{"id":103191,"date":"2019-12-06T07:00:00","date_gmt":"2019-12-06T15:00:00","guid":{"rendered":"http:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103191"},"modified":"2019-12-06T05:02:48","modified_gmt":"2019-12-06T13:02:48","slug":"20191206-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20191206-00\/?p=103191","title":{"rendered":"In C++\/WinRT, what happens when I treat an IInspectable<\/CODE> as or convert one to a bool<\/CODE>"},"content":{"rendered":"

Last time, we looked at weirdness in how C++\/CX treats hat pointers in a bool<\/code> context<\/a>. Fortunately, C++\/WinRT is much less weird.<\/p>\n

The IInspectable<\/code> type supports a conversion to bool<\/code> which tests whether the underlying pointer is null. It also supports comparison against nullptr<\/code> which tests the same thing. And, unlike C++\/CX, C++\/WinRT uses this conversion for both explicit and contextual conversions.<\/p>\n

IInspectable p = winrt::box_value(false);\r\nIInspectable q = winrt::box_value(false);\r\n\r\nif (p)                    std::cout << 1;\r\nif ((bool)p)              std::cout << 2;\r\nif (static_cast<bool>(p)) std::cout << 3;\r\nif (p == q)               std::cout << 4;\r\nif (p == false)           std::cout << 5;\r\nif (!p)                   std::cout << 6;\r\nif ((bool)p == (bool)q)   std::cout << 7;\r\n<\/pre>\n\n\n\n\n\n\n\n\n\n\n
Condition<\/th>\nWhat’s happening<\/th>\nResult<\/th>\n<\/tr>\n
if (p)<\/tt><\/td>\nTests p<\/tt> against nullptr<\/tt>.<\/td>\nprints 1<\/tt><\/td>\n<\/tr>\n
if ((bool)p)<\/tt><\/td>\nTests p<\/tt> against nullptr<\/tt>.<\/td>\nprints 2<\/tt><\/td>\n<\/tr>\n
if (static_cast<bool>(p))<\/tt><\/td>\nTests p<\/tt> against nullptr<\/tt>.<\/td>\nprints 3<\/tt><\/td>\n<\/tr>\n
if (p == q)<\/tt><\/td>\nCompares two objects for identity.<\/td>\ndoes not print<\/td>\n<\/tr>\n
if (p == false)<\/tt><\/td>\nNot allowed (compiler error).<\/td>\n<\/tr>\n
if (!p)<\/tt><\/td>\nTests p<\/tt> against nullptr<\/tt>.<\/td>\ndoes not print<\/td>\n<\/tr>\n
if ((bool)p == (bool)q)<\/tt><\/td>\nTests p<\/tt> and q<\/tt> against nullptr<\/tt>.<\/td>\nprints 7<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Note that the last case prints 7<\/tt> but not for the reason you think. It’s not doing any unboxing at all. It’s just checking whether both variables are non-null.<\/p>\n
IInspectable t = winrt::box_value(true);\r\nif ((bool)p == (bool)t)   std::cout << 8; \/\/ prints 8!\r\n<\/pre>\n
Bonus chatter<\/b>: There is a little quirk in the p == false<\/code> case. My understanding is that prior to C++11, false<\/code> was a legal null pointer constant<\/i>, but the rules in C++11 were tightened so that false<\/code> is no longer a null pointer context.<\/p>\n
Microsoft’s Visual Studio C++ compiler, however, continues to accept false<\/code> as a null pointer constant, even in non-permissive mode. This means that if you’re using Microsoft’s Visual Studio C++ compiler, the fifth row of the table is slightly different:<\/p>\n\n\n\n\n
Condition<\/th>\nWhat’s happening<\/th>\nResult<\/th>\n<\/tr>\n
if (p == false)<\/tt><\/td>\nfalse<\/code> converted to IInspectable{ nullptr }<\/code> 
\nand compared with p<\/code><\/td>\n		does not print<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
 <\/p>\n","protected":false},"excerpt":{"rendered":"
Much less weird than C++\/CX.<\/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-103191","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Much less weird than C++\/CX.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103191","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=103191"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103191\/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=103191"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103191"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103191"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}