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c:preprocessor:undef [2024/02/16 01:06] (current) |
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+ | ====== #undef ====== | ||
+ | |||
+ | ===== Description ===== | ||
+ | With the # undef a macro can be overridden.\\ | ||
+ | Status "defined" or "undefined" is an important characteristic of an identifier, regardless of its actual definition.\\ | ||
+ | The # ifdef and # ifndef can be used to check whether an identifier is currently defined or not.\\ | ||
+ | |||
+ | Syntax\\ | ||
+ | <code c> | ||
+ | #undef macro_name | ||
+ | </code> | ||
+ | |||
+ | ===== c undef example ===== | ||
+ | <code c> | ||
+ | #include <stdio.h> | ||
+ | |||
+ | int main(void) | ||
+ | { | ||
+ | #define TESTDEFINE 1 | ||
+ | printf("%d\n", TESTDEFINE); | ||
+ | |||
+ | #undef TESTDEFINE | ||
+ | |||
+ | #define TESTDEFINE 42 | ||
+ | printf("%d\n", TESTDEFINE); | ||
+ | |||
+ | return 0; | ||
+ | } | ||
+ | </code> | ||
+ | |||
+ | === output === | ||
+ | 1 | ||
+ | 42 | ||