gecko-dev/xpcom/glue/nsCOMPtr.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 nsCOMPtr_h___
#define nsCOMPtr_h___

/*
 * Having problems?
 *
 * See the User Manual at:
 *   http://www.mozilla.org/projects/xpcom/nsCOMPtr.html
 *
 *
 * nsCOMPtr
 *   better than a raw pointer
 * for owning objects
 *                      -- scc
 */

#include "mozilla/AlreadyAddRefed.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/Move.h"
#include "mozilla/TypeTraits.h"

#include "nsDebug.h" // for |NS_ASSERTION|
#include "nsISupportsUtils.h" // for |nsresult|, |NS_ADDREF|, |NS_GET_TEMPLATE_IID| et al

#include "nsCycleCollectionNoteChild.h"


/*
 * WARNING: This file defines several macros for internal use only. These
 * macros begin with the prefix |NSCAP_|. Do not use these macros in your own
 * code. They are for internal use only for cross-platform compatibility, and
 * are subject to change without notice.
 */


#ifdef _MSC_VER
  // Under VC++, we win by inlining StartAssignment.
  #define NSCAP_FEATURE_INLINE_STARTASSIGNMENT

  // Also under VC++, at the highest warning level, we are overwhelmed with
  // warnings about (unused) inline functions being removed. This is to be
  // expected with templates, so we disable the warning.
  #pragma warning( disable: 4514 )
#endif

#define NSCAP_FEATURE_USE_BASE

#ifdef DEBUG
  #define NSCAP_FEATURE_TEST_DONTQUERY_CASES
  #undef NSCAP_FEATURE_USE_BASE
//#define NSCAP_FEATURE_TEST_NONNULL_QUERY_SUCCEEDS
#endif

#ifdef __GNUC__
  // Our use of nsCOMPtr_base::mRawPtr violates the C++ standard's aliasing
  // rules. Mark it with the may_alias attribute so that gcc 3.3 and higher
  // don't reorder instructions based on aliasing assumptions for
  // this variable.  Fortunately, gcc versions < 3.3 do not do any
  // optimizations that break nsCOMPtr.

  #define NS_MAY_ALIAS_PTR(t)    t*  __attribute__((__may_alias__))
#else
  #define NS_MAY_ALIAS_PTR(t)    t*
#endif

#if defined(NSCAP_DISABLE_DEBUG_PTR_TYPES)
  #define NSCAP_FEATURE_USE_BASE
#endif

/*
 * The following three macros (NSCAP_ADDREF, NSCAP_RELEASE, and
 * NSCAP_LOG_ASSIGNMENT) allow external clients the ability to add logging or
 * other interesting debug facilities. In fact, if you want |nsCOMPtr| to
 * participate in the standard logging facility, you provide
 * (e.g., in "nsISupportsImpl.h") suitable definitions
 *
 *   #define NSCAP_ADDREF(this, ptr)         NS_ADDREF(ptr)
 *   #define NSCAP_RELEASE(this, ptr)        NS_RELEASE(ptr)
 */

#ifndef NSCAP_ADDREF
  #define NSCAP_ADDREF(this, ptr)     (ptr)->AddRef()
#endif

#ifndef NSCAP_RELEASE
  #define NSCAP_RELEASE(this, ptr)    (ptr)->Release()
#endif

// Clients can define |NSCAP_LOG_ASSIGNMENT| to perform logging.
#ifdef NSCAP_LOG_ASSIGNMENT
  // Remember that |NSCAP_LOG_ASSIGNMENT| was defined by some client so that we
  // know to instantiate |~nsGetterAddRefs| in turn to note the external
  // assignment into the |nsCOMPtr|.
  #define NSCAP_LOG_EXTERNAL_ASSIGNMENT
#else
    // ...otherwise, just strip it out of the code
  #define NSCAP_LOG_ASSIGNMENT(this, ptr)
#endif

#ifndef NSCAP_LOG_RELEASE
  #define NSCAP_LOG_RELEASE(this, ptr)
#endif

namespace mozilla {

struct unused_t;

} // namespace mozilla

template<class T>
inline already_AddRefed<T>
dont_AddRef(T* aRawPtr)
{
  return already_AddRefed<T>(aRawPtr);
}

template<class T>
inline already_AddRefed<T>&&
dont_AddRef(already_AddRefed<T>&& aAlreadyAddRefedPtr)
{
  return mozilla::Move(aAlreadyAddRefedPtr);
}


/*
 * An nsCOMPtr_helper transforms commonly called getters into typesafe forms
 * that are more convenient to call, and more efficient to use with |nsCOMPtr|s.
 * Good candidates for helpers are |QueryInterface()|, |CreateInstance()|, etc.
 *
 * Here are the rules for a helper:
 *   - it implements |operator()| to produce an interface pointer
 *   - (except for its name) |operator()| is a valid [XP]COM `getter'
 *   - the interface pointer that it returns is already |AddRef()|ed (as from
 *     any good getter)
 *   - it matches the type requested with the supplied |nsIID| argument
 *   - its constructor provides an optional |nsresult*| that |operator()| can
 *     fill in with an error when it is executed
 *
 * See |class nsGetInterface| for an example.
 */
class MOZ_STACK_CLASS nsCOMPtr_helper
{
public:
  virtual nsresult NS_FASTCALL operator()(const nsIID&, void**) const = 0;
};

/*
 * nsQueryInterface could have been implemented as an nsCOMPtr_helper to avoid
 * adding specialized machinery in nsCOMPtr, but do_QueryInterface is called
 * often enough that the codesize savings are big enough to warrant the
 * specialcasing.
 */
class MOZ_STACK_CLASS nsQueryInterface final
{
public:
  explicit
  nsQueryInterface(nsISupports* aRawPtr) : mRawPtr(aRawPtr) {}

  nsresult NS_FASTCALL operator()(const nsIID& aIID, void**) const;

private:
  nsISupports* MOZ_OWNING_REF mRawPtr;
};

class nsQueryInterfaceWithError
{
public:
  nsQueryInterfaceWithError(nsISupports* aRawPtr, nsresult* aError)
    : mRawPtr(aRawPtr)
    , mErrorPtr(aError)
  {
  }

  nsresult NS_FASTCALL operator()(const nsIID& aIID, void**) const;

private:
  nsISupports* MOZ_OWNING_REF mRawPtr;
  nsresult* mErrorPtr;
};

inline nsQueryInterface
do_QueryInterface(nsISupports* aRawPtr)
{
  return nsQueryInterface(aRawPtr);
}

inline nsQueryInterfaceWithError
do_QueryInterface(nsISupports* aRawPtr, nsresult* aError)
{
  return nsQueryInterfaceWithError(aRawPtr, aError);
}

template<class T>
inline void
do_QueryInterface(already_AddRefed<T>&)
{
  // This signature exists solely to _stop_ you from doing the bad thing.
  // Saying |do_QueryInterface()| on a pointer that is not otherwise owned by
  // someone else is an automatic leak. See bug 8221.
}

template<class T>
inline void
do_QueryInterface(already_AddRefed<T>&, nsresult*)
{
  // This signature exists solely to _stop_ you from doing the bad thing.
  // Saying |do_QueryInterface()| on a pointer that is not otherwise owned by
  // someone else is an automatic leak. See bug 8221.
}


////////////////////////////////////////////////////////////////////////////
// Using servicemanager with COMPtrs
class nsGetServiceByCID
{
public:
  explicit nsGetServiceByCID(const nsCID& aCID) : mCID(aCID) {}

  nsresult NS_FASTCALL operator()(const nsIID&, void**) const;

private:
  const nsCID& mCID;
};

class nsGetServiceByCIDWithError
{
public:
  nsGetServiceByCIDWithError(const nsCID& aCID, nsresult* aErrorPtr)
    : mCID(aCID)
    , mErrorPtr(aErrorPtr)
  {
  }

  nsresult NS_FASTCALL operator()(const nsIID&, void**) const;

private:
  const nsCID& mCID;
  nsresult* mErrorPtr;
};

class nsGetServiceByContractID
{
public:
  explicit nsGetServiceByContractID(const char* aContractID)
    : mContractID(aContractID)
  {
  }

  nsresult NS_FASTCALL operator()(const nsIID&, void**) const;

private:
  const char* mContractID;
};

class nsGetServiceByContractIDWithError
{
public:
  nsGetServiceByContractIDWithError(const char* aContractID, nsresult* aErrorPtr)
    : mContractID(aContractID)
    , mErrorPtr(aErrorPtr)
  {
  }

  nsresult NS_FASTCALL operator()(const nsIID&, void**) const;

private:
  const char* mContractID;
  nsresult* mErrorPtr;
};

/**
 * Factors implementation for all template versions of nsCOMPtr.
 *
 * Here's the way people normally do things like this:
 *
 *   template<class T> class Foo { ... };
 *   template<> class Foo<void*> { ... };
 *   template<class T> class Foo<T*> : private Foo<void*> { ... };
 */
class nsCOMPtr_base
{
public:
  explicit nsCOMPtr_base(nsISupports* aRawPtr = 0) : mRawPtr(aRawPtr) {}

  NS_CONSTRUCTOR_FASTCALL ~nsCOMPtr_base()
  {
    NSCAP_LOG_RELEASE(this, mRawPtr);
    if (mRawPtr) {
      NSCAP_RELEASE(this, mRawPtr);
    }
  }

  void NS_FASTCALL
  assign_with_AddRef(nsISupports*);
  void NS_FASTCALL
  assign_from_qi(const nsQueryInterface, const nsIID&);
  void NS_FASTCALL
  assign_from_qi_with_error(const nsQueryInterfaceWithError&, const nsIID&);
  void NS_FASTCALL
  assign_from_gs_cid(const nsGetServiceByCID, const nsIID&);
  void NS_FASTCALL
  assign_from_gs_cid_with_error(const nsGetServiceByCIDWithError&, const nsIID&);
  void NS_FASTCALL
  assign_from_gs_contractid(const nsGetServiceByContractID, const nsIID&);
  void NS_FASTCALL
  assign_from_gs_contractid_with_error(const nsGetServiceByContractIDWithError&,
                                       const nsIID&);
  void NS_FASTCALL
  assign_from_helper(const nsCOMPtr_helper&, const nsIID&);
  void** NS_FASTCALL
  begin_assignment();

protected:
  NS_MAY_ALIAS_PTR(nsISupports) MOZ_OWNING_REF mRawPtr;

  void assign_assuming_AddRef(nsISupports* aNewPtr)
  {
    // |AddRef()|ing the new value (before entering this function) before
    // |Release()|ing the old lets us safely ignore the self-assignment case.
    // We must, however, be careful only to |Release()| _after_ doing the
    // assignment, in case the |Release()| leads to our _own_ destruction,
    // which would, in turn, cause an incorrect second |Release()| of our old
    // pointer. Thank <waterson@netscape.com> for discovering this.
    nsISupports* oldPtr = mRawPtr;
    mRawPtr = aNewPtr;
    NSCAP_LOG_ASSIGNMENT(this, aNewPtr);
    NSCAP_LOG_RELEASE(this, oldPtr);
    if (oldPtr) {
      NSCAP_RELEASE(this, oldPtr);
    }
  }
};

// template<class T> class nsGetterAddRefs;

template<class T>
class nsCOMPtr final
#ifdef NSCAP_FEATURE_USE_BASE
  : private nsCOMPtr_base
#endif
{

#ifdef NSCAP_FEATURE_USE_BASE
  #define NSCAP_CTOR_BASE(x) nsCOMPtr_base(x)
#else
  #define NSCAP_CTOR_BASE(x) mRawPtr(x)

private:
  void assign_with_AddRef(nsISupports*);
  void assign_from_qi(const nsQueryInterface, const nsIID&);
  void assign_from_qi_with_error(const nsQueryInterfaceWithError&, const nsIID&);
  void assign_from_gs_cid(const nsGetServiceByCID, const nsIID&);
  void assign_from_gs_cid_with_error(const nsGetServiceByCIDWithError&,
                                     const nsIID&);
  void assign_from_gs_contractid(const nsGetServiceByContractID, const nsIID&);
  void assign_from_gs_contractid_with_error(
    const nsGetServiceByContractIDWithError&, const nsIID&);
  void assign_from_helper(const nsCOMPtr_helper&, const nsIID&);
  void** begin_assignment();

  void assign_assuming_AddRef(T* aNewPtr)
  {
    T* oldPtr = mRawPtr;
    mRawPtr = aNewPtr;
    NSCAP_LOG_ASSIGNMENT(this, aNewPtr);
    NSCAP_LOG_RELEASE(this, oldPtr);
    if (oldPtr) {
      NSCAP_RELEASE(this, oldPtr);
    }
  }

private:
  T* MOZ_OWNING_REF mRawPtr;
#endif

public:
  typedef T element_type;

#ifndef NSCAP_FEATURE_USE_BASE
  ~nsCOMPtr()
  {
    NSCAP_LOG_RELEASE(this, mRawPtr);
    if (mRawPtr) {
      NSCAP_RELEASE(this, mRawPtr);
    }
  }
#endif

#ifdef NSCAP_FEATURE_TEST_DONTQUERY_CASES
  void Assert_NoQueryNeeded()
  {
    if (mRawPtr) {
      nsCOMPtr<T> query_result(do_QueryInterface(mRawPtr));
      NS_ASSERTION(query_result.get() == mRawPtr, "QueryInterface needed");
    }
  }

  #define NSCAP_ASSERT_NO_QUERY_NEEDED() Assert_NoQueryNeeded();
#else
  #define NSCAP_ASSERT_NO_QUERY_NEEDED()
#endif


  // Constructors

  nsCOMPtr()
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
  }

  nsCOMPtr(const nsCOMPtr<T>& aSmartPtr)
    : NSCAP_CTOR_BASE(aSmartPtr.mRawPtr)
  {
    if (mRawPtr) {
      NSCAP_ADDREF(this, mRawPtr);
    }
    NSCAP_LOG_ASSIGNMENT(this, aSmartPtr.mRawPtr);
  }

  MOZ_IMPLICIT nsCOMPtr(T* aRawPtr)
    : NSCAP_CTOR_BASE(aRawPtr)
  {
    if (mRawPtr) {
      NSCAP_ADDREF(this, mRawPtr);
    }
    NSCAP_LOG_ASSIGNMENT(this, aRawPtr);
    NSCAP_ASSERT_NO_QUERY_NEEDED();
  }

  MOZ_IMPLICIT nsCOMPtr(already_AddRefed<T>& aSmartPtr)
    : NSCAP_CTOR_BASE(aSmartPtr.take())
  {
    NSCAP_LOG_ASSIGNMENT(this, mRawPtr);
    NSCAP_ASSERT_NO_QUERY_NEEDED();
  }

  // Construct from |otherComPtr.forget()|.
  MOZ_IMPLICIT nsCOMPtr(already_AddRefed<T>&& aSmartPtr)
    : NSCAP_CTOR_BASE(aSmartPtr.take())
  {
    NSCAP_LOG_ASSIGNMENT(this, mRawPtr);
    NSCAP_ASSERT_NO_QUERY_NEEDED();
  }

  // Construct from |already_AddRefed|.
  template<typename U>
  MOZ_IMPLICIT nsCOMPtr(already_AddRefed<U>& aSmartPtr)
    : NSCAP_CTOR_BASE(static_cast<T*>(aSmartPtr.take()))
  {
    // But make sure that U actually inherits from T.
    static_assert(mozilla::IsBaseOf<T, U>::value,
                  "U is not a subclass of T");
    NSCAP_LOG_ASSIGNMENT(this, static_cast<T*>(mRawPtr));
    NSCAP_ASSERT_NO_QUERY_NEEDED();
  }

  // Construct from |otherComPtr.forget()|.
  template<typename U>
  MOZ_IMPLICIT nsCOMPtr(already_AddRefed<U>&& aSmartPtr)
    : NSCAP_CTOR_BASE(static_cast<T*>(aSmartPtr.take()))
  {
    // But make sure that U actually inherits from T.
    static_assert(mozilla::IsBaseOf<T, U>::value,
                  "U is not a subclass of T");
    NSCAP_LOG_ASSIGNMENT(this, static_cast<T*>(mRawPtr));
    NSCAP_ASSERT_NO_QUERY_NEEDED();
  }

  // Construct from |do_QueryInterface(expr)|.
  MOZ_IMPLICIT nsCOMPtr(const nsQueryInterface aQI)
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_qi(aQI, NS_GET_TEMPLATE_IID(T));
  }

  // Construct from |do_QueryInterface(expr, &rv)|.
  MOZ_IMPLICIT nsCOMPtr(const nsQueryInterfaceWithError& aQI)
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_qi_with_error(aQI, NS_GET_TEMPLATE_IID(T));
  }

  // Construct from |do_GetService(cid_expr)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByCID aGS)
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_cid(aGS, NS_GET_TEMPLATE_IID(T));
  }

  // Construct from |do_GetService(cid_expr, &rv)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByCIDWithError& aGS)
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_cid_with_error(aGS, NS_GET_TEMPLATE_IID(T));
  }

  // Construct from |do_GetService(contractid_expr)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByContractID aGS)
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_contractid(aGS, NS_GET_TEMPLATE_IID(T));
  }

  // Construct from |do_GetService(contractid_expr, &rv)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByContractIDWithError& aGS)
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_contractid_with_error(aGS, NS_GET_TEMPLATE_IID(T));
  }

  // And finally, anything else we might need to construct from can exploit the
  // nsCOMPtr_helper facility.
  MOZ_IMPLICIT nsCOMPtr(const nsCOMPtr_helper& aHelper)
    : NSCAP_CTOR_BASE(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_helper(aHelper, NS_GET_TEMPLATE_IID(T));
    NSCAP_ASSERT_NO_QUERY_NEEDED();
  }


  // Assignment operators

  nsCOMPtr<T>& operator=(const nsCOMPtr<T>& aRhs)
  {
    assign_with_AddRef(aRhs.mRawPtr);
    return *this;
  }

  nsCOMPtr<T>& operator=(T* aRhs)
  {
    assign_with_AddRef(aRhs);
    NSCAP_ASSERT_NO_QUERY_NEEDED();
    return *this;
  }

  // Assign from |already_AddRefed|.
  template<typename U>
  nsCOMPtr<T>& operator=(already_AddRefed<U>& aRhs)
  {
    // Make sure that U actually inherits from T
    static_assert(mozilla::IsBaseOf<T, U>::value,
                  "U is not a subclass of T");
    assign_assuming_AddRef(static_cast<T*>(aRhs.take()));
    NSCAP_ASSERT_NO_QUERY_NEEDED();
    return *this;
  }

  // Assign from |otherComPtr.forget()|.
  template<typename U>
  nsCOMPtr<T>& operator=(already_AddRefed<U>&& aRhs)
  {
    // Make sure that U actually inherits from T
    static_assert(mozilla::IsBaseOf<T, U>::value,
                  "U is not a subclass of T");
    assign_assuming_AddRef(static_cast<T*>(aRhs.take()));
    NSCAP_ASSERT_NO_QUERY_NEEDED();
    return *this;
  }

  // Assign from |do_QueryInterface(expr)|.
  nsCOMPtr<T>& operator=(const nsQueryInterface aRhs)
  {
    assign_from_qi(aRhs, NS_GET_TEMPLATE_IID(T));
    return *this;
  }

  // Assign from |do_QueryInterface(expr, &rv)|.
  nsCOMPtr<T>& operator=(const nsQueryInterfaceWithError& aRhs)
  {
    assign_from_qi_with_error(aRhs, NS_GET_TEMPLATE_IID(T));
    return *this;
  }

  // Assign from |do_GetService(cid_expr)|.
  nsCOMPtr<T>& operator=(const nsGetServiceByCID aRhs)
  {
    assign_from_gs_cid(aRhs, NS_GET_TEMPLATE_IID(T));
    return *this;
  }

  // Assign from |do_GetService(cid_expr, &rv)|.
  nsCOMPtr<T>& operator=(const nsGetServiceByCIDWithError& aRhs)
  {
    assign_from_gs_cid_with_error(aRhs, NS_GET_TEMPLATE_IID(T));
    return *this;
  }

  // Assign from |do_GetService(contractid_expr)|.
  nsCOMPtr<T>& operator=(const nsGetServiceByContractID aRhs)
  {
    assign_from_gs_contractid(aRhs, NS_GET_TEMPLATE_IID(T));
    return *this;
  }

  // Assign from |do_GetService(contractid_expr, &rv)|.
  nsCOMPtr<T>& operator=(const nsGetServiceByContractIDWithError& aRhs)
  {
    assign_from_gs_contractid_with_error(aRhs, NS_GET_TEMPLATE_IID(T));
    return *this;
  }

  // And finally, anything else we might need to assign from can exploit the
  // nsCOMPtr_helper facility.
  nsCOMPtr<T>& operator=(const nsCOMPtr_helper& aRhs)
  {
    assign_from_helper(aRhs, NS_GET_TEMPLATE_IID(T));
    NSCAP_ASSERT_NO_QUERY_NEEDED();
    return *this;
  }

  // Exchange ownership with |aRhs|; can save a pair of refcount operations.
  void swap(nsCOMPtr<T>& aRhs)
  {
#ifdef NSCAP_FEATURE_USE_BASE
    nsISupports* temp = aRhs.mRawPtr;
#else
    T* temp = aRhs.mRawPtr;
#endif
    NSCAP_LOG_ASSIGNMENT(&aRhs, mRawPtr);
    NSCAP_LOG_ASSIGNMENT(this, temp);
    NSCAP_LOG_RELEASE(this, mRawPtr);
    NSCAP_LOG_RELEASE(&aRhs, temp);
    aRhs.mRawPtr = mRawPtr;
    mRawPtr = temp;
    // |aRhs| maintains the same invariants, so we don't need to |NSCAP_ASSERT_NO_QUERY_NEEDED|
  }

  // Exchange ownership with |aRhs|; can save a pair of refcount operations.
  void swap(T*& aRhs)
  {
#ifdef NSCAP_FEATURE_USE_BASE
    nsISupports* temp = aRhs;
#else
    T* temp = aRhs;
#endif
    NSCAP_LOG_ASSIGNMENT(this, temp);
    NSCAP_LOG_RELEASE(this, mRawPtr);
    aRhs = reinterpret_cast<T*>(mRawPtr);
    mRawPtr = temp;
    NSCAP_ASSERT_NO_QUERY_NEEDED();
  }


  // Other pointer operators

  // Return the value of mRawPtr and null out mRawPtr. Useful for
  // already_AddRefed return values.
  already_AddRefed<T> forget()
  {
    T* temp = 0;
    swap(temp);
    return already_AddRefed<T>(temp);
  }

  // Set the target of aRhs to the value of mRawPtr and null out mRawPtr.
  // Useful to avoid unnecessary AddRef/Release pairs with "out" parameters
  // where aRhs bay be a T** or an I** where I is a base class of T.
  template<typename I>
  void forget(I** aRhs)
  {
    NS_ASSERTION(aRhs, "Null pointer passed to forget!");
    NSCAP_LOG_RELEASE(this, mRawPtr);
    *aRhs = get();
    mRawPtr = 0;
  }

  // Prefer the implicit conversion provided automatically by
  // |operator T*() const|. Use |get()| to resolve ambiguity or to get a
  // castable pointer.
  T* get() const { return reinterpret_cast<T*>(mRawPtr); }

  // Makes an nsCOMPtr act like its underlying raw pointer type whenever it is
  // used in a context where a raw pointer is expected. It is this operator
  // that makes an nsCOMPtr substitutable for a raw pointer.
  //
  // Prefer the implicit use of this operator to calling |get()|, except where
  // necessary to resolve ambiguity.
  operator T*() const { return get(); }

  T* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN
  {
    MOZ_ASSERT(mRawPtr != 0,
               "You can't dereference a NULL nsCOMPtr with operator->().");
    return get();
  }

  // These are not intended to be used by clients. See |address_of| below.
  nsCOMPtr<T>* get_address() { return this; }
  const nsCOMPtr<T>* get_address() const { return this; }

public:
  T& operator*() const
  {
    MOZ_ASSERT(mRawPtr != 0,
               "You can't dereference a NULL nsCOMPtr with operator*().");
    return *get();
  }

  T** StartAssignment()
  {
#ifndef NSCAP_FEATURE_INLINE_STARTASSIGNMENT
    return reinterpret_cast<T**>(begin_assignment());
#else
    assign_assuming_AddRef(0);
    return reinterpret_cast<T**>(&mRawPtr);
#endif
  }
};


/*
 * Specializing nsCOMPtr for nsISupports allows us to use nsCOMPtr<nsISupports>
 * the same way people use nsISupports* and void*, i.e., as a `catch-all'
 * pointing to any valid [XP]COM interface. Otherwise, an nsCOMPtr<nsISupports>
 * would only be able to point to the single [XP]COM-correct nsISupports
 * instance within an object; extra querying ensues. Clients need to be able to
 * pass around arbitrary interface pointers, without hassles, through
 * intermediary code that doesn't know the exact type.
 */
template<>
class nsCOMPtr<nsISupports>
  : private nsCOMPtr_base
{
public:
  typedef nsISupports element_type;

  // Constructors

  nsCOMPtr()
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
  }

  nsCOMPtr(const nsCOMPtr<nsISupports>& aSmartPtr)
    : nsCOMPtr_base(aSmartPtr.mRawPtr)
  {
    if (mRawPtr) {
      NSCAP_ADDREF(this, mRawPtr);
    }
    NSCAP_LOG_ASSIGNMENT(this, aSmartPtr.mRawPtr);
  }

  MOZ_IMPLICIT nsCOMPtr(nsISupports* aRawPtr)
    : nsCOMPtr_base(aRawPtr)
  {
    if (mRawPtr) {
      NSCAP_ADDREF(this, mRawPtr);
    }
    NSCAP_LOG_ASSIGNMENT(this, aRawPtr);
  }

  // Construct from |already_AddRefed|.
  MOZ_IMPLICIT nsCOMPtr(already_AddRefed<nsISupports>& aSmartPtr)
    : nsCOMPtr_base(aSmartPtr.take())
  {
    NSCAP_LOG_ASSIGNMENT(this, mRawPtr);
  }

  // Construct from |otherComPtr.forget()|.
  MOZ_IMPLICIT nsCOMPtr(already_AddRefed<nsISupports>&& aSmartPtr)
    : nsCOMPtr_base(aSmartPtr.take())
  {
    NSCAP_LOG_ASSIGNMENT(this, mRawPtr);
  }

  // Construct from |do_QueryInterface(expr)|.
  MOZ_IMPLICIT nsCOMPtr(const nsQueryInterface aQI)
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_qi(aQI, NS_GET_IID(nsISupports));
  }

  // Construct from |do_QueryInterface(expr, &rv)|.
  MOZ_IMPLICIT nsCOMPtr(const nsQueryInterfaceWithError& aQI)
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_qi_with_error(aQI, NS_GET_IID(nsISupports));
  }

  // Construct from |do_GetService(cid_expr)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByCID aGS)
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_cid(aGS, NS_GET_IID(nsISupports));
  }

  // Construct from |do_GetService(cid_expr, &rv)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByCIDWithError& aGS)
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_cid_with_error(aGS, NS_GET_IID(nsISupports));
  }

  // Construct from |do_GetService(contractid_expr)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByContractID aGS)
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_contractid(aGS, NS_GET_IID(nsISupports));
  }

  // Construct from |do_GetService(contractid_expr, &rv)|.
  MOZ_IMPLICIT nsCOMPtr(const nsGetServiceByContractIDWithError& aGS)
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_gs_contractid_with_error(aGS, NS_GET_IID(nsISupports));
  }

  // And finally, anything else we might need to construct from can exploit
  // the |nsCOMPtr_helper| facility
  MOZ_IMPLICIT nsCOMPtr(const nsCOMPtr_helper& aHelper)
    : nsCOMPtr_base(0)
  {
    NSCAP_LOG_ASSIGNMENT(this, 0);
    assign_from_helper(aHelper, NS_GET_IID(nsISupports));
  }


  // Assignment operators

  nsCOMPtr<nsISupports>& operator=(const nsCOMPtr<nsISupports>& aRhs)
  {
    assign_with_AddRef(aRhs.mRawPtr);
    return *this;
  }

  nsCOMPtr<nsISupports>& operator=(nsISupports* aRhs)
  {
    assign_with_AddRef(aRhs);
    return *this;
  }

  // Assign from |already_AddRefed|.
  nsCOMPtr<nsISupports>& operator=(already_AddRefed<nsISupports>& aRhs)
  {
    assign_assuming_AddRef(aRhs.take());
    return *this;
  }

  // Assign from |otherComPtr.forget()|.
  nsCOMPtr<nsISupports>& operator=(already_AddRefed<nsISupports>&& aRhs)
  {
    assign_assuming_AddRef(aRhs.take());
    return *this;
  }

  // Assign from |do_QueryInterface(expr)|.
  nsCOMPtr<nsISupports>& operator=(const nsQueryInterface aRhs)
  {
    assign_from_qi(aRhs, NS_GET_IID(nsISupports));
    return *this;
  }

  // Assign from |do_QueryInterface(expr, &rv)|.
  nsCOMPtr<nsISupports>& operator=(const nsQueryInterfaceWithError& aRhs)
  {
    assign_from_qi_with_error(aRhs, NS_GET_IID(nsISupports));
    return *this;
  }

  // Assign from |do_GetService(cid_expr)|.
  nsCOMPtr<nsISupports>& operator=(const nsGetServiceByCID aRhs)
  {
    assign_from_gs_cid(aRhs, NS_GET_IID(nsISupports));
    return *this;
  }

  // Assign from |do_GetService(cid_expr, &rv)|.
  nsCOMPtr<nsISupports>& operator=(const nsGetServiceByCIDWithError& aRhs)
  {
    assign_from_gs_cid_with_error(aRhs, NS_GET_IID(nsISupports));
    return *this;
  }

  // Assign from |do_GetService(contractid_expr)|.
  nsCOMPtr<nsISupports>& operator=(const nsGetServiceByContractID aRhs)
  {
    assign_from_gs_contractid(aRhs, NS_GET_IID(nsISupports));
    return *this;
  }

  // Assign from |do_GetService(contractid_expr, &rv)|.
  nsCOMPtr<nsISupports>& operator=(const nsGetServiceByContractIDWithError& aRhs)
  {
    assign_from_gs_contractid_with_error(aRhs, NS_GET_IID(nsISupports));
    return *this;
  }

  // And finally, anything else we might need to assign from can exploit the
  // nsCOMPtr_helper facility
  nsCOMPtr<nsISupports>& operator=(const nsCOMPtr_helper& aRhs)
  {
    assign_from_helper(aRhs, NS_GET_IID(nsISupports));
    return *this;
  }

  // Exchange ownership with |aRhs|; can save a pair of refcount operations.
  void swap(nsCOMPtr<nsISupports>& aRhs)
  {
    nsISupports* temp = aRhs.mRawPtr;
    NSCAP_LOG_ASSIGNMENT(&aRhs, mRawPtr);
    NSCAP_LOG_ASSIGNMENT(this, temp);
    NSCAP_LOG_RELEASE(this, mRawPtr);
    NSCAP_LOG_RELEASE(&aRhs, temp);
    aRhs.mRawPtr = mRawPtr;
    mRawPtr = temp;
  }

  // Exchange ownership with |aRhs|; can save a pair of refcount operations.
  void swap(nsISupports*& aRhs)
  {
    nsISupports* temp = aRhs;
    NSCAP_LOG_ASSIGNMENT(this, temp);
    NSCAP_LOG_RELEASE(this, mRawPtr);
    aRhs = mRawPtr;
    mRawPtr = temp;
  }

  // Return the value of mRawPtr and null out mRawPtr. Useful for
  // already_AddRefed return values.
  already_AddRefed<nsISupports> forget()
  {
    nsISupports* temp = 0;
    swap(temp);
    return already_AddRefed<nsISupports>(temp);
  }

  // Set the target of aRhs to the value of mRawPtr and null out mRawPtr.
  // Useful to avoid unnecessary AddRef/Release pairs with "out"
  // parameters.
  void forget(nsISupports** aRhs)
  {
    NS_ASSERTION(aRhs, "Null pointer passed to forget!");
    *aRhs = 0;
    swap(*aRhs);
  }

  // Other pointer operators

  // Prefer the implicit conversion provided automatically by
  // |operator nsISupports*() const|. Use |get()| to resolve ambiguity or to
  // get a castable pointer.
  nsISupports* get() const { return reinterpret_cast<nsISupports*>(mRawPtr); }

  // Makes an nsCOMPtr act like its underlying raw pointer type whenever it is
  // used in a context where a raw pointer is expected. It is this operator
  // that makes an nsCOMPtr substitutable for a raw pointer.
  //
  // Prefer the implicit use of this operator to calling |get()|, except where
  // necessary to resolve ambiguity/
  operator nsISupports* () const { return get(); }

  nsISupports* operator->() const MOZ_NO_ADDREF_RELEASE_ON_RETURN
  {
    MOZ_ASSERT(mRawPtr != 0,
               "You can't dereference a NULL nsCOMPtr with operator->().");
    return get();
  }

  // These are not intended to be used by clients. See |address_of| below.
  nsCOMPtr<nsISupports>* get_address() { return this; }
  const nsCOMPtr<nsISupports>* get_address() const { return this; }

public:

  nsISupports& operator*() const
  {
    MOZ_ASSERT(mRawPtr != 0,
               "You can't dereference a NULL nsCOMPtr with operator*().");
    return *get();
  }

  nsISupports** StartAssignment()
  {
#ifndef NSCAP_FEATURE_INLINE_STARTASSIGNMENT
    return reinterpret_cast<nsISupports**>(begin_assignment());
#else
    assign_assuming_AddRef(0);
    return reinterpret_cast<nsISupports**>(&mRawPtr);
#endif
  }
};

template<typename T>
inline void
ImplCycleCollectionUnlink(nsCOMPtr<T>& aField)
{
  aField = nullptr;
}

template<typename T>
inline void
ImplCycleCollectionTraverse(nsCycleCollectionTraversalCallback& aCallback,
                            nsCOMPtr<T>& aField,
                            const char* aName,
                            uint32_t aFlags = 0)
{
  CycleCollectionNoteChild(aCallback, aField.get(), aName, aFlags);
}

#ifndef NSCAP_FEATURE_USE_BASE
template<class T>
void
nsCOMPtr<T>::assign_with_AddRef(nsISupports* aRawPtr)
{
  if (aRawPtr) {
    NSCAP_ADDREF(this, aRawPtr);
  }
  assign_assuming_AddRef(reinterpret_cast<T*>(aRawPtr));
}

template<class T>
void
nsCOMPtr<T>::assign_from_qi(const nsQueryInterface aQI, const nsIID& aIID)
{
  void* newRawPtr;
  if (NS_FAILED(aQI(aIID, &newRawPtr))) {
    newRawPtr = 0;
  }
  assign_assuming_AddRef(static_cast<T*>(newRawPtr));
}

template<class T>
void
nsCOMPtr<T>::assign_from_qi_with_error(const nsQueryInterfaceWithError& aQI,
                                       const nsIID& aIID)
{
  void* newRawPtr;
  if (NS_FAILED(aQI(aIID, &newRawPtr))) {
    newRawPtr = 0;
  }
  assign_assuming_AddRef(static_cast<T*>(newRawPtr));
}

template<class T>
void
nsCOMPtr<T>::assign_from_gs_cid(const nsGetServiceByCID aGS, const nsIID& aIID)
{
  void* newRawPtr;
  if (NS_FAILED(aGS(aIID, &newRawPtr))) {
    newRawPtr = 0;
  }
  assign_assuming_AddRef(static_cast<T*>(newRawPtr));
}

template<class T>
void
nsCOMPtr<T>::assign_from_gs_cid_with_error(const nsGetServiceByCIDWithError& aGS,
                                           const nsIID& aIID)
{
  void* newRawPtr;
  if (NS_FAILED(aGS(aIID, &newRawPtr))) {
    newRawPtr = 0;
  }
  assign_assuming_AddRef(static_cast<T*>(newRawPtr));
}

template<class T>
void
nsCOMPtr<T>::assign_from_gs_contractid(const nsGetServiceByContractID aGS,
                                       const nsIID& aIID)
{
  void* newRawPtr;
  if (NS_FAILED(aGS(aIID, &newRawPtr))) {
    newRawPtr = 0;
  }
  assign_assuming_AddRef(static_cast<T*>(newRawPtr));
}

template<class T>
void
nsCOMPtr<T>::assign_from_gs_contractid_with_error(
    const nsGetServiceByContractIDWithError& aGS, const nsIID& aIID)
{
  void* newRawPtr;
  if (NS_FAILED(aGS(aIID, &newRawPtr))) {
    newRawPtr = 0;
  }
  assign_assuming_AddRef(static_cast<T*>(newRawPtr));
}

template<class T>
void
nsCOMPtr<T>::assign_from_helper(const nsCOMPtr_helper& helper, const nsIID& aIID)
{
  void* newRawPtr;
  if (NS_FAILED(helper(aIID, &newRawPtr))) {
    newRawPtr = 0;
  }
  assign_assuming_AddRef(static_cast<T*>(newRawPtr));
}

template<class T>
void**
nsCOMPtr<T>::begin_assignment()
{
  assign_assuming_AddRef(0);
  union
  {
    T** mT;
    void** mVoid;
  } result;
  result.mT = &mRawPtr;
  return result.mVoid;
}
#endif

template<class T>
inline nsCOMPtr<T>*
address_of(nsCOMPtr<T>& aPtr)
{
  return aPtr.get_address();
}

template<class T>
inline const nsCOMPtr<T>*
address_of(const nsCOMPtr<T>& aPtr)
{
  return aPtr.get_address();
}

/**
 * This class is designed to be used for anonymous temporary objects in the
 * argument list of calls that return COM interface pointers, e.g.,
 *
 *   nsCOMPtr<IFoo> fooP;
 *   ...->QueryInterface(iid, getter_AddRefs(fooP))
 *
 * DO NOT USE THIS TYPE DIRECTLY IN YOUR CODE. Use |getter_AddRefs()| instead.
 *
 * When initialized with a |nsCOMPtr|, as in the example above, it returns
 * a |void**|, a |T**|, or an |nsISupports**| as needed, that the outer call
 * (|QueryInterface| in this case) can fill in.
 *
 * This type should be a nested class inside |nsCOMPtr<T>|.
 */
template<class T>
class nsGetterAddRefs
{
public:
  explicit nsGetterAddRefs(nsCOMPtr<T>& aSmartPtr)
    : mTargetSmartPtr(aSmartPtr)
  {
  }

#if defined(NSCAP_FEATURE_TEST_DONTQUERY_CASES) || defined(NSCAP_LOG_EXTERNAL_ASSIGNMENT)
  ~nsGetterAddRefs()
  {
#ifdef NSCAP_LOG_EXTERNAL_ASSIGNMENT
    NSCAP_LOG_ASSIGNMENT(reinterpret_cast<void*>(address_of(mTargetSmartPtr)),
                         mTargetSmartPtr.get());
#endif

#ifdef NSCAP_FEATURE_TEST_DONTQUERY_CASES
    mTargetSmartPtr.Assert_NoQueryNeeded();
#endif
  }
#endif

  operator void**()
  {
    return reinterpret_cast<void**>(mTargetSmartPtr.StartAssignment());
  }

  operator T**() { return mTargetSmartPtr.StartAssignment(); }
  T*& operator*() { return *(mTargetSmartPtr.StartAssignment()); }

private:
  nsCOMPtr<T>& mTargetSmartPtr;
};


template<>
class nsGetterAddRefs<nsISupports>
{
public:
  explicit nsGetterAddRefs(nsCOMPtr<nsISupports>& aSmartPtr)
    : mTargetSmartPtr(aSmartPtr)
  {
  }

#ifdef NSCAP_LOG_EXTERNAL_ASSIGNMENT
  ~nsGetterAddRefs()
  {
    NSCAP_LOG_ASSIGNMENT(reinterpret_cast<void*>(address_of(mTargetSmartPtr)),
                         mTargetSmartPtr.get());
  }
#endif

  operator void**()
  {
    return reinterpret_cast<void**>(mTargetSmartPtr.StartAssignment());
  }

  operator nsISupports**() { return mTargetSmartPtr.StartAssignment(); }
  nsISupports*& operator*() { return *(mTargetSmartPtr.StartAssignment()); }

private:
  nsCOMPtr<nsISupports>& mTargetSmartPtr;
};

template<class T>
inline nsGetterAddRefs<T>
getter_AddRefs(nsCOMPtr<T>& aSmartPtr)
{
  return nsGetterAddRefs<T>(aSmartPtr);
}

template<class T, class DestinationType>
inline nsresult
CallQueryInterface(T* aSource, nsGetterAddRefs<DestinationType> aDestination)
{
  return CallQueryInterface(aSource,
                            static_cast<DestinationType**>(aDestination));
}


// Comparing two |nsCOMPtr|s

template<class T, class U>
inline bool
operator==(const nsCOMPtr<T>& aLhs, const nsCOMPtr<U>& aRhs)
{
  return static_cast<const T*>(aLhs.get()) == static_cast<const U*>(aRhs.get());
}


template<class T, class U>
inline bool
operator!=(const nsCOMPtr<T>& aLhs, const nsCOMPtr<U>& aRhs)
{
  return static_cast<const T*>(aLhs.get()) != static_cast<const U*>(aRhs.get());
}


// Comparing an |nsCOMPtr| to a raw pointer

template<class T, class U>
inline bool
operator==(const nsCOMPtr<T>& aLhs, const U* aRhs)
{
  return static_cast<const T*>(aLhs.get()) == aRhs;
}

template<class T, class U>
inline bool
operator==(const U* aLhs, const nsCOMPtr<T>& aRhs)
{
  return aLhs == static_cast<const T*>(aRhs.get());
}

template<class T, class U>
inline bool
operator!=(const nsCOMPtr<T>& aLhs, const U* aRhs)
{
  return static_cast<const T*>(aLhs.get()) != aRhs;
}

template<class T, class U>
inline bool
operator!=(const U* aLhs, const nsCOMPtr<T>& aRhs)
{
  return aLhs != static_cast<const T*>(aRhs.get());
}

template<class T, class U>
inline bool
operator==(const nsCOMPtr<T>& aLhs, U* aRhs)
{
  return static_cast<const T*>(aLhs.get()) == const_cast<const U*>(aRhs);
}

template<class T, class U>
inline bool
operator==(U* aLhs, const nsCOMPtr<T>& aRhs)
{
  return const_cast<const U*>(aLhs) == static_cast<const T*>(aRhs.get());
}

template<class T, class U>
inline bool
operator!=(const nsCOMPtr<T>& aLhs, U* aRhs)
{
  return static_cast<const T*>(aLhs.get()) != const_cast<const U*>(aRhs);
}

template<class T, class U>
inline bool
operator!=(U* aLhs, const nsCOMPtr<T>& aRhs)
{
  return const_cast<const U*>(aLhs) != static_cast<const T*>(aRhs.get());
}



// Comparing an |nsCOMPtr| to |0|

class NSCAP_Zero;

// Specifically to allow |smartPtr == 0|.
template<class T>
inline bool
operator==(const nsCOMPtr<T>& aLhs, NSCAP_Zero* aRhs)
{
  return static_cast<const void*>(aLhs.get()) == reinterpret_cast<const void*>(aRhs);
}

// Specifically to allow |0 == smartPtr|.
template<class T>
inline bool
operator==(NSCAP_Zero* aLhs, const nsCOMPtr<T>& aRhs)
{
  return reinterpret_cast<const void*>(aLhs) == static_cast<const void*>(aRhs.get());
}

// Specifically to allow |smartPtr != 0|.
template<class T>
inline bool
operator!=(const nsCOMPtr<T>& aLhs, NSCAP_Zero* aRhs)
{
  return static_cast<const void*>(aLhs.get()) != reinterpret_cast<const void*>(aRhs);
}

// Specifically to allow |0 != smartPtr|.
template<class T>
inline bool
operator!=(NSCAP_Zero* aLhs, const nsCOMPtr<T>& aRhs)
{
  return reinterpret_cast<const void*>(aLhs) != static_cast<const void*>(aRhs.get());
}


// Comparing any two [XP]COM objects for identity

inline bool
SameCOMIdentity(nsISupports* aLhs, nsISupports* aRhs)
{
  return nsCOMPtr<nsISupports>(do_QueryInterface(aLhs)) ==
    nsCOMPtr<nsISupports>(do_QueryInterface(aRhs));
}



template<class SourceType, class DestinationType>
inline nsresult
CallQueryInterface(nsCOMPtr<SourceType>& aSourcePtr, DestinationType** aDestPtr)
{
  return CallQueryInterface(aSourcePtr.get(), aDestPtr);
}

#endif // !defined(nsCOMPtr_h___)