{"id":111490,"date":"2025-08-20T07:00:00","date_gmt":"2025-08-20T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=111490"},"modified":"2025-08-15T16:11:52","modified_gmt":"2025-08-15T23:11:52","slug":"20250820-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20250820-00\/?p=111490","title":{"rendered":"Thoughts on creating a tracking pointer class, part 8: Tracking const objects"},"content":{"rendered":"

Last time, we added the ability to create non-modifying tracking pointers<\/a>, but it required that you start with a non-const trackable object. But what if you want to create a non-modifying tracking pointer to an object to which you have only a const reference?<\/p>\n

It took me a few tries before I hit upon the simple solution.\u00b9<\/p>\n

We just need to make the trackable object’s m_trackers<\/code> mutable, and then patch up the consequences.<\/p>\n

template<typename T>\r\nstruct trackable_object\r\n{\r\n    \u27e6 ... \u27e7\r\n\r\n    tracking_ptr<T> track() noexcept {\r\n        return { owner() };\r\n    }\r\n\r\n    tracking_ptr<const T> track() const noexcept {<\/span>\r\n        return { owner() };                       <\/span>\r\n    }                                             <\/span>\r\n\r\n    tracking_ptr<const T> ctrack() const<\/span> noexcept {\r\n        return { owner() };\r\n    }\r\n\r\nprivate:\r\n    friend struct tracking_ptr_base<T>;\r\n\r\n    T* owner() const<\/span> noexcept {\r\n        return const_cast<T*><\/span>(static_cast<const<\/span> T*>(this));\r\n    }\r\n\r\n    tracking_node mutable<\/span> m_trackers;\r\n\r\n    \u27e6 ... \u27e7\r\n};\r\n<\/pre>\n
After making the m_trackers<\/code> mutable, we can make the ctrack()<\/code> method const<\/code>. We may as well also add a track() const<\/code> that produces a read-only tracker from a const object. This is analogous to how C++ standard library container begin()<\/code> and and end()<\/code> methods produce read-only iterators if obtained from const containers.<\/p>\n
Casting away const when creating and updating the tracking_ptr<const T><\/code> is okay because the tracking_ptr<const T><\/code>‘s get()<\/code> method will reapply const before giving it to the client, and the only other use of the pointer is to access the m_trackers<\/code>, which is now mutable.<\/p>\n
There’s still a problem: Although you can convert a T*<\/code> to a const T*<\/code>, a std::unique_ptr<T><\/code> to a std::unique_ptr<const T><\/code>, and a std::shared_ptr<T><\/code> to a std::shared_ptr<const T><\/code>, you cannot convert a tracking_ptr<T><\/code> to a tracking_ptr<const T><\/code>. We’ll fix that next time.<\/p>\n
Bonus chatter<\/b>: I considered whether I should also support tracking_ptr<volatile T><\/code> and tracking_ptr<const volatile T><\/code>. It wouldn’t be hard, but it would be extra typing. I decided not to, on the grounds that the C++ standard library typically doesn’t bother with volatile<\/code> either: Standard containers have begin()<\/code> and cbegin()<\/code> but not vbegin()<\/code> or cvbegin()<\/code>.<\/p>\n
\u00b9 As Blaise Pascal is reported to have written, “If I had more time, I would have written a shorter letter.” Sometimes it takes a lot of work to come up with what ends up looking easy.<\/p>\n","protected":false},"excerpt":{"rendered":"
Making tracking pointers to const objects.<\/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-111490","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Making tracking pointers to const objects.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/111490","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=111490"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/111490\/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=111490"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=111490"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=111490"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}