{"id":109552,"date":"2024-03-19T07:00:00","date_gmt":"2024-03-19T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109552"},"modified":"2024-03-19T11:32:42","modified_gmt":"2024-03-19T18:32:42","slug":"20240319-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240319-00\/?p=109552","title":{"rendered":"How well does C++\/WinRT com_ptr<\/CODE> support class template argument deduction (CTAD)?"},"content":{"rendered":"

Continuing our investigation of which C++ COM wrappers support class template argument deduction<\/a> (CTAD), next up is the C++\/WinRT winrt::com_ptr<\/code>.<\/p>\n

This one is easy: C++\/WinRT’s com_ptr<\/code> doesn’t support CTAD, and it doesn’t even try. There is no com_ptr<T><\/code> constructor that takes a T*<\/code>. If you want a com_ptr<\/code> to copy an existing pointer, you call com_ptr::copy_from<\/code>:<\/p>\n

IWidget* p;\r\n\r\nwinrt::com_ptr<IWidget> smart;\r\nsmart.copy_from(p);\r\n<\/pre>\n
C++\/WinRT eschews the constructor that makes a copy of an inbound pointer on the theory that it’s unclear whether it is taking ownership of the pointer or merely sharing it.<\/p>\n
The library author could have provided a deduction guide for the “take ownership” constructor:<\/p>\n
template<typename T>\r\ncom_ptr(T*, take_ownership_from_abi_t) -> com_ptr<T>;\r\n<\/pre>\n
As a consumer, though, you shouldn’t be creating deduction guides for somebody else’s classes. Instead, you can use a maker function.<\/p>\n
template<typename T>\r\nwinrt::com_ptr<\r\n    std::enable_if_t<std::<is_base_of_v<::IUnknown, T>, T>>\r\n    make_com_ptr(T* p, winrt::take_ownership_from_abi_t)\r\n{\r\n    return { p, winrt::take_ownership_from_abi };\r\n}\r\n<\/pre>\n
There is some extra magic in the return value to activate the function only if T<\/code> derives from ::IUnknown<\/code>. This avoids accidentally creating a com_ptr<\/code> from a C++\/WinRT ABI pointer (which are in the impl<\/code> namespace and are therefore off-limits).<\/p>\n
If you feel so brave, you can also create a maker function that copies the ABI pointer.<\/p>\n
template<typename T>\r\nwinrt::com_ptr<T>\r\n    std::enable_if_t<std::<is_base_of_v<::IUnknown, T>, T>>\r\n    make_com_ptr_from_copy(T* p)\r\n{\r\n    winrt::com_ptr<T> result;\r\n    result.copy_from(p);\r\n    return result;\r\n}\r\n<\/pre>\n
Bonus chatter<\/b>: The last time I made a survey of C++ COM wrappers, someone asked me to include C++\/CX in the list. But C++\/CX doesn’t support wrappers around classic COM interfaces; it supports only Windows Runtime interfaces. Therefore, the answer for C++\/CX is always “Not supported. Nothing to see here.”<\/p>\n","protected":false},"excerpt":{"rendered":"
It doesn’t even try, and probably doesn’t want you to do it.<\/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-109552","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
It doesn’t even try, and probably doesn’t want you to do it.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109552","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=109552"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109552\/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=109552"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109552"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109552"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}