{"id":108466,"date":"2023-07-20T07:00:00","date_gmt":"2023-07-20T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=108466"},"modified":"2023-07-20T05:49:41","modified_gmt":"2023-07-20T12:49:41","slug":"20230720-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230720-00\/?p=108466","title":{"rendered":"How to clone a Windows Runtime map in the face of possible concurrent modification, part 2"},"content":{"rendered":"

Although we’ve figured how to clone a Windows Runtime map in the face of possible concurrent modification<\/a>, we’re still fiddling around with the best design for the function that does the cloning.<\/p>\n

Last time, we wrote a one-shot function that does everything, but maybe the answer is to not try to jam everything into one function.<\/p>\n

We can break things into two functions, one for using a std::map<\/code> and one for using a std::unordered_map<\/code>. (This also avoids problems if the user tries to call the original function with something like clone_as_map<std::vector<int>><\/code>.)<\/p>\n

template<typename M>\r\nauto clone_as_kvp_vector(M const& m)\r\n{\r\n    using KVP = decltype(m.First().Current());\r\n    std::vector<KVP> pairs;\r\n    uint32_t expected;\r\n    uint32_t actual;\r\n    do {\r\n        expected = m.Size();\r\n        pairs.resize(expected + 1);\r\n        try {\r\n            actual = m.First().GetMany(pairs);\r\n        }\r\n        catch (winrt::hresult_changed_state const&) {\r\n            continue;\r\n        }\r\n    } while (actual > expected);\r\n    pairs.resize(actual);\r\n    return pairs;\r\n}\r\n<\/pre>\n
This worker function does the real work of cloning the Windows Runtime map into a C++ vector of IKey\u00adValue\u00adPair<\/code> objects.<\/p>\n
The next step is to write some helpers to cut down on the repetitive typing that happens when you do  type deduction from the future:<\/p>\n
template<typename Override, typename Fallback>\r\nusing override_or_fallback_t =\r\n    std::conditional_t<\r\n        std::is_same_v<Override, void>,\r\n        Fallback,\r\n        Override>;\r\n\r\ntemplate<typename M,\r\n    typename KeyOverride,\r\n    typename ValueOverride>\r\nstruct inferred_runtime_map_traits\r\n{\r\n    using KVP = decltype(std::declval<M>().First().Current());\r\n\r\n    using Key = override_or_fallback_t<\r\n        KeyOverride,\r\n        decltype(KVP().Key())>;\r\n    using Value = override_or_fallback_t<\r\n        ValueOverride,\r\n        decltype(KVP().Value())>;\r\n};\r\n\r\ntemplate<typename M,\r\n    typename KeyOverride,\r\n    typename ValueOverride,\r\n    typename CompareOverride>\r\nstruct inferred_map_traits\r\n    {\r\n    using base = inferred_runtime_map_traits\r\n        <M, KeyOverride, ValueOverride>;\r\n    using Key = typename base::Key;\r\n    using Value = typename base::Value;\r\n    using Compare = override_or_fallback_t<\r\n        CompareOverride, std::less<Key>>;\r\n    using type = std::map\r\n        <Key, Value, Compare>;\r\n};\r\n\r\ntemplate<typename M,\r\n    typename KeyOverride,\r\n    typename ValueOverride,\r\n    typename HashOverride,\r\n    typename KeyEqualOverride>\r\nstruct inferred_unordered_map_traits\r\n{\r\n    using base = inferred_runtime_map_traits\r\n        <M, KeyOverride, ValueOverride>;\r\n    using Key = typename base::Key;\r\n    using Value = typename base::Value;\r\n    using Hash = override_or_fallback_t<\r\n        HashOverride, std::hash<Key>>;\r\n    using KeyEqual = override_or_fallback_t<\r\n        KeyEqualOverride, std::equal_to<Key>>;\r\n    using type = std::unordered_map\r\n        <Key, Value, Hash, KeyEqual>;\r\n};\r\n<\/pre>\n
We start with override_or_fallback_t<\/code>, a type alias that encapsulates the “use the type provided if it is not void<\/code>; otherwise use a fallback type.” We use this to build up an inferred_runtime_map_traits<\/code> which plucks out the KVP<\/code>, Key<\/code>, and Value<\/code> from a Windows Runtime map. We also build an inferred_map_traits<\/code> inferred_unordered_map_traits<\/code> which does the same for the additional template type parameters of a std::map<\/code> and std::unordered_map<\/code>.<\/p>\n
We can use these traits types to save ourselves typing in the next few functions.<\/p>\n
template<typename Key = void,\r\n    typename Value = void,\r\n    typename Compare = void,\r\n    typename M,\r\n    typename Traits = inferred_map_traits\r\n        <M, Key, Value, Compare>>\r\nauto clone_as_map(M const& m,\r\n    typename Traits::Compare const& compare = {})\r\n{\r\n    auto pairs = clone_as_kvp_vector(m);\r\n    typename Traits::type map(compare);\r\n    for (auto&& pair : pairs) {\r\n        map.emplace_hint(\r\n            map.end(), pair.Key(), pair.Value());\r\n    }\r\n    return map;\r\n}\r\n\r\ntemplate<typename Key = void,\r\n    typename Value = void,\r\n    typename Hash = void,\r\n    typename KeyEqual = void,\r\n    typename M,\r\n    typename Traits = inferred_unordered_map_traits\r\n        <M, Key, Value, Hash, KeyEqual>>\r\nauto clone_as_unordered_map(M const& m,\r\n    typename Traits::Hash const& hash = {},\r\n    typename Traits::KeyEqual const& equal = {})\r\n{\r\n    auto pairs = clone_as_kvp_vector(m);\r\n    typename Traits::type map(\r\n        static_cast<uint32_t>(pairs.size()),\r\n        hash, equal);\r\n    for (auto&& pair : pairs) {\r\n        map.emplace(pair.Key(), pair.Value());\r\n    }\r\n    return map;\r\n}\r\n\r\ntemplate<typename Key = void,\r\n    typename Value = void,\r\n    typename Compare = void,\r\n    typename M,\r\n    typename Traits = inferred_map_traits\r\n        <M, Key, Value, Compare>>\r\nauto CloneMap(M const& m,\r\n    typename Traits::Compare const& compare = {})\r\n{\r\n    return winrt::multi_threaded_map(\r\n        clone_as_map<Key, Value, Compare>\r\n        (m, compare));\r\n}\r\n\r\ntemplate<typename Key = void,\r\n    typename Value = void,\r\n    typename Hash = void,\r\n    typename KeyEqual = void,\r\n    typename M,\r\n    typename Traits = inferred_unordered_map_traits\r\n        <M, Key, Value, Hash, KeyEqual>>\r\nauto CloneUnorderedMap(M const& m,\r\n    typename Traits::Hash const& hash = {},\r\n    typename Traits::KeyEqual const& equal = {})\r\n{\r\n    return winrt::multi_threaded_map(\r\n        clone_as_unordered_map<Key, Value, Hash, KeyEqual>\r\n        (m, hash, equal));\r\n}\r\n<\/pre>\n
We front-load the template type parameters that belong to the map or unordered map, so that you can write something that looks almost natural.<\/p>\n
\/\/ Create a std::unordered_map from strings to strings\r\n\/\/ but where the string keys are treated as case-insensitive.\r\nauto clone = clone_unordered_map<\r\n    winrt::hstring, winrt::hstring,\r\n    case_insensitive_string_hash,\r\n    case_insensitive_string_equal>(original);\r\n<\/pre>\n
Most of this wackiness was just C++ template metaprogramming nonsense.<\/p>\n
Notice that we used std::map::emplace_hint<\/code> to optimize for the case that the keys are returned in sorted order.<\/p>\n
Next time, we’ll port this to C++\/CX.<\/p>\n","protected":false},"excerpt":{"rendered":"
Applying what we learned about vectors.<\/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-108466","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Applying what we learned about vectors.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108466","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=108466"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/108466\/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=108466"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=108466"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=108466"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}