deletion operation is
  86  * customizable.  An optional second template parameter specifies a class that
  87  * (through its operator()(T*)) implements the desired deletion policy.  If no
  88  * policy is specified, mozilla::DefaultDelete<T> is used -- which will either
  89  * |delete| or |delete[]| the resource, depending whether the resource is an
  90  * array.  Custom deletion policies ideally should be empty classes (no member
  91  * fields, no member fields in base classes, no virtual methods/inheritance),
  92  * because then UniquePtr can be just as efficient as a raw pointer.
  93  *
  94  * Use of UniquePtr proceeds like so:
  95  *
  96  *   UniquePtr<int> g1; // initializes to nullptr
  97  *   g1.reset(new int); // switch resources using reset()
  98  *   g1 = nullptr; // clears g1, deletes the int
  99  *
 100  *   UniquePtr<int> g2(new int); // owns that int
 101  *   int* p = g2.release(); // g2 leaks its int -- still requires deletion
 102  *   delete p; // now freed
 103  *
 104  *   struct S { int x; S(int x) : x(x) {} };
 105  *   UniquePtr<S> g3, g4(new S(5));
 106  *   g3 = std::move(g4); // g3 owns the S, g4 cleared
 107  *   S* p = g3.get(); // g3 still owns |p|
 108  *   assert(g3->x == 5); // operator-> works (if .get() != nullptr)
 109  *   assert((*g3).x == 5); // also operator* (again, if not cleared)
 110  *   std::swap(g3, g4); // g4 now owns the S, g3 cleared
 111  *   g3.swap(g4);  // g3 now owns the S, g4 cleared
 112  *   UniquePtr<S> g5(std::move(g3)); // g5 owns the S, g3 cleared
 113  *   g5.reset(); // deletes the S, g5 cleared
 114  *
 115  *   struct FreePolicy { void operator()(void* p) { free(p); } };
 116  *   UniquePtr<int, FreePolicy> g6(static_cast<int*>(malloc(sizeof(int))));
 117  *   int* ptr = g6.get();
 118  *   g6 = nullptr; // calls free(ptr)
 119  *
 120  * Now, carefully note a few things you *can't* do:
 121  *
 122  *   UniquePtr<int> b1;
 123  *   b1 = new int; // BAD: can only assign another UniquePtr
 124  *   int* ptr = b1; // BAD: no auto-conversion to pointer, use get()
 125  *
 126  *   UniquePtr<int> b2(b1); // BAD: can't copy a UniquePtr
 127  *   UniquePtr<int> b3 = b1; // BAD: can't copy-assign a UniquePtr
 128  *
 129  * (Note that changing a UniquePtr to store a direct |new| expression is
 130  * permitted, but usually you should use MakeUnique, defined at the end of this
 131  * header.)
 132  *
 133  * A few miscellaneous notes:
 134  *
 135  * UniquePtr, when not instantiated for an array type, can be move-constructed
 136  * and move-assigned, not only from itself but from "derived" UniquePtr<U, E>
 137  * instantiations where U converts to T and E converts to D.  If you want to use
 138  * this, you're going to have to specify a deletion policy for both UniquePtr
 139  * instantations, and T pretty much has to have a virtual destructor.  In other
 140  * words, this doesn't work:
 141  *
 142  *   struct Base { virtual ~Base() {} };
 143  *   struct Derived : Base {};
 144  *
 145  *   UniquePtr<Base> b1;
 146  *   // BAD: DefaultDelete<Base> and DefaultDelete<Derived> don't interconvert
 147  *   UniquePtr<Derived> d1(std::move(b));
 148  *
 149  *   UniquePtr<Base> b2;
 150  *   UniquePtr<Derived, DefaultDelete<Base>> d2(std::move(b2)); // okay
 151  *
 152  * UniquePtr is specialized for array types.  Specializing with an array type
 153  * creates a smart-pointer version of that array -- not a pointer to such an
 154  * array.
 155  *
 156  *   UniquePtr<int[]> arr(new int[5]);
 157  *   arr[0] = 4;
 158  *
 159  * What else is different?  Deletion of course uses |delete[]|.  An operator[]
 160  * is provided.  Functionality that doesn't make sense for arrays is removed.
 161  * The constructors and mutating methods only accept array pointers (not T*, U*
 162  * that converts to T*, or UniquePtr<U[]> or UniquePtr<U>) or |nullptr|.
 163  *
 164  * It's perfectly okay for a function to return a UniquePtr. This transfers
 165  * the UniquePtr's sole ownership of the data, to the fresh UniquePtr created
 166  * in the calling function, that will then solely own that data. Such functions
 167  * can return a local variable UniquePtr, |nullptr|, |UniquePtr(ptr)| where
 168  * |ptr| is a |T*|, or a UniquePtr |Move()|'d from elsewhere.
 169  *
 170  * UniquePtr will commonly be a member of a class, with lifetime equivalent to
 171  * that of that class.  If you want to expose the related resource, you could
 172  * expose a raw pointer via |get()|, but ownership of a raw pointer is
 173  * inherently unclear.  So it's better to expose a |const UniquePtr&| instead.
 174  * This prohibits mutation but still allows use of |get()| when needed (but
 175  * operator-> is preferred).  Of course, you can only use this smart pointer as
 176  * long as the enclosing class instance remains live -- no different than if you
 177  * exposed the |get()| raw pointer.
 178  *
 179  * To pass a UniquePtr-managed resource as a pointer, use a |const UniquePtr&|
 180  * argument.  To specify an inout parameter (where the method may or may not
 181  * take ownership of the resource, or reset it), or to specify an out parameter
 182  * (where simply returning a |UniquePtr| isn't possible), use a |UniquePtr&|
 183  * argument.  To unconditionally transfer ownership of a UniquePtr
 184  * into a method, use a |UniquePtr| argument.  To conditionally transfer
 185  * ownership of a resource into a method, should the method want it, use a
 186  * |UniquePtr&&| argument.
 187  */
 188 template <typename T, class D>
 189 class UniquePtr {
 190  public:
 191   typedef T ElementType;
 192   typedef D DeleterType;
 193   typedef typename detail::PointerType<T, DeleterType>::Type Pointer;
 194
 195  private:
 196   mozilla::CompactPair<Pointer, DeleterType> mTuple;
 197
 198   Pointer& ptr() { return mTuple.first(); }
 199   const Pointer& ptr() const { return mTuple.first(); }
 200
 201   DeleterType& del() { return mTuple.second(); }
 202   const DeleterType& del() const { return mTuple.second(); }
 203
 204  public:
 205   /**
 206    * Construct a UniquePtr containing |nullptr|.
 207    */
 208   constexpr UniquePtr() : mTuple(static_cast<Pointer>(nullptr), DeleterType()) {
 209     static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
 210     static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
 211   }
 212
 213   /**
 214    * Construct a UniquePtr containing |aPtr|.
 215    */
 216   explicit UniquePtr(Pointer aPtr) : mTuple(aPtr, DeleterType()) {
 217     static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
 218     static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
 219   }
 220
 221   UniquePtr(Pointer aPtr,
 222             std::conditional_t<std::is_reference_v<D>, D, const D&> aD1)
 223       : mTuple(aPtr, aD1) {}
 224
 225   UniquePtr(Pointer aPtr, std::remove_reference_t<D>&& aD2)
 226       : mTuple(aPtr, std::move(aD2)) {
 227     static_assert(!std::is_reference_v<D>,
 228                   "rvalue deleter can't be stored by reference");
 229   }
 230
 231   UniquePtr(UniquePtr&& aOther)
 232       : mTuple(aOther.release(),
 233                std::forward<DeleterType>(aOther.get_deleter())) {}
 234
 235   MOZ_IMPLICIT constexpr UniquePtr(decltype(nullptr)) : UniquePtr() {}
 236
 237   template <typename U, class E>
 238   MOZ_IMPLICIT UniquePtr(
 239       UniquePtr<U, E>&& aOther,
 240       std::enable_if_t<
 241           std::is_convertible_v<typename UniquePtr<U, E>::Pointer, Pointer> &&
 242               !std::is_array_v<U> &&
 243               (std::is_reference_v<D> ? std::is_same_v<D, E>
 244                                       : std::is_convertible_v<E, D>),
 245           int>
 246           aDummy = 0)
 247       : mTuple(aOther.release(), std::forward<E>(aOther.get_deleter())) {}
 248
 249   ~UniquePtr() { reset(nullptr); }
 250
 251   UniquePtr& operator=(UniquePtr&& aOther) {
 252     reset(aOther.release());
 253     get_deleter() = std::forward<DeleterType>(aOther.get_deleter());
 254     return *this;
 255   }
 256
 257   template <typename U, typename E>
 258   UniquePtr& operator=(UniquePtr<U, E>&& aOther) {
 259     static_assert(
 260         std::is_convertible_v<typename UniquePtr<U, E>::Pointer, Pointer>,
 261         "incompatible UniquePtr pointees");
 262     static_assert(!std::is_array_v<U>,
 263                   "can't assign from UniquePtr holding an array");
 264
 265     reset(aOther.release());
 266     get_deleter() = std::forward<E>(aOther.get_deleter());
 267     return *this;
 268   }
 269
 270   UniquePtr& operator=(decltype(nullptr)) {
 271     reset(nullptr);
 272     return *this;
 273   }
 274
 275   std::add_lvalue_reference_t<T> operator*() const {
 276     MOZ_ASSERT(get(), "dereferencing a UniquePtr containing nullptr with *");
 277     return *get();
 278   }
 279   Pointer operator->() const {
 280     MOZ_ASSERT(get(), "dereferencing a UniquePtr containing nullptr with ->");
 281     return get();
 282   }
 283
 284   explicit operator bool() const { return get() != nullptr; }
 285
 286   Pointer get() const { return ptr(); }
 287
 288   DeleterType& get_deleter() { return del(); }
 289   const DeleterType& get_deleter() const { return del(); }
 290
 291   [[nodiscard]] Pointer release() {
 292     Pointer p = ptr();
 293     ptr() = nullptr;
 294     return p;
 295   }
 296
 297   void reset(Pointer aPtr = Pointer()) {
 298     Pointer old = ptr();
 299     ptr() = aPtr;
 300     if (old != nullptr) {
 301       get_deleter()(old);
 302     }
 303   }
 304
 305   void swap(UniquePtr& aOther) { mTuple.swap(aOther.mTuple); }
 306
 307   UniquePtr(const UniquePtr& aOther) = delete;  // construct using std::move()!
 308   void operator=(const UniquePtr& aOther) =
 309       delete;  // assign using std::move()!
 310 };
 311
 312 // In case you didn't read the comment by the main definition (you should!): the
 313 // UniquePtr<T[]> specialization exists to manage array pointers.  It deletes
 314 // such pointers using delete[], it will reject construction and modification
 315 // attempts using U* or U[].  Otherwise it works like the normal UniquePtr.
 316 template <typename T, class D>
 317 class UniquePtr<T[], D> {
 318  public:
 319   typedef T* Pointer;
 320   typedef T ElementType;
 321   typedef D DeleterType;
 322
 323  private:
 324   mozilla::CompactPair<Pointer, DeleterType> mTuple;
 325
 326  public:
 327   /**
 328    * Construct a UniquePtr containing nullptr.
 329    */
 330   constexpr UniquePtr() : mTuple(static_cast<Pointer>(nullptr), DeleterType()) {
 331     static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
 332     static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
 333   }
 334
 335   /**
 336    * Construct a UniquePtr containing |aPtr|.
 337    */
 338   explicit UniquePtr(Pointer aPtr) : mTuple(aPtr, DeleterType()) {
 339     static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
 340     static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
 341   }
 342
 343   // delete[] knows how to handle *only* an array of a single class type.  For
 344   // delete[] to work correctly, it must know the size of each element, the
 345   // fields and base classes of each element requiring destruction, and so on.
 346   // So forbid all overloads which would end up invoking delete[] on a pointer
 347   // of the wrong type.
 348   template <typename U>
 349   UniquePtr(U&& aU,
 350             std::enable_if_t<
 351                 std::is_pointer_v<U> && std::is_convertible_v<U, Pointer>, int>
 352                 aDummy = 0) = delete;
 353
 354   UniquePtr(Pointer aPtr,
 355             std::conditional_t<std::is_reference_v<D>, D, const D&> aD1)
 356       : mTuple(aPtr, aD1) {}
 357
 358   UniquePtr(Pointer aPtr, std::remove_reference_t<D>&& aD2)
 359       : mTuple(aPtr, std::move(aD2)) {
 360     static_assert(!std::is_reference_v<D>,
 361                   "rvalue deleter can't be stored by reference");
 362   }
 363
 364   // Forbidden for the same reasons as stated above.
 365   template <typename U, typename V>
 366   UniquePtr(U&& aU, V&& aV,
 367             std::enable_if_t<
 368                 std::is_pointer_v<U> && std::is_convertible_v<U, Pointer>, int>
 369                 aDummy = 0) = delete;
 370
 371   UniquePtr(UniquePtr&& aOther)
 372       : mTuple(aOther.release(),
 373                std::forward<DeleterType>(aOther.get_deleter())) {}
 374
 375   MOZ_IMPLICIT
 376   UniquePtr(decltype(nullptr)) : mTuple(nullptr, DeleterType()) {
 377     static_assert(!std::is_pointer_v<D>, "must provide a deleter instance");
 378     static_assert(!std::is_reference_v<D>, "must provide a deleter instance");
 379   }
 380
 381   ~UniquePtr() { reset(nullptr); }
 382
 383   UniquePtr& operator=(UniquePtr&& aOther) {
 384     reset(aOther.release());
 385     get_deleter() = std::forward<DeleterType>(aOther.get_deleter());
 386     return *this;
 387   }
 388
 389   UniquePtr& operator=(decltype(nullptr)) {
 390     reset();
 391     return *this;
 392   }
 393
 394   explicit operator bool() const { return get() != nullptr; }
 395
 396   T& operator[](decltype(sizeof(int)) aIndex) const { return get()[aIndex]; }
 397   Pointer get() const { return mTuple.first(); }
 398
 399   DeleterType& get_deleter() { return mTuple.second(); }
 400   const DeleterType& get_deleter() const { return mTuple.second(); }
 401
 402   [[nodiscard]] Pointer release() {
 403     Pointer p = mTuple.first();
 404     mTuple.first() = nullptr;
 405     return p;
 406   }
 407
 408   void reset(Pointer aPtr = Pointer()) {
 409     Pointer old = mTuple.first();
 410     mTuple.first() = aPtr;
 411     if (old != nullptr) {
 412       mTuple.second()(old);
 413     }
 414   }
 415
 416   void reset(decltype(nullptr)) {
 417     Pointer old = mTuple.first();
 418     mTuple.first() = nullptr;
 419     if (old != nullptr) {
 420       mTuple.second()(old);
 421     }
 422   }
 423
 424   template <typename U>
 425   void reset(U) = delete;
 426
 427   void swap(UniquePtr& aOther) { mTuple.swap(aOther.mTuple); }
 428
 429   UniquePtr(const UniquePtr& aOther) = delete;  // construct using std::move()!
 430   void operator=(const UniquePtr& aOther) =
 431       delete;  // assign using std::move()!
 432 };
 433
 434 /**
 435  * A default deletion policy using plain old operator delete.
 436  *
 437  * Note that this type can be specialized, but authors should beware of the risk
 438  * that the specialization may at some point cease to match (either because it
 439  * gets moved to a different compilation unit or the signature changes). If the
 440  * non-specialized (|delete|-based) version compiles for that type but does the
 441  * wrong thing, bad things could happen.
 442  *
 443  * This is a non-issue for types which are always incomplete (i.e. opaque handle
 444  * types), since |delete|-ing such a type will always trigger a compilation
 445  * error.
 446  */
 447 template <typename T>
 448 class DefaultDelete {
 449  public:
 450   constexpr DefaultDelete() = default;
 451
 452   template <typename U>
 453   MOZ_IMPLICIT DefaultDelete(
 454       const DefaultDelete<U>& aOther,
 455       std::enable_if_t<std::is_convertible_v<U*, T*>, int> aDummy = 0) {}
 456
 457   void operator()(T* aPtr) const {
 458     static_assert(sizeof(T) > 0, "T must be complete");
 459     delete aPtr;
 460   }
 461 };
 462
 463 /** A default deletion policy using operator delete[]. */
 464 template <typename T>
 465 class DefaultDelete<T[]> {
 466  public:
 467   constexpr DefaultDelete() = default;
 468
 469   void operator()(T* aPtr) const {
 470     static_assert(sizeof(T) > 0, "T must be complete");
 471     delete[] aPtr;
 472   }
 473
 474   template <typename U>
 475   void operator()(U* aPtr) const = delete;
 476 };
 477
 478 template <typename T, class D, typename U, class E>
 479 bool operator==(const UniquePtr<T, D>& aX, const UniquePtr<U, E>& aY) {
 480   return aX.get() == aY.get();
 481 }
 482
 483 template <typename T, class D, typename U, class E>
 484 bool operator!=(const UniquePtr<T, D>& aX, const UniquePtr<U, E>& aY) {
 485   return aX.get() != aY.get();
 486 }
 487
 488 template <typename T, class D>
 489 bool operator==(const UniquePtr<T, D>& aX, const T* aY) {
 490   return aX.get() == aY;
 491 }
 492
 493 template <typename T, class D>
 494 bool operator==(const T* aY, const UniquePtr<T, D>& aX) {
 495   return aY == aX.get();
 496 }
 497
 498 template <typename T, class D>
 499 bool operator!=(const UniquePtr<T, D>& aX, const T* aY) {
 500   return aX.get() != aY;
 501 }
 502
 503 template <typename T, class D>
 504 bool operator!=(const T* aY, const UniquePtr<T, D>& aX) {
 505   return aY != aX.get();
 506 }
 507
 508 template <typename T, class D>
 509 bool operator==(const UniquePtr<T, D>& aX, decltype(nullptr)) {
 510   return !aX;
 511 }
 512
 513 template <typename T, class D>
 514 bool operator==(decltype(nullptr), const UniquePtr<T, D>& aX) {
 515   return !aX;
 516 }
 517
 518 template <typename T, class D>
 519 bool operator!=(const UniquePtr<T, D>& aX, decltype(nullptr)) {
 520   return bool(aX);
 521 }
 522
 523 template <typename T, class D>
 524 bool operator!=(decltype(nullptr), const UniquePtr<T, D>& aX) {
 525   return bool(aX);
 526 }
 527
 528 // No operator<, operator>, operator<=, operator>= for now because simplicity.
 529
 530 namespace detail {
 531
 532 template <typename T>
 533 struct UniqueSelector {
 534   typedef UniquePtr<T> SingleObject;
 535 };
 536
 537 template <typename T>
 538 struct UniqueSelector<T[]> {
 539   typedef UniquePtr<T[]> UnknownBound;
 540 };
 541
 542 template <typename T, decltype(sizeof(int)) N>
 543 struct UniqueSelector<T[N]> {
 544   typedef UniquePtr<T[N]> KnownBound;
 545 };
 546
 547 }  // namespace detail
 548
 549 /**
 550  * MakeUnique is a helper function for allocating new'd objects and arrays,
 551  * returning a UniquePtr containing the resulting pointer.  The semantics of
 552  * MakeUnique<Type>(...) are as follows.
 553  *
 554  *   If Type is an array T[n]:
 555  *     Disallowed, deleted, no overload for you!
 556  *   If Type is an array T[]:
 557  *     MakeUnique<T[]>(size_t) is the only valid overload.  The pointer returned
 558  *     is as if by |new T[n]()|, which value-initializes each element.  (If T
 559  *     isn't a class type, this will zero each element.  If T is a class type,
 560  *     then roughly speaking, each element will be constructed using its default
 561  *     constructor.  See C++11 [dcl.init]p7 for the full gory details.)
 562  *   If Type is non-array T:
 563  *     The arguments passed to MakeUnique<T>(...) are forwarded into a
 564  *     |new T(...)| call, initializing the T as would happen if executing
 565  *     |T(...)|.
 566  *
 567  * There are various benefits to using MakeUnique instead of |new| expressions.
 568  *
 569  * First, MakeUnique eliminates use of |new| from code entirely.  If objects are
 570  * only created through UniquePtr, then (assuming all explicit release() calls
 571  * are safe, including transitively, and no type-safety casting funniness)
 572  * correctly maintained ownership of the UniquePtr guarantees no leaks are
 573  * possible.  (This pays off best if a class is only ever created through a
 574  * factory method on the class, using a private constructor.)
 575  *
 576  * Second, initializing a UniquePtr using a |new| expression requires repeating
 577  * the name of the new'd type, whereas MakeUnique in concert with the |auto|
 578  * keyword names it only once:
 579  *
 580  *   UniquePtr<char> ptr1(new char()); // repetitive
 581  *   auto ptr2 = MakeUnique<char>();   // shorter
 582  *
 583  * Of course this assumes the reader understands the operation MakeUnique
 584  * performs.  In the long run this is probably a reasonable assumption.  In the
 585  * short run you'll have to use your judgment about what readers can be expected
 586  * to know, or to quickly look up.
 587  *
 588  * Third, a call to MakeUnique can be assigned directly to a UniquePtr.  In
 589  * contrast you can't assign a pointer into a UniquePtr without using the
 590  * cumbersome reset().
 591  *
 592  *   UniquePtr<char> p;
 593  *   p = new char;           // ERROR
 594  *   p.reset(new char);      // works, but fugly
 595  *   p = MakeUnique<char>(); // preferred
 596  *
 597  * (And third, although not relevant to Mozilla: MakeUnique is exception-safe.
 598  * An exception thrown after |new T| succeeds will leak that memory, unless the
 599  * pointer is assigned to an object that will manage its ownership.  UniquePtr
 600  * ably serves this function.)
 601  */
 602
 603 template <typename T, typename... Args>
 604 typename detail::UniqueSelector<T>::SingleObject MakeUnique(Args&&... aArgs) {
 605   return UniquePtr<T>(new T(std::forward<Args>(aArgs)...));
 606 }
 607
 608 template <typename T>
 609 typename detail::UniqueSelector<T>::UnknownBound MakeUnique(
 610     decltype(sizeof(int)) aN) {
 611   using ArrayType = std::remove_extent_t<T>;
 612   return UniquePtr<T>(new ArrayType[aN]());
 613 }
 614
 615 template <typename T, typename... Args>
 616 typename detail::UniqueSelector<T>::KnownBound MakeUnique(Args&&... aArgs) =
 617     delete;
 618
 619 /**
 620  * WrapUnique is a helper function to transfer ownership from a raw pointer
 621  * into a UniquePtr<T>. It can only be used with a single non-array type.
 622  *
 623  * It is generally used this way:
 624  *
 625  *   auto p = WrapUnique(new char);
 626  *
 627  * It can be used when MakeUnique is not usable, for example, when the
 628  * constructor you are using is private, or you want to use aggregate
 629  * initialization.
 630  */
 631
 632 template <typename T>
 633 typename detail::UniqueSelector<T>::SingleObject WrapUnique(T* aPtr) {
 634   return UniquePtr<T>(aPtr);
 635 }
 636
 637 }  // namespace mozilla
 638
 639 namespace std {
 640
 641 template <typename T, class D>
 642 void swap(mozilla::UniquePtr<T, D>& aX, mozilla::UniquePtr<T, D>& aY) {
 643   aX.swap(aY);
 644 }
 645
 646 }  // namespace std
 647
 648 #endif /* mozilla_UniquePtr_h */