{"id":739,"date":"2017-12-17T08:52:50","date_gmt":"2017-12-17T08:52:50","guid":{"rendered":"http:\/\/www.rangakrish.com\/?p=739"},"modified":"2017-12-17T09:00:17","modified_gmt":"2017-12-17T09:00:17","slug":"c17-constexpr-if","status":"publish","type":"post","link":"https:\/\/www.rangakrish.com\/index.php\/2017\/12\/17\/c17-constexpr-if\/","title":{"rendered":"C++17: constexpr if"},"content":{"rendered":"

constexpr if <\/i><\/b>is another nice little feature in C++17, which significantly simplifies the way we code, especially in the context of templates. In this article, I will briefly talk about this feature and look at some examples.<\/p>\n

constexpr if <\/i><\/b>is a variant of the standard if<\/i><\/b> statement we have had all along. The primary difference\u00a0 is that the condition part in constexpr if <\/i><\/b>must be a compile-time evaluable expression unlike the standard if<\/i><\/b>, where the condition is evaluated at run-time. Because of this special requirement, the compiler guarantees that the code it generates for the constexpr if <\/i><\/b>will just be the sub-part of the if<\/i><\/b> statement that is reachable. Take a look at the following example.<\/p>\n

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

As you can see, the function checkCondition()<\/i><\/b> in the above example is a constexpr<\/i><\/b> function, in that its return value is known at compile-time. (In this article, we are not going into the details of constexpr <\/i><\/b>in general. If time permits, I will cover that in a separate future article.)<\/p>\n

So, what is the code emitted by the compiler for the constexpr if <\/i><\/b>statement? The figure below shows the assembly code generated by Visual Studio 2017.<\/i><\/b><\/p>\n

\"Generated<\/a>
Generated Code<\/strong><\/figcaption><\/figure>\n

You can see that the false part of the if<\/i><\/b> statement alone has been considered. The reason is, the compiler figured out that the condition is false (always) and hence the true part of the if<\/i><\/b> statement can never get a chance to execute!<\/p>\n

Just to compare with the standard if <\/i><\/b>statement, take a look at the following code:<\/p>\n

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

Here is the assembly code generated for this code:<\/p>\n

\"Generated<\/a>
Generated Code<\/strong><\/figcaption><\/figure>\n

Because the logic for checking the condition is executed at run-time (as opposed to compile-time) in this case, the compiler has to emit code for both the true and false parts of the if<\/i><\/b> statement. It cannot assume that only one of the two will always execute.<\/p>\n

So, compared to the regular if <\/i><\/b>statement, constexpr if <\/i><\/b>results in more compact code. The limitation, obviously, is that the condition must be a compile-time knowable<\/strong><\/em> expression. For example, the following code will not compile because the condition cannot be evaluated at compile time.<\/p>\n

\"Compile-time<\/a>
Compile-time Check Needed<\/strong><\/figcaption><\/figure>\n

Here is another important thing to remember. Even though we might know that one part of the constexpr if <\/i><\/b>statement will not be compiled, we cannot have invalid C++ statements in that part. This will be clear from the following example.<\/p>\n

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

In this respect, it is different from #if<\/strong><\/em> or #ifdef.<\/strong><\/em> Check this out:<\/p>\n

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

constexpr if \u00a0<\/i><\/b>is very useful in the case of templates. It can simplify things considerably. Here is an example of compile-time evaluation of a function (obviously, not suitable for serious context!):<\/p>\n

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

If the above function is to be re-written without constexpr if<\/strong><\/em> (pre C++17), then it would look like this (I have changed the function name for convenience):<\/p>\n

\"Template<\/a>
Template Example (Pre-C++17)<\/strong><\/figcaption><\/figure>\n

It is clear that substantial saving in coding has been achieved by the use of constexpr if<\/strong><\/em>. Not only that, the logic is much clearer!<\/p>\n

You can find many more interesting examples if you dig into the template library. But hopefully, the above gives you an idea.<\/p>\n

OK, that is it for today. Have a great weekend!<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

constexpr if is another nice little feature in C++17, which significantly simplifies the way we code, especially in the context of templates. In this article, I will briefly talk about this feature and look at some examples. constexpr if is a variant of the standard if statement we have had all along. The primary difference\u00a0 […]<\/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":false,"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":[118,125],"class_list":["post-739","post","type-post","status-publish","format-standard","hentry","category-c","category-programming","tag-c17","tag-constexpr-if"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p9OLnF-bV","jetpack-related-posts":[{"id":3216,"url":"https:\/\/www.rangakrish.com\/index.php\/2023\/09\/24\/c23-if-consteval\/","url_meta":{"origin":739,"position":0},"title":"C++23: if consteval","author":"admin","date":"September 24, 2023","format":false,"excerpt":"\u201cIf consteval\u201d is useful in the context of calling a consteval function from within a constexpr function. 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