{"id":1091,"date":"2018-10-14T07:31:28","date_gmt":"2018-10-14T02:01:28","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=1091"},"modified":"2018-10-14T07:39:55","modified_gmt":"2018-10-14T02:09:55","slug":"c17-stdapply-and-stdinvoke","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2018\/10\/14\/c17-stdapply-and-stdinvoke\/","title":{"rendered":"C++17 – std::apply() and std::invoke()"},"content":{"rendered":"

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.<\/p>\n

Lisp, for example, has apply<\/strong><\/em> and funcall<\/strong><\/em>.<\/p>\n

\"Apply<\/a>
Apply and Funcall in Lisp<\/strong><\/figcaption><\/figure>\n

When using apply<\/strong><\/em>, you can see that the arguments are passed via a separate list<\/strong><\/em> object. With funcall<\/strong><\/em>, however, the arguments are passed directly in line.<\/p>\n

Likewise, Mathematica<\/strong><\/em> has Apply<\/strong><\/em> and Construct<\/strong><\/em>:<\/p>\n

\"Apply<\/a>
Apply and Construct in Mathematica<\/strong><\/figcaption><\/figure>\n

As you can see, the idea is quite similar to Lisp<\/strong><\/em>.<\/p>\n

Even in C++ (prior to C++17<\/strong>), it has always been possible to call functions and methods dynamically through respective pointers. Take a look at the following example.<\/p>\n

\"Pointers<\/a>
Pointers in C++<\/strong><\/figcaption><\/figure>\n

What is nice about C++17<\/strong> is that it provides two methods apply<\/strong><\/em> and invoke<\/strong><\/em> (defined in the std<\/strong><\/em> namespace) that make dynamic method invocation more flexible and uniform. You will see that the behavior is quite similar to Lisp<\/strong><\/em> and Mathematica<\/strong><\/em>.<\/p>\n

To make use of std::apply<\/strong><\/em><\/a>\u00a0and std:: invoke<\/strong><\/em><\/a>\u00a0you have to #include <functional><\/strong><\/em><\/p>\n

Let us define a simple function that takes 3 integers and returns their sum. Let us also define a class (actually a struct) that has a public member function and an instance variable.<\/p>\n

\"A<\/a>
A Simple Function and A Class<\/strong><\/figcaption><\/figure>\n

The apply<\/strong><\/em> method takes two arguments namely, the callable<\/strong><\/em> object that needs to be dynamically invoked, and the data structure that contains the arguments<\/strong><\/em> for the callable object. The data structure could be std::pair<\/strong><\/em><\/a>, std::tuple<\/strong><\/em><\/a> or std::array<\/strong><\/em><\/a>.<\/p>\n

So let us create a tuple and array of 3 integers and see how to use apply<\/strong><\/em><\/p>\n

\"Calling<\/a>
Calling std::apply<\/strong><\/figcaption><\/figure>\n

Notice that we cannot pass the original arguments directly in line when calling apply<\/strong><\/em>.<\/p>\n

This is the difference between apply<\/strong><\/em> and invoke<\/strong><\/em>. invoke<\/strong><\/em> requires the arguments to be passed in line as if we are calling the function directly.<\/p>\n

\"Calling<\/a>
Calling std::invoke<\/strong><\/figcaption><\/figure>\n

apply<\/strong><\/em> comes in quite handy if we are calling a varargs<\/strong><\/em> function such as printf<\/strong><\/em>. We can conveniently bundle all the arguments in a tuple<\/strong><\/em> and pass it to apply<\/strong><\/em>.<\/p>\n

\"Calling<\/a>
Calling a Varargs Function<\/strong><\/figcaption><\/figure>\n

Let us consider the case where instead of calling normal functions, we wish to invoke member<\/strong> functions<\/strong><\/em>. Remember that in order to invoke a member function, we need an object recipient<\/strong><\/em> (unless the function is static).<\/p>\n

Both apply<\/strong><\/em> and invoke<\/strong><\/em> make this process quite simple and uniform.<\/p>\n

\"Calling<\/a>
Calling Member Functions<\/strong><\/figcaption><\/figure>\n

Compare this with the traditional call syntax (shown within comments above). Hope you are able to appreciate the benefits of apply<\/strong><\/em> and invoke<\/strong><\/em>.<\/p>\n

Implementations of these two functions depend on some fancy template coding tricks, but fortunately as users, we do not need to know the details.\u00a0<\/span><\/p>\n

You can download my example source code here<\/strong><\/em><\/a>.<\/p>\n

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

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, however, the arguments are passed […]<\/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":[118,160,161],"class_list":["post-1091","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c17","tag-stdapply","tag-stdinvoke"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-hB","jetpack-related-posts":[{"id":122,"url":"https:\/\/www.rangakrish.com\/index.php\/2016\/01\/06\/lisp-returning-multiple-values-from-a-function\/","url_meta":{"origin":1091,"position":0},"title":"Lisp: Returning Multiple Values From A Function","author":"admin","date":"January 6, 2016","format":false,"excerpt":"One of the interesting features of Lisp is its support for returning multiple values from a function, without bundling the values in a special container. Languages such as Python and Ruby support multi-value return (although there are subtle differences from Lisp). In C++11, we can use std::make_tuple() and std::tie(). To\u2026","rel":"","context":"In "LISP"","block_context":{"text":"LISP","link":"https:\/\/www.rangakrish.com\/index.php\/category\/lisp\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":558,"url":"https:\/\/www.rangakrish.com\/index.php\/2017\/07\/07\/clpython-python-in-common-lisp\/","url_meta":{"origin":1091,"position":1},"title":"CLPython – Python in Common Lisp","author":"admin","date":"July 7, 2017","format":false,"excerpt":"My work in the area of NLP requires\u00a0me to work with several frameworks across multiple languages such as Java, Python and Lisp. Sometime ago I got a chance to experiment with CLPython, an open-source implementation of Python in Common Lisp. 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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":1278,"url":"https:\/\/www.rangakrish.com\/index.php\/2018\/12\/02\/book-review-functional-programming-in-c\/","url_meta":{"origin":1091,"position":4},"title":"Book Review: Functional Programming in C++","author":"admin","date":"December 2, 2018","format":false,"excerpt":"Title: Functional Programming in C++: How to improve your C++ programs using functional techniques Author: Ivan Cukic Publisher: Manning Publications Co. 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