{"id":108446,"date":"2023-07-13T07:00:00","date_gmt":"2023-07-13T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=108446"},"modified":"2023-07-13T06:36:59","modified_gmt":"2023-07-13T13:36:59","slug":"20230713-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230713-00\/?p=108446","title":{"rendered":"How to clone a Windows Runtime vector in the face of possible concurrent modification, part 2"},"content":{"rendered":"

Last time, we developed a pattern for cloning a Windows Runtime vector in the face of possible concurrent modification<\/a>. Let’s try to make a function out of this pattern.<\/p>\n

template<typename V>\r\nauto clone_as_vector(V const& v)\r\n-> std::vector<decltype(v.GetAt(0))>\r\n{\r\n    using T = decltype(v.GetAt(0));\r\n    std::vector<T> temp;\r\n    uint32_t expected;\r\n    uint32_t actual;\r\n    do {\r\n        expected = v.Size();\r\n        temp.resize(expected + 1, winrt_empty_value<\/a><T>());\r\n        actual = v.GetMany(0, temp);\r\n    } while (actual > expected);\r\n    temp.erase(temp.begin() + actual, temp.end());\r\n    return temp;\r\n}\r\n\r\ntemplate<typename V>\r\nauto CloneVector(V const& v)\r\n-> winrt::Windows::Foundation::\r\n        Collections::IVector<decltype(v.GetAt(0))>\r\n{\r\n    return winrt::multi_threaded_vector(\r\n        clone_as_vector(v));\r\n}\r\n<\/pre>\n
There are some sneaky bits here.<\/p>\n
First of all, we make clone_as_vector<\/code> generic in that all it requires is that the v<\/code> parameter support GetAt()<\/code> and GetMany()<\/code>. This allows the clone_as_vector<\/code> to work with both IVector<\/code> and IVectorView<\/code>.<\/p>\n
Second, we use an explicit trailing return type instead of just letting the compiler figure it out from the return<\/code> statement. This allow SFINAE to remove the function from consideration, allowing us to define other versions of clone_as_vector<\/code> that work for other collection types.\u00b9<\/p>\n
This seems to work, until we run into IVector<bool><\/code>, because that cause the clone_as_vector<\/code> function to create a std::vector<bool><\/code>.<\/p>\n
Uh-oh.<\/p>\n
We’ll try to fix that next time.<\/p>\n
\u00b9 Though our trailing return type validates only that GetAt()<\/code> works. This means that it will accidentally activate for Bindable\u00adVector<\/code> and Bindable\u00adVector\u00adView<\/code>, which have GetAt()<\/code> but not GetMany()<\/code>. I guess we could fix that by adding more junk:<\/p>\n
template<typename V>\r\nauto clone_as_vector(V const& v)\r\n-> std::enable_if_t<\r\n    std::is_integral_v<decltype(v.GetMany(0, {}))>,\r\n    std::vector<decltype(v.GetAt(0))>>\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
Trying to make a generic solution.<\/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-108446","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Trying to make a generic solution.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108446","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=108446"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108446\/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=108446"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=108446"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=108446"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}