Structure vs Union in C

Both struct and union are user-defined data types in C that group different data types together. However, they differ fundamentally in how they allocate and use memory .

Key Differences Between Structure and Union

Feature	Structure (struct)	Union
Keyword	struct	union
Memory Allocation	Each member gets its own separate memory	All members share the same memory location
Size of the data type	Sum of sizes of all members + padding	Size of the largest member (including padding if any)
Number of members active at a time	All members are active simultaneously	Only one member can be used at a time
Member access	All members can be accessed independently	Accessing one member overwrites others
Initialization	Can initialize all members at once	Only one member can be initialized (usually the first)
Default value	Members are undefined unless initialized	All members share same memory – value depends on last written member
Use case	Group related but independent data (student record, point, book, etc.)	Memory optimization, type punning, variant types, hardware registers
Typical real-world example	struct Student { char name[50]; int roll; float marks; }	union Data { int i; float f; char c; }
Memory efficiency	Consumes more memory	Very memory efficient

Memory Layout Comparison – Visual Example

              
            
C

            #include <stdio.h>

struct Student {
    char grade;     // 1 byte
    int roll;       // 4 bytes
    float marks;    // 4 bytes
};

union Data {
    char grade;     // 1 byte
    int roll;       // 4 bytes
    float marks;    // 4 bytes
};

int main() {
    printf("Size of struct Student: %zu bytes\n", sizeof(struct Student));  // Usually 12 bytes
    printf("Size of union Data: %zu bytes\n", sizeof(union Data));          // 4 bytes
    return 0;
}
          

Note: Structure → 12 bytes (with padding), Union → 4 bytes (largest member = float/int)

Practical Demonstration – How Values Behave

              
            
C

            union Test {
    int a;
    char b;
};

struct TestStruct {
    int a;
    char b;
};

// Union: changing one affects others
union Test u;
u.a = 65;
printf("Union int: %d, char: %c\n", u.a, u.b);  // 65, 'A'

u.b = 'Z';
printf("After char change → int: %d, char: %c\n", u.a, u.b);  // garbage, 'Z'

// Structure: independent
struct TestStruct s;
s.a = 65;
s.b = 'Z';
printf("Struct int: %d, char: %c\n", s.a, s.b);  // 65, 'Z' (both preserved)
          

When to Use Structure vs Union

Use struct when:
- You need to store multiple pieces of related data at the same time
- Examples: Student record, Employee, Point (x,y), Date, Book info
Use union when:
- You want to save memory and only one type of data is needed at a time
- You need to reinterpret the same memory (type punning)
- Examples: Tagged unions (variant types), Network packet parsing, Hardware register access, Low-level memory manipulation

Modern & Safe Usage (Recommended)

              
            
C

            enum Type { INT_TYPE, FLOAT_TYPE, CHAR_TYPE };

struct Variant {
    enum Type type;
    union {
        int i;
        float f;
        char c;
    } value;
};
          