Difference Between Union and Structure in C: 7 Key Distinctions

Structures and unions are both user-defined data types in C that allow you to group multiple variables under a single name. However, they have fundamentally different ways of managing memory and accessing data. Understanding these differences is crucial for making informed choices when designing your C programs. This comprehensive guide will break down the seven key distinctions between unions and structures, providing clear explanations and practical examples to help you utilize these data types effectively.

Structure vs. Union: The Memory Management Showdown

Feature	Structure	Union
Purpose	Store multiple members (variables) of different data types together.	Store multiple members, but only one can be active and accessible at any given time.
Memory Allocation	Each member gets its own unique memory location.	All members share the same memory location, the size of which is determined by the largest member.
Access to Members	All members can be accessed simultaneously and independently.	Only one member can be accessed and modified at a time.
Initialization	You can initialize multiple members at declaration.	Only the first member can be initialized at declaration.
Modification Impact	Changing one member’s value does not affect the values of other members.	Changing one member’s value overwrites the values of all other members.
Memory Efficiency	Less memory efficient, as it allocates memory for all members.	More memory efficient, as it only uses the memory required for the largest member.
Keyword	struct	union

Code Examples: Structure vs. Union in Action

Structure:

struct Student {
    char name[50];
    int age;
    float gpa;
};

struct Student s1 = {"Alice", 20, 3.8};  
// Accessing members:
printf("%s %d %.2f\n", s1.name, s1.age, s1.gpa); // Output: Alice 20 3.80

Union:

union Data {
    int i;
    float f;
};

union Data d1;
d1.i = 42; // Assign to the int member
printf("%d\n", d1.i); // Output: 42
d1.f = 3.14; // Assign to the float member (overwrites the int value)
printf("%.2f\n", d1.f); // Output: 3.14

Choosing the Right Tool: When to Use Structures and Unions

• Structures: Ideal for storing related data items of different types that need to be accessed independently, like student records, employee details, or point coordinates.
• Unions: Suitable for situations where you need to store different types of data in the same memory location, but only one type is relevant at a time. This is often used for memory optimization or when dealing with data that can have multiple representations (e.g., storing a value that could be either an integer or a floating-point number).

FAQs: Difference Between Union and Structure in C