AlreadyAddRefed.h
/* -*- 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 <utility>
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.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. Or, you might want to use this as a parameter type in a function
* that wants to force a transfer-of-ownership from a RefPtr in the caller (for
* example, if the function expects callers to pass in a newly-created object,
* which the function then takes ownership of).
*
* TODO Move already_AddRefed to namespace mozilla. This has not yet been done
* because of the sheer number of usages of already_AddRefed.
*
* When should you use already_AddRefed<>?
* * Ensure a consumer takes ownership of a reference
* * Pass ownership without calling AddRef/Release (sometimes required in
* off-main-thread code)
* * The ref pointer type you're using doesn't support move construction
*
* Otherwise, use std::move(RefPtr/nsCOMPtr/etc).
*/
template <class T>
struct
#if !defined(MOZ_CLANG_PLUGIN) && !defined(XGILL_PLUGIN)
[[nodiscard]]
#endif
MOZ_NON_AUTOABLE already_AddRefed {
already_AddRefed() : mRawPtr(nullptr) {}
// For simplicity, allow returning nullptr from functions returning
// already_AddRefed<T>. Don't permit returning raw T*, though; it's preferred
// to create already_AddRefed<T> from a reference-counting smart pointer.
MOZ_IMPLICIT already_AddRefed(decltype(nullptr)) : 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;
// WARNING: sketchiness ahead.
//
// The x86-64 ABI for Unix-like operating systems requires structures to be
// returned via invisible reference if they are non-trivial for the purposes
// of calls according to the C++ ABI[1]. For our consideration here, that
// means that if we have a non-trivial move constructor or destructor,
// already_AddRefed must be returned by invisible reference. But
// already_AddRefed is small enough and so commonly used that it would be
// beneficial to return it via registers instead. So we need to figure out
// a way to make the move constructor and the destructor trivial.
//
// Our destructor is normally non-trivial, because it asserts that the
// stored pointer has been taken by somebody else prior to destruction.
// However, since the assert in question is compiled only for DEBUG builds,
// we can make the destructor trivial in non-DEBUG builds by simply defining
// it with `= default`.
//
// We now have to make the move constructor trivial as well. It is normally
// non-trivial, because the incoming object has its pointer null-ed during
// the move. This null-ing is done to satisfy the assert in the destructor.
// But since that destructor has no assert in non-DEBUG builds, the clearing
// is unnecessary in such builds; all we really need to perform is a copy of
// the pointer from the incoming object. So we can let the compiler define
// a trivial move constructor for us, and already_AddRefed can now be
// returned in registers rather than needing to allocate a stack slot for
// an invisible reference.
//
// The above considerations apply to Unix-like operating systems only; the
// conditions for the same optimization to apply on x86-64 Windows are much
// more strigent and are basically impossible for already_AddRefed to
// satisfy[2]. But we do get some benefit from this optimization on Windows
// because we removed the nulling of the pointer during the move, so that's
// a codesize win.
//
// [1] https://itanium-cxx-abi.github.io/cxx-abi/abi.html#non-trivial
// [2] https://docs.microsoft.com/en-us/cpp/build/return-values-cpp
already_AddRefed(already_AddRefed<T>&& aOther)
#ifdef DEBUG
: mRawPtr(aOther.take()){}
#else
= default;
#endif
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()
#ifdef DEBUG
{
MOZ_ASSERT(!mRawPtr);
}
#else
= default;
#endif
// 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();
}
[[nodiscard]] 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
