{"id":107269,"date":"2022-10-10T07:00:00","date_gmt":"2022-10-10T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=107269"},"modified":"2022-10-10T07:17:19","modified_gmt":"2022-10-10T14:17:19","slug":"20221010-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20221010-00\/?p=107269","title":{"rendered":"On the overloading of the address-of operator &<\/CODE> in smart pointer classes"},"content":{"rendered":"

Many smart pointer classes overload the address-of operator &<\/code> to give you access to the inner raw pointer.<\/p>\n

Unfortunately, they disagree on what happens to the object being managed by the smart pointer before you get its raw address.<\/p>\n\n\n\n\n\n\n\n\n\n
Library<\/th>\nExisting contents<\/th>\n<\/tr>\n
_com_ptr_t<\/td>\nReleased<\/td>\n<\/tr>\n
ATL (CComPtr)<\/td>\nMust be empty
\n(will assert in Debug)<\/td>\n<\/tr>\n
MFC (IPTR)<\/td>\nReleased<\/td>\n<\/tr>\n
WRL (ComPtr)<\/td>\nReleased<\/td>\n<\/tr>\n
wil (com_ptr)<\/td>\nReleased<\/td>\n<\/tr>\n
C++\/WinRT (com_ptr)<\/td>\nN\/A<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

C++\/WinRT avoids the confusion by simply not having an overloaded operator&<\/code> at all! Not having an overloaded operator&<\/code> also makes it easier to take the address of the smart pointer itself. The put()<\/code> method Releases any managed COM pointer and then returns the address of the raw pointer.<\/p>\n

So let’s finish the table. Let’s say that sp<\/code> is the name of a variable of the corresponding smart pointer type.<\/p>\n\n\n\n\n\n\n\n\n\n
Library<\/th>\nRelease<\/th>\nDon’t release<\/th>\nAssumes empty<\/th>\n<\/tr>\n
_com_ptr_t<\/td>\n&sp<\/code><\/td>\n&sp.GetInterfacePtr()<\/code><\/td>\n <\/td>\n<\/tr>\n
ATL (CComPtr)<\/td>\n <\/td>\n&sp.p<\/code><\/td>\n&sp<\/code><\/td>\n<\/tr>\n
MFC (IPTR)<\/td>\n&sp<\/code><\/td>\n <\/td>\n <\/td>\n<\/tr>\n
WRL (ComPtr)<\/td>\n&sp<\/code>
\np.ReleaseAndGetAddressOf()<\/code><\/td>\n		p.GetAddressOf()<\/code><\/td>\n <\/td>\n<\/tr>\n
wil (com_ptr)<\/td>\n&sp<\/code>
\nsp.put()<\/code><\/td>\n		sp.addressof()<\/code><\/td>\n <\/td>\n<\/tr>\n
C++\/WinRT (com_ptr)<\/td>\nsp.put()<\/code><\/td>\n <\/td>\n <\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Bonus chatter<\/b>: The possibility of an overloaded operator&<\/code> is one of those special cases you tend to forget about when writing template library code.\u00b9 In general, it’s not safe to use the &<\/code> operator to get the address of an object of unknown type, because the operator might be overloaded. You have to use std::addressof<\/code>.<\/p>\n

\u00b9 Hey, at least it’s not an overloaded comma operator<\/a>. That thing is nasty<\/i>.<\/p>\n","protected":false},"excerpt":{"rendered":"

Different patterns, which means you’re never sure what you’re going to get.<\/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-107269","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"

Different patterns, which means you’re never sure what you’re going to get.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107269","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=107269"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107269\/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=107269"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=107269"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=107269"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}