{"id":109529,"date":"2024-03-13T07:00:00","date_gmt":"2024-03-13T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109529"},"modified":"2024-03-13T10:43:01","modified_gmt":"2024-03-13T17:43:01","slug":"20240313-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240313-00\/?p=109529\/","title":{"rendered":"How well does MFC IPTR<\/CODE>\/CIP<\/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 MFC’s IPTR<\/code>\/CIP<\/code>.<\/p>\n

The CIP<\/code> template class fails CTAD because of these constructors:<\/p>\n

template<class _Interface, const IID* _IID>\r\nclass CIP : public _CIP<_Interface, _IID>\r\n{\r\npublic:\r\n    typedef _CIP<_Interface, _IID> BC;\r\n    typedef _Interface Interface;\r\n\r\n    CIP(Interface* pInterface); \/\/ this one\r\n    CIP(Interface* pInterface, BOOL bAddRef); \/\/ and this one\r\n\r\n    \u27e6 other stuff \u27e7\r\n};\r\n<\/pre>\n
In this case, the compiler can deduce the _Interface<\/code> template parameter, but it doesn’t know what _IID<\/code> to use. Again, adding some deduction guides can fix that by saying that it should use the __uuidof(T)<\/code> as the _IID<\/code>.<\/p>\n
template<typename T> CIP(T*)       -> CIP<T, &__uuidof(T)>;\r\ntemplate<typename T> CIP(T*, BOOL) -> CIP<T, &__uuidof(T)>;\r\n<\/pre>\n
 As noted before<\/a>, injecting your own deduction guides into somebody else’s types is not recommended because it may conflict with deduction guides added by the type owner in the future.<\/p>\n
Fortunately, in the case of MFC IPTR<\/code>\/CIP<\/code>, you rarely use CIP<\/code> directly. Instead, you use the IPTR<\/code> that was generated from the CIP<\/code>, and that type already knows what interface it is for, so there’s no need for CTAD.<\/p>\n
using TestPtr = IPTR(Test);\r\n\r\nvoid sample(Test* p)\r\n{\r\n    auto smart = TestPtr(p);\r\n}\r\n<\/pre>\n
Okay, but what if you still want to create an MFC smart pointer from a raw pointer without having to specify the interface?<\/p>\n
You can do what C++ did before CTAD was invented: “You kids with your CTADs and deduction guides. In my day, we had to use maker template functions,\u00b9 and we liked it!”<\/p>\n
template<typename T>\r\nCIP<T, &__uuidof(T)> make_CIP(T* p)\r\n{\r\n    return p;\r\n}\r\n\r\ntemplate<typename T>\r\nCIP<T, &__uuidof(T)> make_CIP(T* p, BOOL bAddRef)\r\n{\r\n    return { p, bAddRef };\r\n}\r\n\r\nvoid sample(Test* p)\r\n{\r\n    auto smart1 = make_CIP(p);\r\n    auto smart2 = make_CIP(p, FALSE);\r\n}\r\n<\/pre>\n
This trick also works for _com_ptr_t<\/code>, which is  something I teased last time<\/a>.<\/p>\n
template<typename T>\r\n_com_ptr_t<_com_IIID<T, &__uuidof(T)>>\r\n    make_com_ptr_t(T* p)\r\n{\r\n    return p;\r\n}\r\n\r\nvoid sample(Test* p)\r\n{\r\n    auto smart = make_com_ptr_t(p);\r\n}\r\n<\/pre>\n
\u00b9 There are maker template functions all over the C++ standard library. std::back_inserter<\/code>, std::make_reverse_iterator<\/code>, std::make_pair<\/code>, etc<\/i>.<\/p>\n","protected":false},"excerpt":{"rendered":"
Not very well, thanks to requiring the interface ID to be specified explicitly.<\/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-109529","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Not very well, thanks to requiring the interface ID to be specified explicitly.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109529","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=109529"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109529\/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=109529"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109529"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109529"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}