{"id":110865,"date":"2025-02-13T07:00:00","date_gmt":"2025-02-13T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=110865"},"modified":"2025-02-13T08:31:24","modified_gmt":"2025-02-13T16:31:24","slug":"20250213-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20250213-00\/?p=110865","title":{"rendered":"A sample implementation of the weak reference pattern for COM callbacks"},"content":{"rendered":"

Some time ago, I shared some patterns for implementing a COM object that hands out references to itself<\/a>, and the first was something I called the “weak pattern”.<\/p>\n

As a reminder, the weak pattern uses a separate callback object that holds a weak reference to the main object and forwards method calls to the main object.<\/p>\n\n\n\n\n\n\n\n\n\n\n\n
 <\/td>\n <\/td>\n\u00a0<\/td>\n <\/td>\nCallback<\/td>\n<\/tr>\n
 <\/td>\n <\/td>\n\u00a0<\/td>\n <\/td>\nIUnknown<\/td>\nICallback<\/td>\n\u2190<\/td>\nevent source<\/td>\n<\/tr>\n
 <\/td>\n <\/td>\nWidget<\/td>\n <\/td>\nrefcount<\/td>\n<\/tr>\n
client<\/td>\n\u2192<\/td>\nIUnknown<\/td>\nIWidget<\/td>\n\u2190<\/td>\nweak reference<\/td>\n<\/tr>\n
 <\/td>\n <\/td>\nrefcount<\/td>\n<\/tr>\n
 <\/td>\n <\/td>\nstate data<\/td>\n<\/tr>\n
 <\/td>\n<\/tr>\n
 <\/td>\n<\/tr>\n
 <\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Let’s flesh this out with an actual implementation.<\/p>\n

For concreteness, let’s say that the main Widget object is implemented in C++\/WinRT, and there is a callback interface called something like IGadget\u00adCallback<\/code>. (Applying the same logic to other libraries is left as an exercise.)<\/p>\n

We start with the gadget and its callback.<\/p>\n

\/\/ gadgets.idl\r\ninterface IGadgetCallback : IUnknown\r\n{\r\n    HRESULT ColorChanged([in] COLORREF oldColor,\r\n                         [in] COLORREF newColor);\r\n    HRESULT NameChanged([in] PCWSTR oldName,\r\n                        [in] PCWSTR newName);\r\n    HRESULT Refreshed();\r\n};\r\n\r\ninterface IGadget : IUnknown\r\n{\r\n    \u27e6 other Gadget methods \u27e7\r\n\r\n    HRESULT SetCallback([in] IGadgetCallback* callback);\r\n};\r\n<\/pre>\n
The idea is that after you create a Gadget, you can use the Set\u00adCallback()<\/code> method to pass an object that receives gadget callbacks for various things that might happen.<\/p>\n
Here’s how we can create a Widget that sets a callback on the Gadget that calls back into the Widget, but without creating a circular reference.<\/p>\n
namespace winrt::Contoso::implementation\r\n{\r\n    struct Widget : WidgetT<Widget>\r\n    {\r\n        void InitializeComponent();\r\n\r\n        \u27e6 other Widget methods \u27e7\r\n\r\n        \/\/ Used by WidgetGadgetCallback\r\n        void ColorChanged(COLORREF oldColor,\r\n                          COLORREF newColor);\r\n        void NameChanged([PCWSTR oldName,\r\n                          PCWSTR newName);\r\n        void Refreshed();\r\n    private:\r\n\r\n        winrt::com_ptr<IGadget> m_gadget =\r\n            winrt::create_instance(CLSID_Gadget);\r\n    };\r\n\r\n    struct WidgetGadgetCallback :\r\n        winrt::implements<WidgetGadgetCallback,\r\n                          IGadgetCallback>\r\n    {\r\n        WidgetGadgetCallback(implementation::Widget* widget);\r\n\r\n        IFACEMETHODIMP ColorChanged(COLORREF oldColor,\r\n                                    COLORREF newColor);\r\n        IFACEMETHODIMP NameChanged(PCWSTR oldName,\r\n                                   PCWSTR newName);\r\n        IFACEMETHODIMP Refreshed();\r\n\r\n    private:\r\n        winrt::weak_ref<implementation::Widget> m_weakWidget;\r\n\r\n        \u27e6 more to come \u27e7\r\n    };\r\n}\r\n<\/pre>\n
The IGadgetCallback<\/code> is implemented not by the Widget<\/code> but by the Widget\u00adGadget\u00adCallback<\/code>, and the idea is that the Widget\u00adGadget\u00adCallback<\/code> forwards the method calls to the corresponding methods on the Widget<\/code>.<\/p>\n
Let’s hook them up. When the Widget<\/code> initializes, it creates the callback object and sets it as the callback.<\/p>\n
void Widget::InitializeComponent()\r\n{\r\n    winrt::check_hresult(m_gadget->SetCallback(\r\n        winrt::make<WidgetGadgetCallback>(this).get());\r\n}\r\n<\/pre>\n
The real work happens in the callback object.<\/p>\n
The constructor takes the incoming Widget*<\/code> and obtains a weak reference to it.<\/p>\n
WidgetGadgetCallback::WidgetGadgetCallback(\r\n        implementation::Widget* widget) :\r\n    m_weakWidget(widget->get_weak()) {}\r\n<\/pre>\n
When a method on the callback interface is called, we want to forward the call back to the Widget<\/code>. To reduce typing, we’ll templatize the pattern.<\/p>\n
struct WidgetGadgetCallback :\r\n    winrt::implements<WidgetGadgetCallback,\r\n                      IGadgetCallback>\r\n{\r\n    \u27e6 existing public methods \u27e7\r\n\r\nprivate:\r\n    winrt::weak_ref<implementation::Widget> m_weakWidget;\r\n\r\n    template<auto Method, typename... Args>\r\n    HRESULT Forward(Args&&... args) try\r\n    {\r\n        if (auto strong = m_weakWidget.get()) {\r\n            ((*strong).*Method)(\r\n                std::forward<Args>(args)...);\r\n        }\r\n        return S_OK;\r\n    }\r\n    catch (...)\r\n    {\r\n        return winrt::to_hresult();\r\n    }\r\n};\r\n<\/pre>\n
The pattern for forwarding the call is to try to resolve the weak reference to a strong reference, and if it succeeds, call the provided method while forwarding the parameters. We also do some adapting between the HRESULT<\/code>-based COM call and the exception-based method back in the Widget<\/code> object. (This was a quirk of this particular example. Your version may not require adapting.)<\/p>\n
Now, we happen to know that all of the parameters to the COM callback methods are cheap to copy, so we could pass them along by value:<\/p>\n
    template<auto Method, typename... Args>\r\n    HRESULT Forward(Args<\/span>... args) try\r\n    {\r\n        if (auto strong = m_weakWidget.get()) {\r\n            ((*strong).*Method)(args<\/span>...);\r\n        }\r\n        return S_OK;\r\n    }\r\n    catch (...)\r\n    {\r\n        return winrt::to_hresult();\r\n    }\r\n<\/pre>\n
The last piece of the puzzle is telling each method to do the forwarding.<\/p>\n
struct WidgetGadgetCallback :\r\n    winrt::implements<WidgetGadgetCallback,\r\n                      IGadgetCallback>\r\n{\r\n    \u27e6 ... \u27e7\r\n\r\n    IFACEMETHODIMP ColorChanged(COLORREF oldColor,\r\n                                COLORREF newColor)\r\n    {\r\n        return Forward<&implementation::Widget::ColorChanged>(\r\n            oldColor, newColor);\r\n    }\r\n\r\n    IFACEMETHODIMP NameChanged(PCWSTR oldName,\r\n                               PCWSTR newName);\r\n    {\r\n        return Forward<&implementation::Widget::NameChanged>(\r\n            oldName, newName);\r\n    }\r\n\r\n    IFACEMETHODIMP Refreshed();\r\n    {\r\n        return Forward<&implementation::Widget::Refreshed>();\r\n    }\r\n\r\n    \u27e6 ... \u27e7\r\n};\r\n<\/pre>\n
We invoke the Forward<\/code> method with the function we want to forward to and the arguments we want to forward.<\/p>\n
In our example, we gave the forwarded-to method the same name as the forwarded-from method, but there’s no requirement that you do that. You just have to provide the correct method name to the Forward<\/code> template function.<\/p>\n","protected":false},"excerpt":{"rendered":"
A forwarder through a weak reference.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-110865","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A forwarder through a weak reference.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/110865","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=110865"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/110865\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=110865"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=110865"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=110865"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}