What are Unions?

This lesson introduces the concept of unions in C programming. It covers the basic syntax and declaration of unions, and highlights their fundamental purpose: allowing different data types to share the same memory location.

In C, a union is a special data type similar to a structure, but with a crucial difference: all its members share the same memory location. This means that at any given time, only one member of the union can hold a value. When you assign a value to one member, all other members effectively lose their previous values.

Why Use Unions?

Unions are used primarily to:

• Conserve Memory: When you know that only one of several possible data types will be used at a time, a union allows you to store them in the same memory space, reducing memory footprint.
• Represent Overlapping Data: Sometimes you need to represent the same data in different ways (e.g., as an integer or as an array of bytes). Unions can be useful in such scenarios.
• Data Type Flexibility: They allow you to store different types of data in the same variable at different times.

Syntax and Declaration

The syntax for declaring a union is similar to that of a structure:

 union myUnion {
                    int integerValue;
                    float floatValue;
                    char characterValue;
                }; 

In this example, myUnion can hold an integer, a float, or a character. The size of myUnion will be the size of its largest member (in this case, likely the float).

Accessing Union Members

You access union members using the dot (.) operator, just like with structures:

 union myUnion data;

                data.integerValue = 10;
                printf("Integer value: %d\n", data.integerValue);

                data.floatValue = 3.14;
                printf("Float value: %f\n", data.floatValue);

                data.characterValue = 'A';
                printf("Character value: %c\n", data.characterValue);

                // At this point, only characterValue holds the actual assigned value.  integerValue and floatValue are overwritten. 

Important Note: When you assign a value to one union member, you're overwriting the value of all other members. It's your responsibility to track which member is currently valid. Using a separate variable to indicate the "type" of data currently stored in the union is a common practice.