#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.

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

    printf("\nDeclare variable 'n' as 'union Number' and initialize its '.as_double'\n");
    printf("field to 654321.123456.\n");
    union Number n = { .as_double = 654321.123456 };
    enum NumberType type_of_n = NUMBER_DOUBLE;

    printf("n.as_double == %f\n", n.as_double);
    printf("n.as_int    == %d   <<<< WRONG\n", n.as_int);
    printf("type_of_n   == %d\n", type_of_n);

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