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authorMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
committerMatt A. Tobin <mattatobin@localhost.localdomain>2018-02-02 04:16:08 -0500
commit5f8de423f190bbb79a62f804151bc24824fa32d8 (patch)
tree10027f336435511475e392454359edea8e25895d /mfbt/AlreadyAddRefed.h
parent49ee0794b5d912db1f95dce6eb52d781dc210db5 (diff)
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Add m-esr52 at 52.6.0
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+/* -*- 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