What Is the Size of Union Data Structure?


Heather Bennett

When it comes to data structures in programming, the size of a union can often be a topic of confusion. A union is a special type of data structure that allows different types of data to be stored in the same memory location.

But what exactly is the size of a union? Let’s dive in and find out.

The Basics of Unions

To understand the size of a union, we first need to understand how they work. In C and C++, a union is defined using the union keyword, followed by a set of member variables enclosed in curly braces.

union MyUnion {
    int i;
    float f;
    char c;

In this example, we have defined a union called MyUnion, which can hold an integer (i), a float (f), or a character (c). The key thing to note here is that all these variables share the same memory location.

The Size Dilemma

The size of a union is determined by its largest member variable. In other words, the size allocated for a union will be equal to the size required by its largest member variable. Let’s consider an example:

#include <stdio.h>

int main() {
    union MyUnion {
        int i;
        float f;
        char c;

    printf("Size of MyUnion: %lu bytes\n", sizeof(union MyUnion));
    return 0;

If we run this code, the output will be:

Size of MyUnion: 4 bytes

Here, the size of MyUnion is 4 bytes because the largest member variable (in this case, an int) requires 4 bytes of memory.

Memory Overlap

One important thing to keep in mind when working with unions is that the memory allocated for different member variables overlaps. This means that changing the value of one member will affect the values of other members.

This can lead to unexpected behavior if not handled carefully. It is crucial to ensure that you are accessing the correct member variable at any given time.


In summary, the size of a union is determined by its largest member variable. The memory allocated for a union is shared among all its members, leading to potential memory overlap. Understanding these concepts will help you make efficient use of unions in your programs.


  • The size of a union can vary depending on the platform and compiler used.
  • The order in which members are declared in a union does not affect its size.
  • Care should be taken when accessing and modifying union members to avoid unintended consequences.

I hope this article has provided you with a clear understanding of the size of a union. Happy coding!

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