{"id":111557,"date":"2025-09-04T07:00:00","date_gmt":"2025-09-04T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=111557"},"modified":"2025-09-03T08:02:59","modified_gmt":"2025-09-03T15:02:59","slug":"20250904-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20250904-00\/?p=111557","title":{"rendered":"How can I write a C++\/WinRT IAsyncOperation<T><\/CODE> where T<\/CODE> is not a Windows Runtime type?, part 2"},"content":{"rendered":"

Last time, we smuggled an arbitrary C++ value inside an IInspectable<\/code><\/a> but noted the lack of safety. So let’s add some safety.<\/p>\n

To ensure that the object is in-process, you can declare the interface as “local”, meaning that the interface is for use only within a single process, and the MIDL compiler will not generate proxies for it. Even easier is just defining the interface manually, without involving the MIDL compiler at all.<\/p>\n

struct\r\nDECLSPEC_UUID(\"\u27e6some guid\u27e7\")\r\nIValueAsInspectable : ::IUnknown\r\n{\r\n};\r\n<\/pre>\n
The trick here is that manually defining the interface lets us put C++ stuff inside it.<\/p>\n
template<typename T> struct ValueAsInspectable;\r\n\r\nstruct\r\nDECLSPEC_UUID(\"\u27e6some guid\u27e7\")\r\nIValueAsInspectable : ::IUnknown\r\n{\r\n    std::type_info const* type;\r\n\r\n    template<typename T>\r\n    T& get_value()\r\n    {\r\n        if (*type != typeid(T)) {\r\n            throw std::bad_cast();\r\n        }\r\n        return static_cast<ValueAsInspectable<T>*>(this)\r\n            ->value;\r\n    }\r\n};\r\n\r\ntemplate<typename T>\r\nstruct ValueAsInspectable :\r\n    winrt::implements<ValueAsInspectable<T>,\r\n    winrt::Windows::Foundation::IInspectable,\r\n    IValueAsInspectable>\r\n{\r\n    T value;\r\n\r\n    template<typename...Args>\r\n    ValueAsInspectable(Args&&... args) :\r\n        value{ std::forward<Args>(args)... }\r\n    {\r\n        this->>type = std::addressof(typeid(T));\r\n    }\r\n};\r\n<\/pre>\n
The IValueAsInspectable<\/code> has a COM interface ID, so it can be queried for. But once you get it, you can call the C++ method get_value<T>()<\/code>. That method first validates that you are using the matching T<\/code>, throwing a bad_cast<\/code> exception if not. If you pass that test, then it returns a reference to the value hiding inside.<\/p>\n
And for convenience, we can add these helpers. The first two we have seen already:<\/p>\n
template<typename T, typename...Args>\r\nwinrt::Windows::Foundation::IInspectable\r\n    MakeValueAsInspectable(Args&&... args)\r\n{\r\n    return winrt::make<ValueAsInspectable<T>>(\r\n        std::forward<Args>(args)...);\r\n}\r\n\r\ntemplate<typename T>\r\nwinrt::Windows::Foundation::IInspectable\r\n    MakeValueAsInspectable(T&& arg)\r\n{\r\n    return winrt::make<ValueAsInspectable<\r\n        std::remove_reference_t<T>>>(\r\n        std::forward<T>(arg));\r\n}\r\n<\/pre>\n
Next are functions for extracting the value from an object that we believe to be a Value\u00adAs\u00adInspectable<T><\/code>.<\/p>\n
template<typename T>\r\nT& ValueRefFromInspectable(\r\n    winrt::Windows::Foundation::IInspectable const& arg)\r\n{\r\n    return arg.as<IValueAsInspectable>()->\r\n        get_value<T>();\r\n}\r\n\r\ntemplate<typename T>\r\nT CopyValueFromInspectable(\r\n    winrt::Windows::Foundation::IInspectable const& arg)\r\n{\r\n    return ValueRefFromInspectable<T>(arg);\r\n}\r\n\r\ntemplate<typename T>\r\nT MoveValueFromInspectable(\r\n    winrt::Windows::Foundation::IInspectable && arg)\r\n{\r\n    return std::move(ValueRefFromInspectable<T>(arg));\r\n}\r\n<\/pre>\n
The basic function is Value\u00adRef\u00adFrom\u00adInspectable<T><\/code> which produces an lvalue reference from an inspectable that we assume represents a Value\u00adAs\u00adInspectable<T><\/code>. (If we’re wrong, it throws a bad_cast<\/code> exception.) Building on that are two functions which either copy or move the value out of the Value\u00adAs\u00adInspectable<T><\/code>.<\/p>\n
I gave the function that returns an lvalue name that emphasizes that you get a reference from the inspectable, hoping to prevent people from getting a reference to an expiring inspectable:<\/p>\n
\/\/ Code in italics is wrong\r\n\/\/ Don't do this.\r\nauto& value = ValueRefFromInspectablt<Widget>(\r\n    co_await DoSomethingAsync());<\/i>\r\n<\/pre>\n
Note also that if you modify the value reference or move the value out of the inpectable, this affects the underlying object, which means that other people that have a reference to the same inspectable will see the object change state.<\/p>\n
void MutateTheWidget(winrt::Windows::Foundation::IInspectable obj)\r\n{\r\n    auto& widget = ValueRefFromInspectable<Widget>(obj);\r\n    \u27e6 do something that modifies the widget \u27e7\r\n}\r\n\r\nwinrt::fire_and_forget Sample()\r\n{\r\n    auto obj = co_await DoSomethingAsync();\r\n    MutateTheWidget(obj);\r\n    auto& widget = ValueRefFromInspectable<Widget>(obj);\r\n    \/\/ this code sees that the widget has been mutated\r\n}\r\n<\/pre>\n
But really, if you need a coroutine that produces a non-Windows Runtime type, then use a non-Windows Runtime coroutine library. I’m personally partial to wil::task<\/code> (and its COM-aware buddy wil::com_task<\/code>), seeing as I wrote it.<\/p>\n","protected":false},"excerpt":{"rendered":"
Safer smuggling.<\/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-111557","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Safer smuggling.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/111557","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=111557"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/111557\/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=111557"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=111557"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=111557"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}