{"id":2975,"date":"2022-12-25T11:28:03","date_gmt":"2022-12-25T05:58:03","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=2975"},"modified":"2022-12-25T11:28:03","modified_gmt":"2022-12-25T05:58:03","slug":"stdtie","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2022\/12\/25\/stdtie\/","title":{"rendered":"Std::tie"},"content":{"rendered":"

std::tuple<\/strong><\/em><\/a> is a widely used abstraction in C++<\/strong><\/em> and has been around since C++11<\/strong><\/em>. It is a generalization of std::pair<\/strong><\/em><\/a>.<\/p>\n

std::tie<\/strong><\/em><\/a> is convenient when we want to create a tuple of lvalue references<\/em><\/strong> to existing variables. It is a function template commonly used to unpack a tuple into individual objects.<\/p>\n

\"std::tie<\/a>
std::tie Example<\/strong><\/figcaption><\/figure>\n

In the above example, we first define a tuple object comprising an integer, a string and a double. We then use std::tie<\/strong><\/em> to assign the respective values of the tuple to the individual variables \u201cI\u201d<\/strong><\/em> and \u201cd\u201d<\/strong><\/em>. This works because, as mentioned earlier, std::tie<\/strong><\/em> returns a tuple with lvalue references to its parameters. Notice the use of std::ignore<\/strong><\/em><\/a> to skip an element of the RHS tuple.<\/p>\n

The main difference between structured binding<\/strong><\/em><\/a> and std::tie<\/strong><\/em> is that the former both creates <\/strong><\/em>and initializes<\/strong><\/em> at the same time, whereas the latter only assigns<\/strong><\/em> values to already existing variables. The following example shows this.<\/p>\n

\"Compared<\/a>
Compared with Structured Binding<\/strong><\/figcaption><\/figure>\n

The last point to keep in mind about std::tie<\/strong><\/em> is that it requires modifiable lvalue<\/strong><\/em> parameters when it is used to \u201cunpack\u201d<\/strong><\/em> a tuple. Take a look this code:<\/p>\n

\"Parameter<\/a>
Parameter Requirements<\/strong><\/figcaption><\/figure>\n

The reason why line 35<\/strong><\/em> won\u2019t compile is because std::tie<\/strong><\/em> is used on the LHS of assignment to unpack a tuple, but its parameters are literals<\/strong><\/em>. In line 39<\/strong><\/em>, even though the parameter to std::tie<\/strong><\/em> is an lvalue, it is a const<\/strong><\/em> object and hence cannot be modified by the unpacking assignment.<\/p>\n

Line 43<\/strong><\/em> is OK because here std::tie<\/strong><\/em> is used on the RHS and there is no modification to its argument. Line 48<\/strong><\/em> shows that we can use the std::get<\/strong><\/em><\/a> function to access an individual element of the tuple returned by std::tie<\/strong> <\/em>and update it with a new value.<\/p>\n

You can download the sample code here<\/strong><\/em><\/a>.<\/p>\n

Merry Christmas and Happy New Year 2023!<\/strong><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

std::tuple is a widely used abstraction in C++ and has been around since C++11. It is a generalization of std::pair. std::tie is convenient when we want to create a tuple of lvalue references to existing variables. It is a function template commonly used to unpack a tuple into individual objects. In the above example, we […]<\/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":[67,345,346,344],"class_list":["post-2975","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c","tag-stdignore","tag-stdtie","tag-stdtuple"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/s9OLnF-stdtie","jetpack-related-posts":[{"id":1091,"url":"https:\/\/www.rangakrish.com\/index.php\/2018\/10\/14\/c17-stdapply-and-stdinvoke\/","url_meta":{"origin":2975,"position":0},"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":700,"url":"https:\/\/www.rangakrish.com\/index.php\/2017\/11\/21\/c17-stdoptional\/","url_meta":{"origin":2975,"position":1},"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. This failure can be represented as an exception, or as a return value that unamibiguously denotes failure (for example, -1). Throwing an exception is a strong form of failure and\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\/Example1-3.png?resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2017\/11\/Example1-3.png?resize=350%2C200 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2017\/11\/Example1-3.png?resize=525%2C300 1.5x"},"classes":[]},{"id":2889,"url":"https:\/\/www.rangakrish.com\/index.php\/2022\/08\/09\/stdarray-vs-stdvector\/","url_meta":{"origin":2975,"position":2},"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":1928,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/15\/stdis_destructible\/","url_meta":{"origin":2975,"position":3},"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":1973,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/04\/26\/stdis_base_of\/","url_meta":{"origin":2975,"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<>. 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