{"id":104427,"date":"2020-11-06T07:00:00","date_gmt":"2020-11-06T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=104427"},"modified":"2020-11-06T07:03:48","modified_gmt":"2020-11-06T15:03:48","slug":"20201106-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20201106-00\/?p=104427","title":{"rendered":"How do I save a C++\/WinRT array_view as a com_array?"},"content":{"rendered":"

If you have a Windows Runtime class with a property whose Windows Runtime type is an array, then the C++\/WinRT projection expresses the property setter and getter as follows:<\/p>\n

\/\/ Set array\r\nvoid IntArray(winrt::array_view<int32_t const> const& value);\r\n\r\n\/\/ Get array\r\nwinrt::com_array<int32_t> IntArray();\r\n<\/pre>\n
 We saw earlier<\/a> that these correspond to the PassArray<\/code> and ReceiveArray<\/code> patterns, respectively.<\/p>\n
How would you implement the backing store for this property?<\/p>\n
Well, the first thing to note is that the backing store should not<\/i> be an array_view<\/code>, because an array_view<\/code> is a non-owning view into somebody else’s data. If your setter saved just the array_view<\/code>, then it would be left with a dangling pointer, because the array_view<\/code> parameter is valid only for the duration of the call.<\/p>\n
You realize that what you want to do is save a copy of the contents of an array_view<\/code>. One option is to save it in a com_array<\/code>, but there is no obvious way to create a com_array<\/code> that is a copy of the contents of an array_view<\/code>, seeing as there’s no constructor that takes an array_view<\/code>.<\/p>\n
You need to use the two-iterator constructor for com_array<\/code>. This creates a copy of the provided range of data and saves it in a com_array<\/code>.<\/p>\n
Similarly, to return the com_array<\/code>, you need to construct the return value in the same way. The com_array<\/code> is not copyable, so you’ll have to use the two-iterator constructor.<\/p>\n
struct Class : ClassT<Class>\r\n{\r\nprivate:\r\n  winrt::com_array<int32_t> int_array_;\r\n\r\npublic:\r\n  void IntArray(winrt::array_view<int32_t const> const& value)\r\n  {\r\n    int_array_ = { value.begin(), value.end() };<\/span>\r\n  }\r\n\r\n  auto IntArray()\r\n  {\r\n    return winrt::com_array\r\n      { int_array_.begin(), int_array_.end() };<\/span>\r\n  }\r\n};\r\n<\/pre>\n
We take advantage of  class template argument deduction (CTAD)<\/a> to avoid having to repeat the type int32_t<\/code> when constructing the com_array<\/code>. The full version would have been<\/p>\n
    return winrt::com_array<int32_t>\r\n      { int_array_.begin(), int_array_.end() };\r\n<\/pre>\n
The com_array<\/code> is not copyable, but it is<\/i> movable, so if you want to just give it away, you can std::move<\/code> it. You don’t want to do that for a property backing store, but maybe it’ll come in handy in other cases.<\/p>\n
If you intend to do something with the backing store beyond simply using it to hold data, you might want to use a more versatile data structure like a std::vector<\/code>. Fortunately, a com_array<\/code> can construct from a vector, so you can do this:<\/p>\n
struct Class : ClassT<Class>\r\n{\r\nprivate:\r\n  std::vector<int32_t> int_vector_;<\/span>\r\n\r\npublic:\r\n  void IntArray(winrt::array_view<int32_t const> const& value)\r\n  {\r\n    int_vector_ = { value.begin(), value.end() };<\/span>\r\n  }\r\n\r\n  auto IntArray()\r\n  {\r\n    return winrt::com_array{ int_vector_ };<\/span>\r\n  }\r\n};\r\n<\/pre>\n
There’s a not-entirely-obvious constructor for that.<\/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-104427","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
There’s a not-entirely-obvious constructor for that.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/104427","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=104427"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/104427\/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=104427"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=104427"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=104427"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
Here, we take advantage of a deduction guide (introduced in  version 2.0.200601.2<\/a>) to avoid having to specialize the winrt::com_array<\/code>.<\/p>\n","protected":false},"excerpt":{"rendered":"