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148 lines
5.0 KiB
C++
148 lines
5.0 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* Typed temporary pointers for reference-counted smart pointers. */
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#ifndef AlreadyAddRefed_h
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#define AlreadyAddRefed_h
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#include "mozilla/Assertions.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/Move.h"
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namespace mozilla {
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struct unused_t;
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} // namespace mozilla
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/**
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* already_AddRefed cooperates with reference counting smart pointers to enable
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* you to assign in a pointer _without_ |AddRef|ing it. You might want to use
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* this as a return type from a function that returns an already |AddRef|ed
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* pointer.
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*
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* TODO Move already_AddRefed to namespace mozilla. This has not yet been done
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* because of the sheer number of usages of already_AddRefed.
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*/
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template<class T>
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struct MOZ_MUST_USE MOZ_NON_AUTOABLE already_AddRefed
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{
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/*
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* We want to allow returning nullptr from functions returning
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* already_AddRefed<T>, for simplicity. But we also don't want to allow
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* returning raw T*, instead preferring creation of already_AddRefed<T> from
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* a reference counting smart pointer.
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*
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* We address the latter requirement by making the (T*) constructor explicit.
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* But |return nullptr| won't consider an explicit constructor, so we need
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* another constructor to handle it. Plain old (decltype(nullptr)) doesn't
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* cut it, because if nullptr is emulated as __null (with type int or long),
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* passing nullptr to an int/long parameter triggers compiler warnings. We
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* need a type that no one can pass accidentally; a pointer-to-member-function
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* (where no such function exists) does the trick nicely.
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*
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* That handles the return-value case. What about for locals, argument types,
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* and so on? |already_AddRefed<T>(nullptr)| considers both overloads (and
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* the (already_AddRefed<T>&&) overload as well!), so there's an ambiguity.
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* We can target true nullptr using decltype(nullptr), but we can't target
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* emulated nullptr the same way, because passing __null to an int/long
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* parameter triggers compiler warnings. So just give up on this, and provide
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* this behavior through the default constructor.
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*
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* We can revert to simply explicit (T*) and implicit (decltype(nullptr)) when
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* nullptr no longer needs to be emulated to support the ancient b2g compiler.
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* (The () overload could also be removed, if desired, if we changed callers.)
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*/
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already_AddRefed() : mRawPtr(nullptr) {}
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// The return and argument types here are arbitrarily selected so no
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// corresponding member function exists.
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typedef void (already_AddRefed::* MatchNullptr)(double, float);
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MOZ_IMPLICIT already_AddRefed(MatchNullptr aRawPtr) : mRawPtr(nullptr) {}
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explicit already_AddRefed(T* aRawPtr) : mRawPtr(aRawPtr) {}
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// Disallow copy constructor and copy assignment operator: move semantics used instead.
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already_AddRefed(const already_AddRefed<T>& aOther) = delete;
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already_AddRefed<T>& operator=(const already_AddRefed<T>& aOther) = delete;
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already_AddRefed(already_AddRefed<T>&& aOther) : mRawPtr(aOther.take()) {}
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already_AddRefed<T>& operator=(already_AddRefed<T>&& aOther)
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{
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mRawPtr = aOther.take();
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return *this;
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}
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/**
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* This helper is useful in cases like
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*
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* already_AddRefed<BaseClass>
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* Foo()
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* {
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* RefPtr<SubClass> x = ...;
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* return x.forget();
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* }
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*
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* The autoconversion allows one to omit the idiom
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*
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* RefPtr<BaseClass> y = x.forget();
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* return y.forget();
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*
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* Note that nsRefPtr is the XPCOM reference counting smart pointer class.
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*/
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template <typename U>
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MOZ_IMPLICIT already_AddRefed(already_AddRefed<U>&& aOther) : mRawPtr(aOther.take()) {}
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~already_AddRefed() { MOZ_ASSERT(!mRawPtr); }
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// Specialize the unused operator<< for already_AddRefed, to allow
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// nsCOMPtr<nsIFoo> foo;
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// Unused << foo.forget();
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// Note that nsCOMPtr is the XPCOM reference counting smart pointer class.
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friend void operator<<(const mozilla::unused_t& aUnused,
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const already_AddRefed<T>& aRhs)
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{
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auto mutableAlreadyAddRefed = const_cast<already_AddRefed<T>*>(&aRhs);
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aUnused << mutableAlreadyAddRefed->take();
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}
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MOZ_WARN_UNUSED_RESULT T* take()
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{
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T* rawPtr = mRawPtr;
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mRawPtr = nullptr;
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return rawPtr;
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}
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/**
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* This helper provides a static_cast replacement for already_AddRefed, so
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* if you have
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*
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* already_AddRefed<Parent> F();
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*
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* you can write
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*
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* already_AddRefed<Child>
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* G()
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* {
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* return F().downcast<Child>();
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* }
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*/
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template<class U>
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already_AddRefed<U> downcast()
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{
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U* tmp = static_cast<U*>(mRawPtr);
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mRawPtr = nullptr;
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return already_AddRefed<U>(tmp);
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}
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private:
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T* MOZ_OWNING_REF mRawPtr;
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};
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#endif // AlreadyAddRefed_h
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