{"id":2024,"date":"2020-06-21T08:36:01","date_gmt":"2020-06-21T03:06:01","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=2024"},"modified":"2020-06-21T08:36:01","modified_gmt":"2020-06-21T03:06:01","slug":"stdis_standard_layout","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2020\/06\/21\/stdis_standard_layout\/","title":{"rendered":"std::is_standard_layout<>"},"content":{"rendered":"

In our last post<\/strong><\/em><\/a>, we learnt about the type trait std::is_trivial<T><\/strong><\/em>. Today, let us go through another type trait that is quite similar.<\/p>\n

The expression\u00a0is_standard_layout<T>::value<\/strong><\/em> returns true<\/strong><\/em> if the layout of objects of type T<\/strong><\/em> is compiler independent, and hence is of standard<\/strong><\/em> format. Else, it returns false<\/strong><\/em>. This is important if we create objects in C++<\/strong><\/em> and pass them to another language environment that is compatible with plain C<\/strong><\/em> structures. You might be aware that if we define a class with virtual functions, or one that derives from multiple base classes, the memory occupied by an object of that type is not guaranteed to have a standard<\/strong><\/em> structure.\u00a0 <\/span>For example, it might be impossible to copy of objects such a type using the C function memcpy<\/strong><\/em>, as the elements might not be contiguous.<\/p>\n

Let us look at various examples that use is_standard_layout<T>\u00a0<\/strong><\/em>and finally conclude by identifying the difference between this and is_trivial<T><\/strong><\/em>.<\/p>\n

Example-1: Primitive Types<\/h3>\n

This trait can be applied to primitive types as well. Here is the example program:<\/p>\n

\"Working<\/a>
Working with Primitive Types<\/strong><\/figcaption><\/figure>\n

And here is the output from the program:<\/p>\n

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

Example-2: Simple Classes<\/h3>\n

Take a look at the structs A, B, C, <\/strong><\/em>and D\u00a0<\/strong><\/em>below.<\/p>\n

\"Basic<\/a>
Basic Classes<\/strong><\/figcaption><\/figure>\n

The struct A<\/strong><\/em> is a typical POD (Plain Old Data)<\/strong><\/em>, containing only data members. Struct B<\/strong><\/em> includes a static member, but otherwise it is quite similar to A<\/strong><\/em>.\u00a0<\/span><\/p>\n

Struct C<\/strong><\/em> contains a no-arg constructor, a copy constructor, destructor, and a member function. Struct D<\/strong><\/em> is different in that it enforces access control.<\/p>\n

Let us see the output of the program:<\/p>\n

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

Notice that, even though D<\/strong><\/em>‘s members are not public, it still has a standard layout.<\/p>\n

Example-3: Special Cases<\/h3>\n

Let us extend the earlier example by including two special cases:<\/p>\n

1) Class has reference member variable<\/p>\n

2) Class defines more than one access level<\/p>\n

Here is the example:<\/p>\n

\"Special<\/a>
Special Cases<\/strong><\/figcaption><\/figure>\n

Here is the program’s output:<\/p>\n

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

As you can see, both the types have non-standard layout.<\/p>\n

Example-4: Class Hierarchy<\/h3>\n

This section covers classes that have virtual functions or derive from other classes. Go through the example:<\/p>\n

\"Class<\/a>
Class Hierarchy<\/strong><\/figcaption><\/figure>\n

Struct G<\/strong><\/em> has a virtual destructor. We know that objects of such a class need additional storage for keeping track of the virtual table, something that is not the case with a simple struct acting like a POD<\/strong><\/em>.\u00a0<\/span><\/p>\n

Struct I<\/strong><\/em> uses private derivation while inheriting from C<\/strong><\/em>.<\/p>\n

Struct J<\/strong><\/em> differs from H<\/strong><\/em> in that it defines an additional data member in its scope. Struct K<\/strong><\/em> inherits from two different base classes.<\/p>\n

Can you guess the output? Here it is:<\/p>\n

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

Interesting. Even though I<\/strong><\/em> derives privately<\/strong><\/em> from its base class, it has a standard layout.<\/p>\n

The above examples show that there is substantial similarity between is_trivial<T><\/strong><\/em> and is_standard_layout<T><\/strong><\/em>. What is the difference, if any? Well, struct C <\/strong><\/em>in our example,\u00a0<\/strong><\/em>holds the answer.<\/p>\n

\"The<\/a>
The Difference<\/strong><\/figcaption><\/figure>\n

When we run this program, this is what we get:<\/p>\n

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

The presence of constructors and destructor makes struct C<\/strong><\/em> non-trivial, although it has standard layout.<\/p>\n

That is it for today. Hope you found the discussion useful.\u00a0<\/span><\/p>\n

Download the sources from here<\/strong><\/em><\/a>.<\/p>\n

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

In our last post, we learnt about the type trait std::is_trivial<T>. Today, let us go through another type trait that is quite similar. The expression\u00a0is_standard_layout<T>::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 important if we create objects […]<\/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_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":[67,241,235],"class_list":["post-2024","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c","tag-is_standard_layout","tag-type-trait"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-wE","jetpack-related-posts":[{"id":3184,"url":"https:\/\/www.rangakrish.com\/index.php\/2023\/08\/13\/stdis_scoped_enum\/","url_meta":{"origin":2024,"position":0},"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":1928,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/15\/stdis_destructible\/","url_meta":{"origin":2024,"position":1},"title":"std::is_destructible","author":"admin","date":"March 15, 2020","format":false,"excerpt":"In the last article, I explained the deleted destructor in some detail. Today, I would like to talk about a related construct, a type trait called std::is_destructible. Type traits, defined in the header , are a big help when it comes to implementing template metaprogramming. \u00a0See this\u00a0article for a nice\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Private\/Protected Destructor","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/03\/Example7.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/03\/Example7.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/03\/Example7.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":2024,"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":1946,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/28\/stdis_empty\/","url_meta":{"origin":2024,"position":3},"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. As per the specification, is_empty::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\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"Example4: Union and Bit Field","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/03\/Example4-1.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/03\/Example4-1.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2020\/03\/Example4-1.jpg?resize=525%2C300&ssl=1 1.5x"},"classes":[]},{"id":1973,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/04\/26\/stdis_base_of\/","url_meta":{"origin":2024,"position":4},"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":2011,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/06\/06\/stdis_trivial\/","url_meta":{"origin":2024,"position":5},"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":[]}],"_links":{"self":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/posts\/2024","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=2024"}],"version-history":[{"count":0,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/posts\/2024\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/media?parent=2024"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/categories?post=2024"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/tags?post=2024"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}