Example usage of union

union_example.c

#include <stdio.h>
#include <stdlib.h>

typedef union {
    int i;
    double d;
} int_or_double;

typedef enum {
    INT,
    DOUBLE
} number_type;

typedef struct {
    number_type type;
    int_or_double n;
} number;

number add(const number* a, const number* b)
{
    number ret;
    if(a->type == INT && b->type == INT) {
        ret.type = INT;
        ret.n.i = a->n.i + b->n.i;
    } else {
        double x = a->type == INT ? a->n.i : a->n.d;
        double y = b->type == INT ? b->n.i : b->n.d;
        ret.type = DOUBLE;
        ret.n.d = x + y;
    }
    return ret;
}

void print_number(const number* num)
{
    if(num->type == INT) {
        printf("%d\n", num->n.i);
    } else {
        printf("%f\n", num->n.d);
    }
}

number make_int(int i)
{
    int_or_double u;
    u.i = i;
    number num = {INT, u};
    return num;
}

number make_double(double d)
{
    int_or_double u;
    u.d = d;
    number num = {DOUBLE, u};
    return num;
}


int main(void)
{
    number a = make_int(8);
    number b = make_int(6);
    number c = make_double(4.5);
    number res1 = add(&a, &b);
    number res2 = add(&a, &c);

    print_number(&res1);
    print_number(&res2);

    return EXIT_SUCCESS;
}