C can share same memory area by a number of different variables via a union.
A union is usually given a tag name in the same way as structure.
You use the keyword union to define a union.
For example, the following statement declares a union to be shared by three variables:
union U_example { float decval; int *pnum; double my_value; } u1;
This statement declares a union with the tag name U_example, which shares memory between decval, pnum, and my_value.
The statement also defines one instance of the union with a variable name of u1.
You can declare further instances of this union with a statement such as this:
union U_example u2, u3;
Members of a union are accessed in exactly the same way as members of a structure.
For example, to assign values to members of u1 and u2, you can write this:
u1.decval = 2.5; u2.decval = 3.5*u1.decval;
The size of an instance of a union is the memory required for the largest member.
#include <stdio.h>
// ww w . ja va2s . com
typedef union UDate UDate;
typedef struct Date Date;
typedef struct MixedDate MixedDate;
typedef struct NumericDate NumericDate;
void print_date(const Date* date); // Prototype
enum Date_Format{numeric, text, mixed}; // Date formats
struct MixedDate {
char *day;
char *date;
int year;
};
struct NumericDate {
int day;
int month;
int year;
};
union UDate {
char *date_str;
MixedDate day_date;
NumericDate nDate;
};
struct Date {
enum Date_Format format;
UDate date;
};
int main(void) {
NumericDate yesterday = { 11, 11, 2020};
MixedDate today = {"Monday", "12th November", 2020};
char tomorrow[] = "Tues 13th Nov 2020";
// Create Date object with a numeric date
UDate udate = {tomorrow};
Date the_date;
the_date.date = udate;
the_date.format = text;
print_date(&the_date);
// Create Date object with a text date
the_date.date.nDate = yesterday;
the_date.format = numeric;
print_date(&the_date);
// Create Date object with a mixed date
the_date.date.day_date = today;
the_date.format = mixed;
print_date(&the_date);
return 0;
}
void print_date(const Date* date) {
switch(date->format) {
case numeric:
printf_s("The date is %d/%d/%d.\n", date->date.nDate.day,
date->date.nDate.month,
date->date.nDate.year);
break;
case text:
printf_s("The date is %s.\n", date->date.date_str);
break;
case mixed:
printf_s("The date is %s %s %d.\n", date->date.day_date.day,
date->date.day_date.date,
date->date.day_date.year);
break;
default:
printf_s("Invalid date format.\n");
}
}
The code above generates the following result.