{"id":1946,"date":"2020-03-28T18:26:53","date_gmt":"2020-03-28T12:56:53","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=1946"},"modified":"2020-03-28T18:26:53","modified_gmt":"2020-03-28T12:56:53","slug":"stdis_empty","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/28\/stdis_empty\/","title":{"rendered":"std::is_empty"},"content":{"rendered":"

In the previous post<\/strong><\/em><\/a>, we looked at the std::is_destructible<T><\/strong><\/em> type trait. Today, let us try to understand another type trait std::is_empty<T>.<\/strong><\/em><\/a><\/p>\n

As per the specification, is_empty<T>::value<\/strong><\/em> will return true<\/strong><\/em> in the following cases:<\/p>\n

– The class\/struct has no non-static data member<\/span><\/p>\n

– The class\/struct does not define a virtual function<\/span><\/p>\n

– The class\/struct does not derive from a virtual base class<\/span><\/p>\n

– The class\/struct does not derive from non-empty base classes<\/span><\/p>\n

Let us understand this concept by looking at some examples.<\/p>\n

Here is our first example:<\/p>\n

\"Example1\"<\/a>
Example1<\/strong><\/figcaption><\/figure>\n

For convenience, we will use std::is_empty_v<T><\/strong><\/em> instead of is_empty<T>::value<\/strong><\/em>. Also, just to make it a bit more interesting, I am printing the “<\/strong>sizeof”<\/strong><\/em>\u00a0each class.\u00a0<\/span><\/p>\n

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

\"Example1<\/a>
Example1 Output<\/strong><\/figcaption><\/figure>\n

The output is along the expected lines. You can see that even if a class is empty<\/strong><\/em>, its size is always greater than zero<\/strong><\/em> (the actual value is implementation dependent).\u00a0 <\/span>The reason why class E<\/strong><\/em>\u00a0is not empty<\/strong><\/em> is because it has an instance variable (although the class A<\/strong><\/em>\u00a0is empty).<\/p>\n

Let us now look at some examples involving inheritance.<\/p>\n

\"Inheritance<\/a>
Inheritance Examples<\/strong><\/figcaption><\/figure>\n

Here is the corresponding output:<\/p>\n

\"Example2<\/a>
Example2 Output<\/strong><\/figcaption><\/figure>\n

The behaviors of classes A, B, C<\/strong><\/em>, and D<\/strong><\/em> are as expected. However, I am surprised by the output corresponding to class E<\/strong><\/em>. I think this should be true<\/strong><\/em> since, as per the spec, deriving non-virtually from empty<\/strong><\/em> classes makes the class empty<\/strong><\/em>. I believe this is a bug in Microsoft C++ compiler.<\/p>\n

Here are some more examples involving inheritance:<\/p>\n

\"Example3\"<\/a>
Example: Virtual Derivation and Virtual Functions<\/strong><\/figcaption><\/figure>\n

These involve virtual<\/strong> functions<\/strong><\/em> and virtual<\/strong> derivation<\/strong><\/em>. What do we expect in these cases?<\/p>\n

\"Example3<\/a>
Example3 Output<\/strong><\/figcaption><\/figure>\n

Again, there is no surprise here. If a class has a virtual function and\/or derives virtually from another class, it is not<\/strong><\/em> an empty<\/strong><\/em> class as per the spec.<\/p>\n

Before concluding, let us consider the case of a union<\/strong><\/em>\u00a0and a class with bit field<\/strong><\/em>:<\/p>\n

\"Example4:<\/a>
Example4: Union and Bit Field<\/strong><\/figcaption><\/figure>\n

This is the output generated:<\/p>\n

\"Example4<\/a>
Example4 Output<\/strong><\/figcaption><\/figure>\n

As expected, unions<\/strong><\/em> are non-empty. And, a class with a bit field of size zero<\/strong><\/em> is considered empty<\/strong><\/em>.\u00a0<\/span><\/p>\n

I hope the above examples provide some clarity on the behavior of std::is_empty<T><\/strong><\/em> type trait.\u00a0<\/span><\/p>\n

For the examples, I used Microsoft Visual Studio 2019 (ver 16.4.5)<\/strong><\/em>. You can download the examples from here<\/strong><\/em><\/a>.<\/p>\n

Have a nice weekend!\u00a0<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

In the previous post, we looked at the std::is_destructible<T> type trait. Today, let us try to understand another type trait std::is_empty<T>. As per the specification, is_empty<T>::value will return true in the following cases: – The class\/struct has no non-static data member – The class\/struct does not define a virtual function – The class\/struct does not […]<\/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":[67,234,235],"class_list":["post-1946","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c","tag-stdis_empty","tag-type-trait"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-vo","jetpack-related-posts":[{"id":2024,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/06\/21\/stdis_standard_layout\/","url_meta":{"origin":1946,"position":0},"title":"std::is_standard_layout","author":"admin","date":"June 21, 2020","format":false,"excerpt":"In our last post, we learnt about the type trait std::is_trivial. Today, let us go through another type trait that is quite similar. The expression\u00a0is_standard_layout::value returns true if the layout of objects of type T is compiler independent, and hence is of standard format. Else, it returns false. This is\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"The Difference","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/06\/Example4.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/06\/Example4.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/06\/Example4.jpg?resize=525%2C300&ssl=1 1.5x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/06\/Example4.jpg?resize=700%2C400&ssl=1 2x"},"classes":[]},{"id":2011,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/06\/06\/stdis_trivial\/","url_meta":{"origin":1946,"position":1},"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":1946,"position":2},"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":2280,"url":"https:\/\/www.rangakrish.com\/index.php\/2021\/01\/17\/c20-nodiscard-attribute\/","url_meta":{"origin":1946,"position":3},"title":"C++20 [[nodiscard]] Attribute","author":"admin","date":"January 17, 2021","format":false,"excerpt":"[[nodiscard]]\u00a0attribute \u201cencourages\u201d the compiler to issue a warning when the return value from a function is ignored. It was introduced in C++ 17 and enhanced in C++ 20 to include a string literal that can be used as an explanation of the warning. Let us look at different cases one\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Case-1: Enumeration","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2021\/01\/Example1-src-300x261.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2021\/01\/Example1-src-300x261.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2021\/01\/Example1-src-300x261.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":1946,"position":4},"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":1946,"position":5},"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":[]}],"_links":{"self":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/posts\/1946","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/comments?post=1946"}],"version-history":[{"count":0,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/posts\/1946\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/media?parent=1946"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/categories?post=1946"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/tags?post=1946"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}