{"id":1981,"date":"2020-05-09T20:29:01","date_gmt":"2020-05-09T14:59:01","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=1981"},"modified":"2020-05-09T20:32:58","modified_gmt":"2020-05-09T15:02:58","slug":"dynamic_cast-vs-stdis_base_of","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2020\/05\/09\/dynamic_cast-vs-stdis_base_of\/","title":{"rendered":"dynamic_cast<> vs. std::is_base_of<>"},"content":{"rendered":"

In the last article<\/strong><\/em><\/a>, we looked at the std:is_base_of<T1, T2><\/strong><\/em> type trait. One question that a reader asked since that article appeared is “How does is_base_of<><\/span> differ from the dynamic_cast<><\/span> operator?”<\/strong><\/em><\/p>\n

Good question! In today’s post let me try to address the key differences between the two.<\/p>\n

I am not going to cover the dynamic_cast<><\/strong><\/em> operator in detail here. I urge you to go through the primary reference<\/strong><\/em><\/a>.<\/p>\n

One of the common uses of dynamic_cast<><\/strong><\/em> operator is to “safely”<\/strong><\/em> cast a pointer to base class type to a pointer to derived class type, provided it is valid. This might require the use of run-time type information (RTTI<\/strong><\/em>).\u00a0<\/span><\/p>\n

Contrast this with is_base_of<><\/strong><\/em>. This works with type names and not object references. To get a better idea, feel free to go through the earlier post<\/strong><\/em><\/a>.<\/p>\n

Let us look at 4 different cases comparing dynamic_cast<><\/strong><\/em> and is_base_of<><\/strong><\/em>.<\/p>\n

<\/h3>\n

Case -1: Working with Classes<\/h3>\n

dynamic_cast<><\/strong><\/em> can only work with class types, not primitive types. That is not a restriction in the case of is_base_of<><\/strong>. See the following example:<\/p>\n

\"Case-1\"<\/a>
Case-1<\/strong><\/figcaption><\/figure>\n

You can see that dynamic_cast<><\/strong><\/em> will not accept a non-class argument (line 15).\u00a0<\/span><\/p>\n

Case-2: Non-polymorphic Class Hierrachy<\/h3>\n

dynamic_cast<><\/strong><\/em> expects the argument to refer to a polymorphic<\/strong><\/em> class class, i.e., one that has at least one virtual function. The example below illustrates this point.<\/p>\n

\"Case-2\"<\/a>
Case-2<\/strong><\/figcaption><\/figure>\n

is_base_of<><\/strong><\/em> does not have this limitation.<\/p>\n

Case-3: Polymorphic Class Hierarchy<\/h3>\n

This is the common use case for dynamic_cast<><\/strong><\/em> operator. Go through the following example.<\/p>\n

\"Case-3\"<\/a>
Case-3: Polymorphic Hierarchy<\/strong><\/figcaption><\/figure>\n

This shows how dynamic_cast<><\/strong><\/em> uses RTTI<\/strong><\/em> to convert a pointer from base class to derived class pointer. In the case of invalid conversion, it returns NULL.<\/p>\n

The type trait\u00a0is_base_of<><\/strong><\/em>\u00a0works with types and not actual objects.<\/p>\n

Case-4: Class Hierarchy with Twists!<\/h3>\n

As the final example, let us consider a class hierarchy that uses protected<\/strong><\/em> derivation and also multiple derivation<\/strong><\/em> of a common base class.\u00a0<\/span><\/p>\n

\"Case-4:<\/a>
Case-4: Complex Hierarchy<\/strong><\/figcaption><\/figure>\n

Notice that class R<\/strong><\/em> is derived from Q<\/strong><\/em> using “protected”<\/strong><\/em> mode. Due to this, methods outside of R<\/strong><\/em> (global functions or methods of any class not derived from R<\/strong><\/em>) cannot access any member of Q<\/strong><\/em> via R<\/strong><\/em> instance. Interestingly, because of the non-public derivation,\u00a0 <\/span>dynamic_cast<><\/strong><\/em> fails in line 18 by returning NULL.<\/p>\n

Likewise, class S<\/strong><\/em> derives from P<\/strong><\/em> twice, via Q<\/strong><\/em> and R<\/strong><\/em>. This results in S<\/strong><\/em> object having two instances of P<\/strong><\/em> inside it. That is why, in lines 39-40, we first convert to void *<\/strong><\/em> then convert to P *<\/strong><\/em>. Of course, we get a run-time exception when we convert back to derived class through dynamic_cast<><\/strong><\/em>.<\/p>\n

It is possible to consider some minor variations of the above examples, but I think you now have a good understanding of the difference between dynamic_cast<><\/strong><\/em> and is_base_of<><\/strong><\/em>.<\/p>\n

The example programs can be downloaded from here<\/strong><\/em><\/a>. I used\u00a0Visual Studio 2019 (64 bit), Version 16.5.4 <\/strong><\/em>to test these examples.<\/p>\n

Have a nice weekend!<\/p>\n","protected":false},"excerpt":{"rendered":"

In the last article, we looked at the std:is_base_of<T1, T2> type trait. One question that a reader asked since that article appeared is “How does is_base_of<> differ from the dynamic_cast<> operator?” Good question! In today’s post let me try to address the key differences between the two. I am not going to cover the dynamic_cast<> […]<\/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_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_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},"jetpack_post_was_ever_published":false},"categories":[49,17],"tags":[67,238,237,235],"class_list":["post-1981","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c","tag-dynamic_cast","tag-is_base_of","tag-type-trait"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-vX","jetpack-related-posts":[{"id":2011,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/06\/06\/stdis_trivial\/","url_meta":{"origin":1981,"position":0},"title":"std::is_trivial","author":"admin","date":"June 6, 2020","format":false,"excerpt":"Today let us try to understand the type trait std::is_trivial. This trait checks if the given type is a trivial type. For a precise definition of what trivial means, please visit the official page. As usual, we will go through three cases: - Primitive types - Classes without inheritance -\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Program Output","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/06\/output2.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":3184,"url":"https:\/\/www.rangakrish.com\/index.php\/2023\/08\/13\/stdis_scoped_enum\/","url_meta":{"origin":1981,"position":1},"title":"std::is_scoped_enum","author":"admin","date":"August 13, 2023","format":false,"excerpt":"The type trait \"std::is_scoped_enum::value\" was introduced in C++23 to check whether the type \u201cT\u201d is a scoped enum type. Another way to use this is std::is_scoped_enum_v. Before getting into this trait in detail, let us briefly recap the differences between unscoped and scoped enums. Unscoped Enums Unscoped enums are the\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Unscoped Enums","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2023\/08\/unscoped-300x110.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2023\/08\/unscoped-300x110.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2023\/08\/unscoped-300x110.jpg?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":1960,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/04\/12\/stdcommon_type-type-trait\/","url_meta":{"origin":1981,"position":2},"title":"std::common_type Type Trait","author":"admin","date":"April 12, 2020","format":false,"excerpt":"In today's post, I would like to go over the type trait std::common_type<>. This trait was introduced in C++11. As per the specification, std::common_type::type refers to a type Tx in the given list, which the rest of the types in the list can be implicitly converted to. This\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Program Output","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/04\/Fig5.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":1973,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/04\/26\/stdis_base_of\/","url_meta":{"origin":1981,"position":3},"title":"std::is_base_of","author":"admin","date":"April 26, 2020","format":false,"excerpt":"In our on-going series on C++ Type Traits, today's topic is about the trait std::is_base_of<>. For the official description, see this. std::is_base_of::value takes two arguments, both classes (or structs), and returns true if A is a base class of B and false otherwise. Trivially, std::::value is true. Let\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Output","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/04\/Output.jpg?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]},{"id":1946,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/28\/stdis_empty\/","url_meta":{"origin":1981,"position":4},"title":"std::is_empty","author":"admin","date":"March 28, 2020","format":false,"excerpt":"In the previous post, we looked at the std::is_destructible type trait. Today, let us try to understand another type trait std::is_empty. 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