{"id":4746,"date":"2021-05-08T00:01:08","date_gmt":"2021-05-08T07:01:08","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4746"},"modified":"2021-05-01T10:58:51","modified_gmt":"2021-05-01T17:58:51","slug":"the-_generic-keyword","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4746","title":{"rendered":"The _Generic<\/em> Keyword"},"content":{"rendered":"

The C11 standard added the “underscore” bunch to the C language’s traditional keywords:<\/p>\n

_Alignas
\n_Alingof
\n_Atomic
\n_Bool
\n_Complex
\n_Generic
\n_Imaginary
\n_Noreturn
\n_Static_assert
\n_Thread_local<\/em><\/p>\n

I don’t routinely use any of these in my programs, beyond trying a few out to see how they work. The _Bool<\/em><\/a> keyword comes in handy. The rest? Well, they’re worth exploring from a curiosity standpoint. For this week’s Lesson, I reveal the mysteries of the _Generic<\/em> keyword.
\n
\nIt would be neat if computer scientists just threw in these new keywords for fun. They’d say, “This new underscore word will really drive the coders nuts!” But no, these keywords have their roots in problems that were originally solved in other ways. This tale tells true for the _Generic<\/em> keyword, which has its roots in C’s various high-level math functions.<\/p>\n

For example, different math functions are required for specific data types. You may not be aware of these differences because the math function is really a macro. Internally, the macro calls a specific function based on the data type used as an argument. This process explains how the _Generic<\/em> keyword came about; its job is to help determine a data type.<\/p>\n

Here is the format for _Generic<\/em>, which behaves more like a function than a keyword:<\/p>\n

r<\/em> = _Generic( expression<\/em>, list<\/em>... );<\/code><\/p>\n

r<\/em> is an int<\/em> value generated by the _Generic<\/em> keyword.<\/p>\n

expression<\/em> is the variable or constant to be evaluated.<\/p>\n

list<\/em> is a menu of options. It works similar to case<\/em> statements in a switch-case<\/em> structure. The list<\/em> presents a series of data types followed by a colon and an expression. These items are separated by commas, and a default<\/em> item is present to handle no-matches:<\/p>\n

_Generic( var, char: 1, int: 2, long: 3, default: 0 );<\/code><\/p>\n

Above, assume the data type for var<\/code> is int<\/em>. Therefore, the value 2 is generated by the _Generic<\/em> keyword.<\/p>\n

Here’s another, more practical example from a math function standpoint:<\/p>\n

#define cbrt(X) _Generic((X), long double: cbrtl, float: cbrtf, default: cbrt )<\/code><\/p>\n

In this declaration, the cbrt()<\/code> macro can evaluate to either the cbrtl()<\/em>, cbrtf()<\/em> or cbrt()<\/em> function depending on the data type of argument X<\/code>. In this manner, _Generic<\/em> helps the code to implement the proper cubed root function based on the argument’s data type.<\/p>\n

Because I’m mathematically disinclined, here is some sample code you can play with:<\/p>\n

2021_05_08-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char a;\r\n    int r;\r\n\r\n    r = _Generic( a, char: 1, int: 2, long: 3, default: 0 );\r\n    printf(\"'a' is \");\r\n    switch(r)\r\n    {\r\n        case 1:\r\n            puts(\"a char\");\r\n            break;\r\n        case 2:\r\n            puts(\"an int\");\r\n            break;\r\n        case 3:\r\n            puts(\"a long\");\r\n            break;\r\n        default:\r\n            puts(\"unknown\");\r\n    }\r\n\r\n    return(0);\r\n}<\/pre>\n
At Line 8, the _Generic<\/em> keyword evaluates the data type of variable a<\/code>. In this case, it’s a char<\/em>, so int<\/em> value 1 is returned. This value is saved in variable r<\/code> and used in the switch-case<\/em> structure starting at Line 10. Here is the output:<\/p>\n
'a' is a char<\/code><\/p>\n
You can change the declaration of variable a<\/code> at Line 5 to int<\/em>, long<\/em>, and so on to see how output is affected. If you change the data type to float<\/em> or a pointer, the default<\/em> value of 0 is generated.<\/p>\n
This Lesson may not convince you to use _Generic<\/em> in your code, but I hope you appreciate its worth. Perhaps you can work it in somehow, but like the most of the underscore keywords, it remains a curiosity.<\/p>\n","protected":false},"excerpt":{"rendered":"
The C11 standard formalized introduction of the keyword _Generic<\/em>. Why? 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-4746","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\/4746","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=4746"}],"version-history":[{"count":5,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4746\/revisions"}],"predecessor-version":[{"id":4756,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4746\/revisions\/4756"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4746"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4746"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4746"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}