{"id":3691,"date":"2025-05-04T17:29:36","date_gmt":"2025-05-04T11:59:36","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=3691"},"modified":"2025-05-04T17:29:36","modified_gmt":"2025-05-04T11:59:36","slug":"beyond-ownership-understanding-the-role-of-stdweak_ptr-in-modern-c","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2025\/05\/04\/beyond-ownership-understanding-the-role-of-stdweak_ptr-in-modern-c\/","title":{"rendered":"Beyond Ownership: Understanding the Role of std::weak_ptr in Modern C++"},"content":{"rendered":"

Smart Pointers<\/strong><\/em> in C++<\/strong><\/em> are elegant abstractions for managing dynamic memory safely, avoiding dangling pointers and preventing leaks. While std::unique_ptr<\/strong><\/em> and std::shared_ptr<\/strong><\/em> are well understood and widely used, std::weak_ptr<\/strong><\/em> often demands a deeper dive to use correctly. In this article, I will attempt to explain what it is and where it is useful.<\/p>\n

Some basics first. unique_ptr<\/strong><\/em> is useful when we need exclusive ownership of a resource. It cannot be copied, only moved. On the otherhand, we use shared_ptr<\/strong><\/em> when we need shared ownership. Internally it uses reference counting to keep track of shared references.<\/p>\n

weak_ptr<\/strong><\/em> is different from the above two. At its core, it is a smart pointer that holds a non-owning (\u201cweak\u201d)<\/strong><\/em> reference to an object managed by shared_ptr<\/strong><\/em>. The key point to note is that it doesn\u2019t \u201cown\u201d<\/strong><\/em> anything, but can observe whether the object still exists.<\/p>\n

Let us start with a simple example. Assume we have two classes that need to reference each other. See the code below:<\/p>\n

\"Circular<\/a>
Circular Reference Example<\/strong><\/figcaption><\/figure>\n

In the above case, both are using shared_ptr<\/strong><\/em> to reference each other. The problem with this is that it results in a \u201ccyclic reference\u201d<\/strong><\/em> and hence the objects will not be destroyed! This is confirmed by the program output:<\/p>\n

\"Program<\/a>
Program Output<\/strong><\/figcaption><\/figure>\n

You can see that the destructors<\/strong><\/em> are not called.<\/p>\n

The correct solution is to use a weak_ptr<\/strong><\/em> in one of the classes, say \u201cStructB\u201d<\/strong><\/em>. The changed version is here:<\/p>\n

\"Using<\/a>
Using weak_ptr Instead<\/strong><\/figcaption><\/figure>\n

The first thing to note is that a weak_ptr<\/strong><\/em> is created from a shared_ptr<\/strong><\/em>, not from a raw pointer. In this case, since the weak_ptr<\/strong><\/em> doesn\u2019t \u201cown\u201d<\/strong><\/em> the reference, both the objects are correctly destroyed. See the output:<\/p>\n

\"Output<\/a>
Output After Correction<\/strong><\/figcaption><\/figure>\n

The following example shows other interesting aspects of weak_ptr<\/strong><\/em>:<\/p>\n

\"Other<\/a>
Other Features of weak_ptr<\/strong><\/figcaption><\/figure>\n

Here are the key points to understand from the above example:<\/p>\n

\n

1) It is OK to initialize a week_ptr<\/strong><\/em> with another weak_ptr<\/strong><\/em>, but this doesn\u2019t change the reference count of the managed object.<\/p>\n

2) The \u201cuse_count()\u201d<\/strong><\/em> function returns the reference count of the shared_ptr<\/strong><\/em> referenced by this weak_ptr<\/strong><\/em>.\u00a0<\/span><\/p>\n

3) The \u201clock()\u201d<\/strong><\/em> method is required to access the managed object. This will be empty if the shared_ptr<\/strong><\/em> has been destructed.<\/p>\n

4) The \u201cexpired\u201d<\/strong><\/em> method returns \u201ctrue\u201d<\/strong><\/em> if the managed object has been deleted, else it returns \u201cfalse\u201d<\/strong><\/em>.<\/p>\n<\/blockquote>\n

Let us conclude this article with an example of the popular \u201cObserver\u201d<\/strong><\/em> pattern:<\/p>\n

\"Observer<\/a>
Observer Patte<\/strong>rn<\/figcaption><\/figure>\n

The EventDispatcher<\/strong><\/em> class handles registering and notifying multiple listerners. To keep things simple, the \u201clistener\u201d<\/strong><\/em> is implemented as a \u201cfunction\u201d<\/strong> <\/em>object, but it could be a more feature-rich class. Note how the \u201cnotify()\u201d<\/strong><\/em> method removes deleted listeners from the queue.<\/p>\n

When we run the program, this is the output:<\/p>\n

\"Observer<\/a>
Observer Pattern Output<\/strong><\/figcaption><\/figure>\n

In summary, here are the key properties of weak_ptr<\/strong><\/em>:<\/p>\n

1. Non-ownership:<\/strong> It doesn’t own the resource it references and hence doesn’t affect its lifetime<\/p>\n

2. Observation capability:<\/strong> It can observe whether the object still exists<\/p>\n

3. Convertibility:<\/strong> It can be converted to a shared_ptr to access the object (if it still exists)\u00a0<\/span><\/p>\n

4. Safety:<\/strong> It helps in preventing dangling pointer issues by providing safe access mechanisms<\/p><\/blockquote>\n

Hope you found the article useful. You can download the source from here<\/strong><\/em><\/a>. The code was tested in Visual Studio Professional (64 bit) ver 17.13.5<\/strong><\/em>.\u00a0<\/span><\/p>\n

Have a great week ahead!<\/p>\n","protected":false},"excerpt":{"rendered":"

Smart Pointers in C++ are elegant abstractions for managing dynamic memory safely, avoiding dangling pointers and preventing leaks. While std::unique_ptr and std::shared_ptr are well understood and widely used, std::weak_ptr often demands a deeper dive to use correctly. In this article, I will attempt to explain what it is and where it is useful. Some basics […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[49,17],"tags":[371,423,421,422,424],"class_list":["post-3691","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c20","tag-shared_ptr","tag-smart-pointer","tag-unique_ptr","tag-weak_ptr"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-Xx","jetpack-related-posts":[{"id":2889,"url":"https:\/\/www.rangakrish.com\/index.php\/2022\/08\/09\/stdarray-vs-stdvector\/","url_meta":{"origin":3691,"position":0},"title":"std::array vs. std::vector","author":"admin","date":"August 9, 2022","format":false,"excerpt":"One of the common questions when starting to study the container abstractions in C++20 is \"Should I use std::array\u00a0or std::vector?\". The correct answer is \"use what is best suited for the current situation\". That is not good enough, obviously. In this short article, I will try to answer this question\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"std::array Example","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2022\/08\/code1-300x150.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2022\/08\/code1-300x150.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2022\/08\/code1-300x150.jpg?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":2876,"url":"https:\/\/www.rangakrish.com\/index.php\/2022\/07\/27\/understanding-stdspan\/","url_meta":{"origin":3691,"position":1},"title":"Understanding std::span","author":"admin","date":"July 27, 2022","format":false,"excerpt":"Introduced in C++20, std::span\u00a0is a light-weight abstraction that provides a convenient view\u00a0into a collection of contiguous elements. Note that it is not enough for the elements to be logically contiguous, but they must be contiguous in memory too. Thus, span will work with C-style arrays, C++ vectors and arrays. It\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Example-1: span","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2022\/07\/code1-285x300.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":1091,"url":"https:\/\/www.rangakrish.com\/index.php\/2018\/10\/14\/c17-stdapply-and-stdinvoke\/","url_meta":{"origin":3691,"position":2},"title":"C++17 – std::apply() and std::invoke()","author":"admin","date":"October 14, 2018","format":false,"excerpt":"Calling a function (or function object) dynamically, through a pointer known at runtime, is a common programming scenario. Almost all languages support this use case. Lisp, for example, has apply and funcall. When using apply, you can see that the arguments are passed via a separate list object. With funcall,\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Calling Member Functions","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2018\/10\/Code5.png?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2018\/10\/Code5.png?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2018\/10\/Code5.png?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2018\/10\/Code5.png?resize=700%2C400&ssl=1 2x"},"classes":[]},{"id":1905,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/04\/deleted-destructor-in-c\/","url_meta":{"origin":3691,"position":3},"title":"Deleted Destructor in C++","author":"admin","date":"March 4, 2020","format":false,"excerpt":"Since C++ 11, we can use the keyword delete to declare functions as deleted and thus prohibit the use of these functions in the code. See this article\u00a0for a brief overview. In particular, the destructor of a class\/struct\/union can be declared as deleted. In today\u2019s article, I am going to\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Example-8","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/03\/Example-8.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":683,"url":"https:\/\/www.rangakrish.com\/index.php\/2017\/11\/05\/c17-stdvariant\/","url_meta":{"origin":3691,"position":4},"title":"C++17: std::variant","author":"admin","date":"November 5, 2017","format":false,"excerpt":"C++17 introduces a new type-safe union in the form of std::variant. At any time, it can store a single value from one of many types. We need to include \u00a0to use this feature. Let us look at a simple example to get started: First, we define a variant object v1\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Example1","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2017\/11\/Example1.png?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2017\/11\/Example1.png?resize=350%2C200 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2017\/11\/Example1.png?resize=525%2C300 1.5x"},"classes":[]},{"id":700,"url":"https:\/\/www.rangakrish.com\/index.php\/2017\/11\/21\/c17-stdoptional\/","url_meta":{"origin":3691,"position":5},"title":"C++17: std::optional","author":"admin","date":"November 21, 2017","format":false,"excerpt":"Suppose we want to write a function that returns a value, but with the possibility that\u00a0 the computation might fail. 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