Flags

C/C++

Last updated on 2019-10-20

You’ll see API’s use flags alot, they are a great way of storing multiple options in a single variable. Then you can query the variable to see if the option has been set.

/* flags.c */

#include <stdio.h>

/* flags must be power of two */
enum flags {
        OPT_RESIZE = 1 << 0,
        OPT_CLOSE = 1 << 1,
        OPT_DRAG = 1 << 2,
        OPT_MIN = 1 << 3,
};

int
main() {
        /* use bitwise 'or' to combine the options */
        int opts = OPT_RESIZE | OPT_DRAG | OPT_MIN; 

        /* then we can check for flag(s) with '&' */
        if(opts & OPT_RESIZE) {
                printf("Resize supported\n");
        }
        if(opts & (OPT_DRAG | OPT_MIN)) {
                printf("Drag and Minimize supported\n");
        }
        if(opts & OPT_CLOSE) {
                printf("Close supported\n");
        }

        return 0;
}

Compile and run,

gcc flags.c && ./a.out

outputs …

Resize Supported
Drag and Minimize supported