{"id":107986,"date":"2023-03-30T07:00:00","date_gmt":"2023-03-30T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=107986"},"modified":"2023-03-28T22:04:42","modified_gmt":"2023-03-29T05:04:42","slug":"20230330-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230330-00\/?p=107986","title":{"rendered":"How can I box a std::optional<\/CODE> into a C++\/WinRT IInspectable<\/CODE>?"},"content":{"rendered":"

Say you have a std::optional<T><\/code> in your hand, and you want to convert it to an IInspectable<\/code>, say because you want to put it inside a PropertySet<\/code>.<\/p>\n

void SaveHeight(winrt::PropertySet const& set)\r\n{\r\n    std::optional<int> height = GetHeight();\r\n\r\n    set.Insert(L\"height\", \/* what goes here? *\/);\r\n}\r\n<\/pre>\n
Well, one thing you shouldn’t<\/i> do is go right for the value<\/code>:<\/p>\n
\/\/ Code in italics is wrong.\r\nvoid SaveHeight(winrt::PropertySet const& set)\r\n{\r\n    std::optional<int> height = GetHeight();\r\n\r\n    set.Insert(L\"height\",\r\n        winrt::box_value(height.value())<\/i><\/span>);\r\n}\r\n<\/pre>\n
Calling value()<\/code> on an empty std::optional<\/code> throws the std::bad_optional_access<\/code> exception. In that case, your SaveHeight<\/code> method throws an exception instead of saving a nullptr<\/code> into the property set. If this exception crosses an ABI boundary, C++\/WinRT and C++\/CX will convert it to E_FAIL<\/code>, and WIL will convert it to ERROR_UNHANDLED_EXCEPTION<\/code>. But really, it doesn’t matter how the C++ exception is converted to an ABI HRESULT<\/code> because you didn’t want an exception in the first place. You wanted the std::nullopt<\/code> to convert to a null pointer.<\/p>\n
You could manually check for an empty std::optional<\/code>:<\/p>\n
void SaveHeight(winrt::PropertySet const& set)\r\n{\r\n    std::optional<int> height = GetHeight();\r\n\r\n    set.Insert(L\"height\",\r\n        height ? winrt::box_value(*height) : nullptr<\/span>);\r\n}\r\n<\/pre>\n
But there’s an even easier way.<\/p>\n
C++\/WinRT provides a conversion operator from std::optional<T><\/code> to IReference<T><\/code> which does the obvious thing: An empty std::optional<\/code> becomes nullptr<\/code> and a std::optional<\/code> with a value becomes an IReference<\/code> that holds the value.<\/p>\n
void SaveHeight(winrt::PropertySet const& set)\r\n{\r\n    std::optional<int> height = GetHeight();\r\n\r\n    set.Insert(L\"height\",\r\n        winrt::IReference(height)<\/span>);\r\n}\r\n<\/pre>\n
We are taking advantage here of class template argument deduction (CTAD), which lets us write just winrt::IReference(...)<\/code> and let the compiler infer that we are constructing a winrt::IReference<int><\/code>.<\/p>\n","protected":false},"excerpt":{"rendered":"
There’s a handy conversion for that, but you have to know where to look.<\/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-107986","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
There’s a handy conversion for that, but you have to know where to look.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107986","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=107986"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107986\/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=107986"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=107986"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=107986"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}