gecko-dev/mfbt/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 "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.
 *
 * 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 Move(RefPtr/nsCOMPtr/etc).
 */
template<class T>
struct MOZ_TEMPORARY_CLASS 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) {}

  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();
  }

  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