{"id":3106,"date":"2023-05-13T10:42:25","date_gmt":"2023-05-13T05:12:25","guid":{"rendered":"https:\/\/www.rangakrish.com\/?p=3106"},"modified":"2023-05-13T10:42:25","modified_gmt":"2023-05-13T05:12:25","slug":"stdexpected","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2023\/05\/13\/stdexpected\/","title":{"rendered":"std::expected<>"},"content":{"rendered":"

I had written about std::optional<><\/strong><\/em><\/a> in an earlier article. C++23<\/strong><\/em> introduces std::expected<><\/strong><\/em><\/a> as an interesting extension to std::optional<><\/strong><\/em>. Whereas std::optional<><\/strong><\/em> contains a value or none at all, std::expected<><\/strong><\/em> contains a value<\/strong><\/em> or an error code<\/strong><\/em> associated with it. This gives better control to the caller in terms of handling the outcome of the function call.
\nLet us look at an example to get started:<\/p>\n

\"An<\/a>
An Example<\/strong><\/figcaption><\/figure>\n

In the above example, function \u201cfoo()\u201d<\/strong><\/em> takes an integer and returns either an integer or an error code. If the supplied argument is between 10 and 100 (both included), then it returns a computed value, else it returns an error code of type \u201cErrCode\u201d<\/strong><\/em>.<\/p>\n

Let us see how this function is called inside \u201cmain()\u201d<\/strong><\/em>. The first call (line 25<\/strong><\/em>) passes \u201c-100\u201d<\/strong><\/em>, which is outside the acceptable range. In this case, obviously, we expect the function to return an error code. The \u201cresult\u201d<\/strong><\/em> variable will be \u201ctrue\u201d<\/strong><\/em> if the function returned a computed value, false if an error code was returned. This is checked in lines 26 – 29<\/strong><\/em> and corresponding value printed. Notice the use of \u201cvalue()\u201d<\/strong><\/em> and \u201cerror()\u201d<\/strong><\/em> member function calls to access the computed value and the error code respectively. This is convenient.<\/p>\n

Next, Line 32<\/strong><\/em> passes a valid argument to the \u201cfoo()\u201d<\/strong><\/em> function. In this case, the \u201cresult\u201d<\/strong><\/em> variable will be \u201ctrue\u201d<\/strong><\/em> and hence line 34<\/strong><\/em> will be executed.<\/p>\n

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

As in the case of std::optional<><\/strong><\/em>, there is a \u201cvalue_or()\u201d<\/strong><\/em> member function that returns either the normal value, or in the case of an error return, the user-supplied value. This is shown in line 39<\/strong><\/em>.<\/p>\n

\"Use<\/a>
Use of “value_or”<\/strong><\/figcaption><\/figure>\n

In the case, because the supplied argument is outside the permitted range, an error code will be returned. Hence the \u201cvalue_or()\u201d<\/strong><\/em> function will return \u201c99999\u201d<\/strong><\/em> instead of the error code. This is convenient when we cascade function calls.<\/p>\n

\"Output<\/a>
Output from “value_or()”<\/figcaption><\/figure>\n

We have seen how the member function \u201cvalue()\u201d<\/strong><\/em> allows us to retrieve the normal return value from the function. What happens if we call this function when the function returns an error code? This is a fatal error and hence the program will be aborted. See line 44<\/strong><\/em> below:<\/p>\n

\"Incorrect<\/a>
Incorrect Use of “value()”<\/strong><\/figcaption><\/figure>\n

Here is what happens when we run the program:<\/p>\n

\"Abnormal<\/a>
Abnormal Program Termination<\/strong><\/figcaption><\/figure>\n

Finally, can std::expected<><\/strong><\/em> be used with \u201cvoid\u201d<\/strong><\/em> returning functions? Yes, that is possible. See this code snippet:<\/p>\n

\"Using<\/a>
Using with “void”-returning Function<\/strong><\/figcaption><\/figure>\n

Of course, in this case, there is no \u201cnormal\u201d<\/strong><\/em> return value, but we can access the error code, if any, as usual.<\/p>\n

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

It is a matter of debate\/style whether such functions should return the error code via the normal return type or should use std::expected<><\/strong><\/em>. I personally prefer the former.<\/p>\n

I should stress at this point that std::expected<><\/strong><\/em> does not replace the correct use of exceptions in the code. When to \u201creturn\u201d<\/strong><\/em> an error code or \u201cthrow\u201d<\/strong><\/em> an exception is an important design decision and must be taken seriously.<\/p>\n

I tried the above example in Visual Studio Professional (64 bit) Ver 17.5.5<\/strong><\/em> on Windows 10<\/strong><\/em>. You can download the source from here<\/strong><\/em><\/a>.<\/p>\n

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

I had written about std::optional<> in an earlier article. C++23 introduces std::expected<> as an interesting extension to std::optional<>. Whereas std::optional<> contains a value or none at all, std::expected<> contains a value or an error code associated with it. This gives better control to the caller in terms of handling the outcome of the function call. […]<\/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":[361,362],"class_list":["post-3106","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c23","tag-stdexpected"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-O6","jetpack-related-posts":[{"id":700,"url":"https:\/\/www.rangakrish.com\/index.php\/2017\/11\/21\/c17-stdoptional\/","url_meta":{"origin":3106,"position":0},"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":1928,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/15\/stdis_destructible\/","url_meta":{"origin":3106,"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":2889,"url":"https:\/\/www.rangakrish.com\/index.php\/2022\/08\/09\/stdarray-vs-stdvector\/","url_meta":{"origin":3106,"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":1946,"url":"https:\/\/www.rangakrish.com\/index.php\/2020\/03\/28\/stdis_empty\/","url_meta":{"origin":3106,"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":3184,"url":"https:\/\/www.rangakrish.com\/index.php\/2023\/08\/13\/stdis_scoped_enum\/","url_meta":{"origin":3106,"position":4},"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":2975,"url":"https:\/\/www.rangakrish.com\/index.php\/2022\/12\/25\/stdtie\/","url_meta":{"origin":3106,"position":5},"title":"Std::tie","author":"admin","date":"December 25, 2022","format":false,"excerpt":"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.\u2026","rel":"","context":"In "C++"","block_context":{"text":"C++","link":"https:\/\/www.rangakrish.com\/index.php\/category\/c\/"},"img":{"alt_text":"std::tie Example","src":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2022\/12\/Example1-300x142.jpg?resize=350%2C200&ssl=1","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2022\/12\/Example1-300x142.jpg?resize=350%2C200&ssl=1 1x, https:\/\/i0.wp.com\/www.rangakrish.com\/wp-content\/uploads\/2022\/12\/Example1-300x142.jpg?resize=525%2C300&ssl=1 1.5x"},"classes":[]}],"_links":{"self":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/posts\/3106","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=3106"}],"version-history":[{"count":0,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/posts\/3106\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/media?parent=3106"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/categories?post=3106"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.rangakrish.com\/index.php\/wp-json\/wp\/v2\/tags?post=3106"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}