How to Declare Union in C: Syntax and Examples

In C, you declare a union using the union keyword followed by a name and a block of member variables inside braces. Each member shares the same memory location, so only one member can hold a value at a time.

Syntax

A union is declared with the keyword union, followed by the union name and a block containing its members. Each member can be of different types, but they share the same memory space.

• union: keyword to declare a union
• Name: identifier for the union type
• Members: variables inside braces that share memory

c

union UnionName {
    type1 member1;
    type2 member2;
    type3 member3;
};

Example

This example shows how to declare a union with an integer and a float. It assigns a value to one member and prints it, then assigns a value to the other member and prints it. Notice how the value changes because both members share the same memory.

c

#include <stdio.h>

union Number {
    int i;
    float f;
};

int main() {
    union Number num;

    num.i = 10;
    printf("num.i = %d\n", num.i);

    num.f = 220.5f;
    printf("num.f = %.2f\n", num.f);

    // After assigning to num.f, num.i value changes
    printf("num.i after assigning num.f = %d\n", num.i);

    return 0;
}

Output

num.i = 10
num.f = 220.50
num.i after assigning num.f = 1120403456

Common Pitfalls

One common mistake is assuming all members of a union hold their values independently. In reality, all members share the same memory, so changing one member overwrites the others. Another pitfall is not initializing the union before reading a member, which can lead to undefined behavior.

Wrong way example:

union Data {
    int x;
    float y;
};

union Data d;
printf("d.y = %f\n", d.y);  // Reading uninitialized member

Right way example:

union Data d;
d.x = 5;
printf("d.x = %d\n", d.x);

Quick Reference

• Use union to save memory when only one of several variables is needed at a time.
• Only one member can hold a valid value at any moment.
• Accessing a member different from the last assigned one leads to unpredictable results.
• Unions can be anonymous or named.

Key Takeaways

Declare a union with the union keyword followed by member variables inside braces.

All union members share the same memory location; only one member holds a valid value at a time.

Assigning a value to one member overwrites the others.

Always initialize a union member before reading it to avoid undefined behavior.

Use unions to save memory when you need to store different types but only one at a time.