mfbt/AlreadyAddRefed.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 /* Typed temporary pointers for reference-counted smart pointers. */
   8
   9 #ifndef AlreadyAddRefed_h
  10 #define AlreadyAddRefed_h
  11
  12 #include "mozilla/Assertions.h"
  13 #include "mozilla/Attributes.h"
  14 #include "mozilla/Move.h"
  15
  16 namespace mozilla {
  17
  18 struct unused_t;
  19
  20 } // namespace mozilla
  21
  22 /**
  23  * already_AddRefed cooperates with reference counting smart pointers to enable
  24  * you to assign in a pointer _without_ |AddRef|ing it.  You might want to use
  25  * this as a return type from a function that returns an already |AddRef|ed
  26  * pointer.
  27  *
  28  * TODO Move already_AddRefed to namespace mozilla.  This has not yet been done
  29  * because of the sheer number of usages of already_AddRefed.
  30  *
  31  * When should you use already_AddRefed<>?
  32  * * Ensure a consumer takes ownership of a reference
  33  * * Pass ownership without calling AddRef/Release (sometimes required in
  34  *   off-main-thread code)
  35  * * The ref pointer type you're using doesn't support move construction
  36  *
  37  * Otherwise, use Move(RefPtr/nsCOMPtr/etc).
  38  */
  39 template<class T>
  40 struct MOZ_MUST_USE_TYPE MOZ_NON_AUTOABLE already_AddRefed
  41 {
  42   /*
  43    * We want to allow returning nullptr from functions returning
  44    * already_AddRefed<T>, for simplicity.  But we also don't want to allow
  45    * returning raw T*, instead preferring creation of already_AddRefed<T> from
  46    * a reference counting smart pointer.
  47    *
  48    * We address the latter requirement by making the (T*) constructor explicit.
  49    * But |return nullptr| won't consider an explicit constructor, so we need
  50    * another constructor to handle it.  Plain old (decltype(nullptr)) doesn't
  51    * cut it, because if nullptr is emulated as __null (with type int or long),
  52    * passing nullptr to an int/long parameter triggers compiler warnings.  We
  53    * need a type that no one can pass accidentally; a pointer-to-member-function
  54    * (where no such function exists) does the trick nicely.
  55    *
  56    * That handles the return-value case.  What about for locals, argument types,
  57    * and so on?  |already_AddRefed<T>(nullptr)| considers both overloads (and
  58    * the (already_AddRefed<T>&&) overload as well!), so there's an ambiguity.
  59    * We can target true nullptr using decltype(nullptr), but we can't target
  60    * emulated nullptr the same way, because passing __null to an int/long
  61    * parameter triggers compiler warnings.  So just give up on this, and provide
  62    * this behavior through the default constructor.
  63    *
  64    * We can revert to simply explicit (T*) and implicit (decltype(nullptr)) when
  65    * nullptr no longer needs to be emulated to support the ancient b2g compiler.
  66    * (The () overload could also be removed, if desired, if we changed callers.)
  67    */
  68   already_AddRefed() : mRawPtr(nullptr) {}
  69
  70   MOZ_IMPLICIT already_AddRefed(decltype(nullptr)) : mRawPtr(nullptr) {}
  71
  72   explicit already_AddRefed(T* aRawPtr) : mRawPtr(aRawPtr) {}
  73
  74   // Disallow copy constructor and copy assignment operator: move semantics used instead.
  75   already_AddRefed(const already_AddRefed<T>& aOther) = delete;
  76   already_AddRefed<T>& operator=(const already_AddRefed<T>& aOther) = delete;
  77
  78   already_AddRefed(already_AddRefed<T>&& aOther) : mRawPtr(aOther.take()) {}
  79
  80   already_AddRefed<T>& operator=(already_AddRefed<T>&& aOther)
  81   {
  82     mRawPtr = aOther.take();
  83     return *this;
  84   }
  85
  86   /**
  87    * This helper is useful in cases like
  88    *
  89    *  already_AddRefed<BaseClass>
  90    *  Foo()
  91    *  {
  92    *    RefPtr<SubClass> x = ...;
  93    *    return x.forget();
  94    *  }
  95    *
  96    * The autoconversion allows one to omit the idiom
  97    *
  98    *    RefPtr<BaseClass> y = x.forget();
  99    *    return y.forget();
 100    *
 101    * Note that nsRefPtr is the XPCOM reference counting smart pointer class.
 102    */
 103   template <typename U>
 104   MOZ_IMPLICIT already_AddRefed(already_AddRefed<U>&& aOther) : mRawPtr(aOther.take()) {}
 105
 106   ~already_AddRefed() { MOZ_ASSERT(!mRawPtr); }
 107
 108   // Specialize the unused operator<< for already_AddRefed, to allow
 109   // nsCOMPtr<nsIFoo> foo;
 110   // Unused << foo.forget();
 111   // Note that nsCOMPtr is the XPCOM reference counting smart pointer class.
 112   friend void operator<<(const mozilla::unused_t& aUnused,
 113                          const already_AddRefed<T>& aRhs)
 114   {
 115     auto mutableAlreadyAddRefed = const_cast<already_AddRefed<T>*>(&aRhs);
 116     aUnused << mutableAlreadyAddRefed->take();
 117   }
 118
 119   MOZ_MUST_USE T* take()
 120   {
 121     T* rawPtr = mRawPtr;
 122     mRawPtr = nullptr;
 123     return rawPtr;
 124   }
 125
 126   /**
 127    * This helper provides a static_cast replacement for already_AddRefed, so
 128    * if you have
 129    *
 130    *   already_AddRefed<Parent> F();
 131    *
 132    * you can write
 133    *
 134    *   already_AddRefed<Child>
 135    *   G()
 136    *   {
 137    *     return F().downcast<Child>();
 138    *   }
 139    */
 140   template<class U>
 141   already_AddRefed<U> downcast()
 142   {
 143     U* tmp = static_cast<U*>(mRawPtr);
 144     mRawPtr = nullptr;
 145     return already_AddRefed<U>(tmp);
 146   }
 147
 148 private:
 149   T* MOZ_OWNING_REF mRawPtr;
 150 };
 151
 152 #endif // AlreadyAddRefed_h