Understanding the Difference Between Structure and Union in C

Understanding the Difference Between Structure and Union in C

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Atul
Atul Harsha
Senior Manager Content
Updated on Sep 14, 2023 11:25 IST

A structure is a custom data type that holds multiple members of different data type under a single unit where union is a user defined data type that combine object of different data type in the exact memory location. In this article, we will explore the difference between structure and union in C.

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When diving into the world of C programming, one of the fundamental concepts to grasp is the difference between structure and union in C. Both structures and unions allow you to group different data types together, but they serve distinct purposes. Let’s understand the main difference between them. Later, I used a relatable analogy to clarify the concept: a multifunctional vending machine to explain the difference between structure and union in C.

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Difference Between Structure and Union

The main difference between union and structure is how specific information is stored and accessed. The table below highlights the key differences between structure and union in C is as follows:

Point of Difference Structure Union
Memory Allocation Allocates memory for all its members. Allocates memory only for the largest member.
Total Size Sum of sizes of all members. Size of the largest member.
Data Storage Can store values for all members simultaneously. Can store value for only one member at a time.
Use Case When you want to group different data types and access all of them at once. When you want to save memory and only need one data type at a time.
Example struct example {
int a;
float b;
}
union example {
int a;
float b;
}
Accessing Members All members can be accessed at any time. Only the last stored member can be accessed.
Modification Impact Modifying a member doesn’t affect other members. Modifying one member may overwrite other members.
 

Structures in C: The Multi-Slot Vending Machine

Analogy: Think of a structure in C as a vending machine with multiple slots.

  • Each slot is dedicated to a specific product: one for cola, one for chips, one for a toy, and so on.
  • All products are visible simultaneously, each in its designated slot.

Technical Insight: In C, a structure operates similarly to this multi-slot vending machine. It can store multiple values of different data types concurrently, with each value occupying its own space.

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How to Define a Structure in C?

A structure is defined using the struct keyword. The struct keyword defines a new data type with more than one member.

Syntax of Declaring a Structure in C

struct [structure name]
{
type member_1;
type member_2;
. . .
type member_n;
};

Example:


 
struct vendingMachine {
float cola;
char chips[10];
int toy;
};
Copy code

In the above example, the vendingMachine structure encapsulates the price of cola, the type of chips, and the toy code, all at the same time. This showcases the primary difference between structure and union in C when it comes to data storage.


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Unions in C: The Single-Slot Vending Machine

Analogy: A union in C is akin to a vending machine with just one slot.

  • This slot can accommodate any product, but only one at a given moment.
  • If you opt for cola, the slot displays cola. If you then select a toy, it replaces the cola.

Technical Insight: Similarly, in C, a union is like this single-slot vending machine. It can hold a value of any data type, but only one value at any given time.

How to Define a Union in C?

The union statement is used to define a union. It defines a new data type that can store multiple member variables of different data types in the exact memory location. The syntax to define a union using the union keyword is similar to defining a structure.

Syntax of Declaring a Union

union [union name]
{
type member_1;
type member_2;
. . .
type member_n;
};

Example:


 
union singleSlotMachine {
float cola;
char chips[10];
int toy;
};
Copy code

Here, the singleSlotMachine union can either store the price of cola, the type of chips, or the toy code, but not simultaneously. This is a crucial aspect of the difference between structure and union in C.

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Highlighting the Key Differences Between Structure and Union in C:

  • Space Allocation: Structures allocate space for all its members, like a vending machine reserving slots for every product. In contrast, unions use space only for the largest member, much like a single-slot vending machine.
  • Data Storage: In structures, all members can store data simultaneously. However, in unions, only one member can hold data at any moment.
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Similarities between Structure and Union in C

The following are the similarities between structure and union:

  1. Accessing Members: Members of both structures and unions are accessed using the dot (.) operator.
  2. Pointer Access: If you have a pointer to a structure or union, you access its members using the arrow (->) operator.
  3. Custom Data Types: Both structures and unions can be used to define custom data types. Once defined, you can declare variables of the structure or union type.
  4. Nested Definitions: Structures can contain unions, and unions can contain structures. This allows for complex data structures to be created.
  5. Memory Allocation: Both structures and unions are stored in contiguous memory locations.
  6. Function Parameters: Both structures and unions can be passed to functions as parameters and can also be returned from functions.
  7. Array Declaration: You can declare arrays of both structures and unions.
  8. Initialization: Both structures and unions can be initialized at the time of declaration.

Also Read: Top C Programming Interview Questions and Answers

Difference between Structure and Union in C – Scenarios

Scenario Use Structure Union
Grouping related data attributes of an entity (e.g., details of a student). ✔️  
Saving memory by ensuring only one of the data attributes is valid at any time.   ✔️
Representing a record in a database with multiple fields. ✔️  
Storing different data types in the same memory location at different times.   ✔️
Representing a complex object with multiple attributes (e.g., a car). ✔️  
Handling different data representations using the same memory (e.g., RGB vs HSV).   ✔️
Grouping data for a collection of items (e.g., a list of books in a library). ✔️  
Storing variant data types in systems with limited memory (e.g., embedded systems).   ✔️

Conclusion

The difference between structure and union in C is analogous to the distinction between multi-slot and single-slot vending machines. While structures in C allow for storing multiple data types concurrently, unions prioritize space efficiency, storing only one data type at a time. This understanding is pivotal for anyone looking to master C programming.

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FAQs

Which is better to work with, structure or union?

Structures are known to be more versatile while unions are easier to debug. Structures are more suitable for consistent performance and are recommended to use when working with multiple data types. Unions are more suitable when working with a huge number of members or objects. Unions are more suitable for situations where you want to use the same memory location for two or more members. Structures are usually used when working with objects with different attributes while unions are recommended when using data types that are unknown.

What is the main difference between a structure and a union?

The syntaxes to declare or define a structure and a union are similar. The major difference between a structure and a union is storage. In a structure, each member has its distinct storage location while the members of a union utilize a shared memory location that is equal to the size of its largest data member.

What are the disadvantages of structure in C?

In a structure in C, the memory is allocated to all data members. Thus, a structure takes more storage space compared to a union that takes only the memory size required by the largest data size. Also, with the increasing complexity of a project, it becomes difficult to manage all the data members of a structure. Another disadvantage of structure is that altering one data structure in a program makes it necessary to change at several other places, which becomes hard to track.

What is an Anonymous union?

An anonymous union does not have a tag or a name. It is a member of another union or structure. It cannot be followed by a declarator. It is not a type rather it defines an unnamed object. The names of a unionu2019s members shall be distinct from other names within the scope in which the union is declared.

What are the disadvantages of Union in C?

Some of the disadvantages of the union in C are: u2022 They cannot store multiple values at a time. u2022 They assign one common storage space for all their members.

What is a union of structure in C language?

In C programming, we can nest a structure inside a union. This is known as a union of structures. We can also nest a union inside a structure.

What are the advantages of Structure in C?

Every member is assigned to a unique memory location and multiple member can be initialize at a time. Structure can store multiple values of the different members.

About the Author
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Atul Harsha
Senior Manager Content

Experienced AI and Machine Learning content creator with a passion for using data to solve real-world challenges. I specialize in Python, SQL, NLP, and Data Visualization. My goal is to make data science engaging an... Read Full Bio