🟡 Intermediate · Lesson 28

Unions & Enumerations

Unions और Enumerations

What is a union?

A union is a user-defined data type like a structure, but all members share the same memory location. At a time, normally only one member holds a meaningful value.

Syntax

union Data {
    int i;
    float f;
    char str[20];
};

Memory Sharing Example

C Language

#include <stdio.h>
#include <string.h>

union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    union Data d;

    d.i = 10;
    printf("i = %d\n", d.i);

    d.f = 25.5;
    printf("f = %.1f\n", d.f);

    strcpy(d.str, "C Language");
    printf("str = %s\n", d.str);

    printf("Size of union = %zu bytes", sizeof(d));
    return 0;
}

i = 10 f = 25.5 str = C Language Size of union = 20 bytes

💡 Why size is 20?

The largest member is char str[20], so the union needs enough memory for that member.

Structure vs Union

Feature	Structure	Union
Memory	Separate memory for all members	Shared memory for all members
Size	Sum of member sizes plus padding	Size of largest member plus padding
Use	Store many values together	Store one of many possible values
Access	All members can hold valid values	Only last assigned member is reliable

Enumerations

An enum gives names to integer constants. It makes programs more readable.

C Language

#include <stdio.h>

enum Week { SUN, MON, TUE, WED, THU, FRI, SAT };

enum Result { FAIL = 0, PASS = 1 };

int main() {
    enum Week today = WED;
    enum Result r = PASS;

    printf("Today value = %d\n", today);
    printf("Result value = %d", r);
    return 0;
}

Today value = 3 Result value = 1

Summary

Union members share same memory
Union size depends on its largest member
Use union when only one value is needed at a time
Enum creates readable names for integer constants
Enums improve code clarity in menus, status and options

Union क्या है?

Union structure जैसा user-defined data type है, लेकिन इसके सभी members same memory share करते हैं। एक समय पर सामान्यतः एक ही member की value meaningful होती है।

Syntax

union Data {
    int i;
    float f;
    char name[20];
};

Memory Sharing

C Language

#include <stdio.h>
#include <string.h>

union Data {
    int i;
    float f;
    char name[20];
};

int main() {
    union Data d;
    d.i = 10;
    d.f = 20.5;
    strcpy(d.name, "Amit");

    printf("Name = %s", d.name);
    return 0;
}

⚠️ ध्यान दें

Union में last assigned member की value ही reliable होती है क्योंकि memory same होती है।

Structure vs Union

Feature	Structure	Union
Memory	हर member की अलग memory	सभी members same memory share करते हैं
Size	लगभग सभी members का total	सबसे बड़े member के बराबर
Use	कई values साथ रखनी हों	एक समय पर एक value रखनी हो

Enumeration

enum integer constants को readable names देता है।

C Language

enum Week { SUN, MON, TUE, WED, THU, FRI, SAT };
enum Week today = MON;
printf("%d", today);   // 1

सारांश

Union में members same memory share करते हैं
Union का size largest member पर depend करता है
Enum integer constants को names देता है
Union memory saving के लिए useful है
Enum menu/status values के लिए अच्छा है

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