Comprehensive Guide to C Language Unions: Differences Between Unions and Structures

In the C language, a union and a structure are two important data types used to store different types of data. This article will detail their basic concepts, usage, and explain the differences between them.

1. Structure (Struct)

1. What is a Structure?

A structure is a user-defined data type that allows combining different types of data into a single entity. Using a structure makes it easier to manage related data.

2. How to Define and Use a Structure?

We use the <span>struct</span> keyword to define a structure. For example, we can define a structure representing student information:

#include <stdio.h>
#include <string.h>
// Define a structure for student information
struct Student {
    char name[50];
    int age;
    float score;
};

int main() {
    // Create and initialize a variable of type Student
    struct Student student1;
    strcpy(student1.name, "Alice");
    student1.age = 20;
    student1.score = 85.0;
    // Output student information
    printf("Name: %s\n", student1.name);
    printf("Age: %d\n", student1.age);
    printf("Score: %.2f\n", student1.score);
    return 0;
}

3. Considerations

• A structure can have multiple members and can accommodate various data types.
• All members occupy space in memory, with their size equal to the sum of all member sizes.

2. Union

1. What is a Union?

A union is similar to a regular variable, but it can hold only one value at a time, and the memory it occupies is equal to that of its largest member. In other words, all members share the same memory area, thus saving space.

2. How to Define and Use a Union?

We use the <span>union</span> keyword to define a union. For example, we can define a union for vehicle information:

#include <stdio.h>
#include <string.h>
// Define a union for vehicle information
union Vehicle {
    char car[50];
    char bike[50];
};

int main() {
   // Create and initialize a variable of type Vehicle
    union Vehicle vehicle;
   // Use car information
   strcpy(vehicle.car, "Toyota");
   printf("Car: %s\n", vehicle.car);
   // Use bike information - this overwrites the previous car information
   strcpy(vehicle.bike, "Harley Davidson");
   printf("Bike: %s\n", vehicle.bike);
   return 0;
}

3. Considerations

• Only one member can be accessed at a time; changing one will affect the others.
• The memory size is determined by the largest element, not the sum of all elements, making it very memory efficient.

3. Differences Between Union and Structure

Feature	Union	Structure
Memory Allocation	Memory allocated based on the largest element	Memory allocated based on the sum of all elements
Data Access	Only one value can be accessed at a time	Multiple values can be accessed simultaneously
Member Sharing Method	All members share the same memory	Each member has its own independent space
Usage	Space-saving, used when only one data type needs to be retained.	Stores related data, more general-purpose.

4. Conclusion

Through this article, you should have gained an understanding of the basic concepts and code examples in C language, as well as how to use structures and unions appropriately. In daily programming, you should choose the appropriate method based on your needs: if you need to frequently manipulate multiple related fields, it is recommended to use a structure; whereas if you want to reduce the required space and only need to save one of them, you should prioritize using a union. I hope this article helps you.