{"id":7254,"date":"2025-11-29T00:01:18","date_gmt":"2025-11-29T08:01:18","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=7254"},"modified":"2025-11-22T12:55:46","modified_gmt":"2025-11-22T20:55:46","slug":"consistently-constant","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=7254","title":{"rendered":"Consistently Constant"},"content":{"rendered":"

A new keyword added with the C23 standard is constexpr<\/em>. It’s a storage class specifier that sets a constant value. Unlike the original C language qualifier, const<\/em>, storage declared with the constexpr<\/em> is truly constant and cannot be altered, as was demonstrated in last week’s Lesson<\/a>.
\n
\nThough the C23 standard is dated for the year 2023, not every compiler is current with it; implementation is happening slowly. Even then, the -std=c23<\/code> switch (or -std=c2x<\/code>) may be required to activate various C23 features, including recognition of the constexpr<\/em> keyword.<\/p>\n

In a terminal window, use the command clang --version<\/code> or gcc --version<\/code> to obtain the compiler’s version. On my main computer, clang<\/em> is version 14.0.0. I’ve installed later versions for testing, but nothing that fully supports C23 on this system.<\/p>\n

On my Linux test computer, clang<\/em> is version 18.1.3, which doesn’t recognize the constexpr<\/em> keyword even with the -std=c23<\/code> switch applied. But I have installed clang<\/em> version 19 (19.1.1), which does recognize constexpr<\/em> but (again) only with the -std=c23<\/code> switch used.<\/p>\n

Here is sample code:<\/p>\n

2025_11_29-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    constexpr int size = 16;\r\n\r\n    printf(\"The value is %d\\n\",size);\r\n\r\n    return 0;\r\n}<\/pre>\n
As with the const<\/em> qualifier, you must assign a value immediately when using constexpr<\/em> to declare storage. In this code, int<\/em> variable size<\/code> is declared a constant expression and assigned a value of 16. Here’s the output:<\/p>\n
Size is 16<\/code><\/p>\n
To prove how bulletproof the constexpr<\/em> is, I tried the same trick from last week’s Lesson to reset its value. Here’s an update to the code presented:<\/p>\n
2025_11_29-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    constexpr int size = 16;\r\n    int *s;\r\n\r\n    printf(\"The value is %d\\n\",size);\r\n    s = (int *)&size;\r\n    *s = 0;\r\n    printf(\"The value is %d\\n\",size);\r\n\r\n    return 0;\r\n}<\/pre>\n
This code builds with no warnings or errors. It runs when compiled with C23 compatibility as described earlier. Here’s the output:<\/p>\n
Size is 16
\nSize is 16<\/code><\/p>\n
No errors are generated on output, but the constant size<\/code> is unaltered. I suppose this result confirms the constant’s consistency on a superficial level; at least the storage seems more protected than shown in last week’s Lesson.<\/p>\n
I’ve read other interesting about how constexpr<\/em> can be used, along with some limitations and proper procedures. I’ll most likely cover these details in the future when the C23 standard is more widely implemented. It’s been two years since I last wrote about C23<\/a>. Things are moving slowly, but at least they’re moving.<\/p>\n","protected":false},"excerpt":{"rendered":"
The only true constant declaration in C is found in the C23 standard and the constexpr<\/em> qualifier. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-7254","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7254","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=7254"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7254\/revisions"}],"predecessor-version":[{"id":7282,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7254\/revisions\/7282"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7254"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7254"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7254"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}