gecko-dev/ipc/mscom/Aggregation.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/. */

#ifndef mozilla_mscom_Aggregation_h
#define mozilla_mscom_Aggregation_h

#include "mozilla/Attributes.h"

#include <stddef.h>
#include <unknwn.h>

namespace mozilla {
namespace mscom {

/**
 * This is used for stabilizing a COM object's reference count during
 * construction when that object aggregates other objects. Since the aggregated
 * object(s) may AddRef() or Release(), we need to artifically boost the
 * refcount to prevent premature destruction. Note that we increment/decrement
 * instead of AddRef()/Release() in this class because we want to adjust the
 * refcount without causing any other side effects (like object destruction).
 */
template <typename RefCntT>
class MOZ_RAII StabilizedRefCount {
 public:
  explicit StabilizedRefCount(RefCntT& aRefCnt) : mRefCnt(aRefCnt) {
    ++aRefCnt;
  }

  ~StabilizedRefCount() { --mRefCnt; }

  StabilizedRefCount(const StabilizedRefCount&) = delete;
  StabilizedRefCount(StabilizedRefCount&&) = delete;
  StabilizedRefCount& operator=(const StabilizedRefCount&) = delete;
  StabilizedRefCount& operator=(StabilizedRefCount&&) = delete;

 private:
  RefCntT& mRefCnt;
};

namespace detail {

template <typename T>
class InternalUnknown : public IUnknown {
 public:
  STDMETHODIMP QueryInterface(REFIID aIid, void** aOutInterface) override {
    return This()->InternalQueryInterface(aIid, aOutInterface);
  }

  STDMETHODIMP_(ULONG) AddRef() override { return This()->InternalAddRef(); }

  STDMETHODIMP_(ULONG) Release() override { return This()->InternalRelease(); }

 private:
  T* This() {
    return reinterpret_cast<T*>(reinterpret_cast<char*>(this) -
                                offsetof(T, mInternalUnknown));
  }
};

}  // namespace detail
}  // namespace mscom
}  // namespace mozilla

#define DECLARE_AGGREGATABLE(Type)                                      \
 public:                                                                \
  STDMETHODIMP QueryInterface(REFIID riid, void** ppv) override {       \
    return mOuter->QueryInterface(riid, ppv);                           \
  }                                                                     \
  STDMETHODIMP_(ULONG) AddRef() override { return mOuter->AddRef(); }   \
  STDMETHODIMP_(ULONG) Release() override { return mOuter->Release(); } \
                                                                        \
 protected:                                                             \
  STDMETHODIMP InternalQueryInterface(REFIID riid, void** ppv);         \
  STDMETHODIMP_(ULONG) InternalAddRef();                                \
  STDMETHODIMP_(ULONG) InternalRelease();                               \
  friend class mozilla::mscom::detail::InternalUnknown<Type>;           \
  mozilla::mscom::detail::InternalUnknown<Type> mInternalUnknown

#endif  // mozilla_mscom_Aggregation_h