Unions

A union lets several members share one storage region. Its size is sufficient for its largest member, but normally only the most recently written member is considered active. It represents “one of several forms,” so the program must separately track which form is current.

#include <stdio.h>

union Value {
    int whole;
    double decimal;
    char letter;
};

int main(void) {
    union Value value;

    value.whole = 42;
    printf("whole: %d\n", value.whole);

    value.decimal = 3.5;
    printf("decimal: %.1f\n", value.decimal);

    value.letter = 'Z';
    printf("letter: %c\n", value.letter);

    return 0;
}

After writing decimal, do not assume the earlier whole still contains 42. Members overlap the same region; reading a non-current member interprets the same object representation as another type and is not a normal data conversion.