How to Declare a Pointer to a Union in C?

Union is a user-defined data type in C language that can contain elements of the different data types and pointers are used to store memory addresses. In this article, we will learn how to declare a pointer to a union in C++.

Pointer to Union in C

To declare a pointer to a union first, define a union and then declare a pointer that points to the union type using the below syntax:

Syntax to Declare Pointer to Union in C

//Define Union
union unionName {
    int var1;
    float var2;
};
//declare pointer to union
union unionName *ptr;

We can also assign the pointer the address of some union variable by using the addressof(&) operator.

C Program to Declare Pointer to Union

The following programs show how to declare pointers to unions in C programming and access members using the arrow operator (->).

// C program to declare pointer to union
#include <stdio.h>

// Defining a union
union myUnion {
    int intValue;
    float floatValue;
    char charValue;
};

int main()
{
    // Creating a union variable
    union myUnion u;

    // Declaring a pointer to the union and assign it the
    // address of the union variable
    union myUnion* ptr = &u;

    // Using the pointer to Set the intValue member of the
    // union
    ptr->intValue = 100;

    // Accessing the intValue member of the union
    printf("The intValue is: %d\n", ptr->intValue);

    // set value for floatValue This will overwrite the
    // intValue
    ptr->floatValue = 3.14;

    // Accessing the floatValue member of the union
    printf("The floatValue is: %f\n", ptr->floatValue);

    return 0;
}

The intValue is: 100
The floatValue is: 3.140000