xpcom/base/nsISupportsUtils.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef nsISupportsUtils_h__
   8 #define nsISupportsUtils_h__
   9
  10 #include <type_traits>
  11
  12 #include "nscore.h"
  13 #include "nsIOutputStream.h"
  14 #include "nsISupports.h"
  15 #include "nsError.h"
  16 #include "nsDebug.h"
  17 #include "nsISupportsImpl.h"
  18 #include "mozilla/RefPtr.h"
  19
  20 /**
  21  * Macro for adding a reference to an interface.
  22  * @param _ptr The interface pointer.
  23  */
  24 #define NS_ADDREF(_ptr) (_ptr)->AddRef()
  25
  26 /**
  27  * Macro for adding a reference to this. This macro should be used
  28  * because NS_ADDREF (when tracing) may require an ambiguous cast
  29  * from the pointers primary type to nsISupports. This macro sidesteps
  30  * that entire problem.
  31  */
  32 #define NS_ADDREF_THIS() AddRef()
  33
  34 // Making this a |inline| |template| allows |aExpr| to be evaluated only once,
  35 // yet still denies you the ability to |AddRef()| an |nsCOMPtr|.
  36 template <class T>
  37 inline void ns_if_addref(T aExpr) {
  38   if (aExpr) {
  39     aExpr->AddRef();
  40   }
  41 }
  42
  43 /**
  44  * Macro for adding a reference to an interface that checks for nullptr.
  45  * @param _expr The interface pointer.
  46  */
  47 #define NS_IF_ADDREF(_expr) ns_if_addref(_expr)
  48
  49 /*
  50  * Given these declarations, it explicitly OK and efficient to end a `getter'
  51  * with:
  52  *
  53  *    NS_IF_ADDREF(*result = mThing);
  54  *
  55  * even if |mThing| is an |nsCOMPtr|.  If |mThing| is an |nsCOMPtr|, however, it
  56  * is still _illegal_ to say |NS_IF_ADDREF(mThing)|.
  57  */
  58
  59 /**
  60  * Macro for releasing a reference to an interface.
  61  * @param _ptr The interface pointer.
  62  */
  63 #define NS_RELEASE(_ptr) \
  64   do {                   \
  65     (_ptr)->Release();   \
  66     (_ptr) = 0;          \
  67   } while (0)
  68
  69 /**
  70  * Macro for releasing a reference to this interface.
  71  */
  72 #define NS_RELEASE_THIS() Release()
  73
  74 /**
  75  * Macro for releasing a reference to an interface, except that this
  76  * macro preserves the return value from the underlying Release call.
  77  * The interface pointer argument will only be NULLed if the reference count
  78  * goes to zero.
  79  *
  80  * @param _ptr The interface pointer.
  81  * @param _rc  The reference count.
  82  */
  83 #define NS_RELEASE2(_ptr, _rc)  \
  84   do {                          \
  85     _rc = (_ptr)->Release();    \
  86     if (0 == (_rc)) (_ptr) = 0; \
  87   } while (0)
  88
  89 /**
  90  * Macro for releasing a reference to an interface that checks for nullptr;
  91  * @param _ptr The interface pointer.
  92  */
  93 #define NS_IF_RELEASE(_ptr) \
  94   do {                      \
  95     if (_ptr) {             \
  96       (_ptr)->Release();    \
  97       (_ptr) = 0;           \
  98     }                       \
  99   } while (0)
 100
 101 /*
 102  * Often you have to cast an implementation pointer, e.g., |this|, to an
 103  * |nsISupports*|, but because you have multiple inheritance, a simple cast
 104  * is ambiguous.  One could simply say, e.g., (given a base |nsIBase|),
 105  * |static_cast<nsIBase*>(this)|; but that disguises the fact that what
 106  * you are really doing is disambiguating the |nsISupports|.  You could make
 107  * that more obvious with a double cast, e.g., |static_cast<nsISupports*>
 108                                                            (*
 109  static_cast<nsIBase*>(this))|, but that is bulky and harder to read...
 110  *
 111  * The following macro is clean, short, and obvious.  In the example above,
 112  * you would use it like this: |NS_ISUPPORTS_CAST(nsIBase*, this)|.
 113  */
 114
 115 #define NS_ISUPPORTS_CAST(__unambiguousBase, __expr) \
 116   static_cast<nsISupports*>(static_cast<__unambiguousBase>(__expr))
 117
 118 // a type-safe shortcut for calling the |QueryInterface()| member function
 119 template <class T, class DestinationType>
 120 inline nsresult CallQueryInterface(T* aSource, DestinationType** aDestination) {
 121   // We permit nsISupports-to-nsISupports here so that one can still obtain
 122   // the canonical nsISupports pointer with CallQueryInterface.
 123   static_assert(
 124       !(std::is_same_v<DestinationType, T> ||
 125         std::is_base_of<DestinationType, T>::value) ||
 126           std::is_same_v<DestinationType, nsISupports>,
 127       "don't use CallQueryInterface for compile-time-determinable casts");
 128
 129   MOZ_ASSERT(aSource, "null parameter");
 130   MOZ_ASSERT(aDestination, "null parameter");
 131
 132   return aSource->QueryInterface(NS_GET_TEMPLATE_IID(DestinationType),
 133                                  reinterpret_cast<void**>(aDestination));
 134 }
 135
 136 template <class SourceType, class DestinationType>
 137 inline nsresult CallQueryInterface(RefPtr<SourceType>& aSourcePtr,
 138                                    DestinationType** aDestPtr) {
 139   return CallQueryInterface(aSourcePtr.get(), aDestPtr);
 140 }
 141
 142 #endif /* __nsISupportsUtils_h */