C Primer Plus, Fourth Edition

Unions: A Quick Look

A union is a type that enables you to store different data types in the same memory space (but not simultaneously). A typical use is a table designed to hold a mixture of types in some order that is neither regular nor known in advance. By using an array of unions, you can create an array of equal-sized units, each of which can hold a variety of data types.

Unions are set up in much the same way as structures. There is a union template and a union variable. They can be defined in one step or, by using a union tag, in two. Here is an example of a union template with a tag:

union hold {
    int digit;
    double bigfl;
    char letter;
};

A structure with a similar declaration would be able to hold an int value and a double value and ...