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

// Q: How can I track which field has been set?
// A: You can use an enum.

union Number {
    int    as_int;
    double as_double;
};   // <<<< SEMICOLON!!! <<<<<

enum NumberType {
    NUMBER_INT,    // = 0
    NUMBER_DOUBLE  // = 1
};   // <<<< SEMICOLON!!! <<<<<

// SIMILAR to #define NUMBER_INT    0
//            #define NUMBER_DOUBLE 1  ... but not the same because we have a real type.

void print_number(union Number n, enum NumberType type_of_n) {
    if(type_of_n == NUMBER_INT) {
        printf("%d\n", n.as_int);
    }
    else if(type_of_n == NUMBER_DOUBLE) {
        printf("%f\n", n.as_double);
    }
    else {
        assert(false);  // Unexpected value of type_of_n
    }
}

int main(int argc, char* argv[]) {

    /* union Number n = { .as_double = 654321.123456 }; */
    /* enum NumberType type_of_n = NUMBER_DOUBLE; */

    union Number n = { .as_int = 5 };
    enum NumberType type_of_n = NUMBER_INT;

    print_number(n, type_of_n);

    return EXIT_SUCCESS;
}
/* vim: set tabstop=4 shiftwidth=4 fileencoding=utf-8 noexpandtab: */