1 files changed, 147 insertions, 0 deletions
diff --git a/mfbt/AlreadyAddRefed.h b/mfbt/AlreadyAddRefed.h
new file mode 100644
index 000000000..0d7b0caed
--- /dev/null
+++ b/mfbt/AlreadyAddRefed.h
@@ -0,0 +1,147 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+/* Typed temporary pointers for reference-counted smart pointers. */
+
+#ifndef AlreadyAddRefed_h
+#define AlreadyAddRefed_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Move.h"
+
+namespace mozilla {
+
+struct unused_t;
+
+} // namespace mozilla
+
+/**
+ * already_AddRefed cooperates with reference counting smart pointers to enable
+ * you to assign in a pointer _without_ |AddRef|ing it.  You might want to use
+ * this as a return type from a function that returns an already |AddRef|ed
+ * pointer.
+ *
+ * TODO Move already_AddRefed to namespace mozilla.  This has not yet been done
+ * because of the sheer number of usages of already_AddRefed.
+ */
+template<class T>
+struct MOZ_MUST_USE_TYPE MOZ_NON_AUTOABLE already_AddRefed
+{
+  /*
+   * We want to allow returning nullptr from functions returning
+   * already_AddRefed<T>, for simplicity.  But we also don't want to allow
+   * returning raw T*, instead preferring creation of already_AddRefed<T> from
+   * a reference counting smart pointer.
+   *
+   * We address the latter requirement by making the (T*) constructor explicit.
+   * But |return nullptr| won't consider an explicit constructor, so we need
+   * another constructor to handle it.  Plain old (decltype(nullptr)) doesn't
+   * cut it, because if nullptr is emulated as __null (with type int or long),
+   * passing nullptr to an int/long parameter triggers compiler warnings.  We
+   * need a type that no one can pass accidentally; a pointer-to-member-function
+   * (where no such function exists) does the trick nicely.
+   *
+   * That handles the return-value case.  What about for locals, argument types,
+   * and so on?  |already_AddRefed<T>(nullptr)| considers both overloads (and
+   * the (already_AddRefed<T>&&) overload as well!), so there's an ambiguity.
+   * We can target true nullptr using decltype(nullptr), but we can't target
+   * emulated nullptr the same way, because passing __null to an int/long
+   * parameter triggers compiler warnings.  So just give up on this, and provide
+   * this behavior through the default constructor.
+   *
+   * We can revert to simply explicit (T*) and implicit (decltype(nullptr)) when
+   * nullptr no longer needs to be emulated to support the ancient b2g compiler.
+   * (The () overload could also be removed, if desired, if we changed callers.)
+   */
+  already_AddRefed() : mRawPtr(nullptr) {}
+
+  // The return and argument types here are arbitrarily selected so no
+  // corresponding member function exists.
+  typedef void (already_AddRefed::* MatchNullptr)(double, float);
+  MOZ_IMPLICIT already_AddRefed(MatchNullptr aRawPtr) : mRawPtr(nullptr) {}
+
+  explicit already_AddRefed(T* aRawPtr) : mRawPtr(aRawPtr) {}
+
+  // Disallow copy constructor and copy assignment operator: move semantics used instead.
+  already_AddRefed(const already_AddRefed<T>& aOther) = delete;
+  already_AddRefed<T>& operator=(const already_AddRefed<T>& aOther) = delete;
+
+  already_AddRefed(already_AddRefed<T>&& aOther) : mRawPtr(aOther.take()) {}
+
+  already_AddRefed<T>& operator=(already_AddRefed<T>&& aOther)
+  {
+    mRawPtr = aOther.take();
+    return *this;
+  }
+
+  /**
+   * This helper is useful in cases like
+   *
+   *  already_AddRefed<BaseClass>
+   *  Foo()
+   *  {
+   *    RefPtr<SubClass> x = ...;
+   *    return x.forget();
+   *  }
+   *
+   * The autoconversion allows one to omit the idiom
+   *
+   *    RefPtr<BaseClass> y = x.forget();
+   *    return y.forget();
+   *
+   * Note that nsRefPtr is the XPCOM reference counting smart pointer class.
+   */
+  template <typename U>
+  MOZ_IMPLICIT already_AddRefed(already_AddRefed<U>&& aOther) : mRawPtr(aOther.take()) {}
+
+  ~already_AddRefed() { MOZ_ASSERT(!mRawPtr); }
+
+  // Specialize the unused operator<< for already_AddRefed, to allow
+  // nsCOMPtr<nsIFoo> foo;
+  // Unused << foo.forget();
+  // Note that nsCOMPtr is the XPCOM reference counting smart pointer class.
+  friend void operator<<(const mozilla::unused_t& aUnused,
+                         const already_AddRefed<T>& aRhs)
+  {
+    auto mutableAlreadyAddRefed = const_cast<already_AddRefed<T>*>(&aRhs);
+    aUnused << mutableAlreadyAddRefed->take();
+  }
+
+  MOZ_MUST_USE T* take()
+  {
+    T* rawPtr = mRawPtr;
+    mRawPtr = nullptr;
+    return rawPtr;
+  }
+
+  /**
+   * This helper provides a static_cast replacement for already_AddRefed, so
+   * if you have
+   *
+   *   already_AddRefed<Parent> F();
+   *
+   * you can write
+   *
+   *   already_AddRefed<Child>
+   *   G()
+   *   {
+   *     return F().downcast<Child>();
+   *   }
+   */
+  template<class U>
+  already_AddRefed<U> downcast()
+  {
+    U* tmp = static_cast<U*>(mRawPtr);
+    mRawPtr = nullptr;
+    return already_AddRefed<U>(tmp);
+  }
+
+private:
+  T* MOZ_OWNING_REF mRawPtr;
+};
+
+#endif // AlreadyAddRefed_h