{"id":108448,"date":"2023-07-14T07:00:00","date_gmt":"2023-07-14T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=108448"},"modified":"2023-07-13T12:11:56","modified_gmt":"2023-07-13T19:11:56","slug":"20230714-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230714-00\/?p=108448","title":{"rendered":"How to clone a Windows Runtime vector in the face of possible concurrent modification, part 3"},"content":{"rendered":"

Last time, we cloned a Windows Runtime vector in the face of possible concurrent modification<\/a>, but we ran into trouble with std::vector<bool><\/code>.<\/p>\n

As I noted some time ago<\/a>, the C++ language defines a specialization std::vector<bool><\/code> which represents a packed bit array, rather than defining a separate type like std::bitvector<\/code>. This has made a lot of people very angry and has been widely regarded<\/a> as a bad move<\/a>.<\/p>\n

In our case, the specialization of std::vector<bool><\/code> breaks our clone_as_vector<\/code> function, since it needs a winrt::array_view<bool><\/code>, which needs a C-style array of bool<\/code> objects, not a packed bit array.<\/p>\n

We’ll have to detect the bool<\/code> case in our function and substitute a std::unique_ptr<bool[]><\/code>.<\/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::conditional_t<         <\/span>\r\n        std::is_same_v<T, bool>,<\/span>\r\n        std::unique_ptr<bool[]>,<\/span>\r\n        std::vector<T>><\/span> temp;   <\/span>\r\n    uint32_t expected;\r\n    uint32_t actual;\r\n    do {\r\n        expected = v.Size();\r\n        if constexpr (std::is_same_v<T, bool>) {             <\/span>\r\n            temp = std::make_unique<bool[]>(expected + 1);   <\/span>\r\n            actual = v.GetMany(0,                            <\/span>\r\n                winrt::array_view(temp.get(), expected + 1));<\/span>\r\n        } else {                                             <\/span>\r\n            temp.resize(expected + 1, winrt_empty_value<\/a><T>());\r\n            actual = v.GetMany(0, temp);\r\n        }\r\n    } while (actual > expected);\r\n    if constexpr (std::is_same_v<T, bool>) {                <\/span>\r\n        return std::vector(temp.get(), temp.get() + actual);<\/span>\r\n    } else {                                                <\/span>\r\n        temp.erase(temp.begin() + actual, temp.end());\r\n        return temp;\r\n    }\r\n}\r\n<\/pre>\n
If the value type of the vector is a bool<\/code>, then the temp<\/code> changes to a std::unique_ptr<bool[]><\/code>, and that means that we have to change how we resize the array (namely by replacing it with a new allocation) and how we generate the array_view<bool><\/code> (by using the pointer + size constructor).<\/p>\n
After the loop has captured the elements, we convert our C-style array of bool<\/code> into a std::vector<bool><\/code>.<\/p>\n
Now that I looked at it some more, it seems that there is barely any shared code at all. May as well just make it two functions glued together.<\/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    uint32_t expected;\r\n    uint32_t actual;\r\n    if constexpr (std::is_same_v<T, bool>) {\r\n        std::unique_ptr<bool[]> temp;\r\n        do {\r\n            expected = v.Size();\r\n            temp = std::make_unique<bool[]>(expected + 1);\r\n            actual = v.GetMany(0,\r\n                winrt::array_view(temp.get(), expected + 1));\r\n        } while (actual > expected);\r\n        return std::vector(temp.get(), temp.get() + actual);\r\n    } else {\r\n        std::vector<T> temp;\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\n<\/pre>\n
Whew, we took care of the pesky std::vector<bool><\/code>.<\/p>\n
Next time, we’ll look at the potential infinite loop and whether it offers a denial of service attack.<\/p>\n","protected":false},"excerpt":{"rendered":"
Dealing with the pesky std::vector<bool><\/CODE>.<\/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-108448","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Dealing with the pesky std::vector<bool><\/CODE>.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108448","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=108448"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108448\/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=108448"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=108448"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=108448"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}