When to use a union and when to use a structure

Question

I know the differences between union and structure. But from a design and coding perspective what are the various use cases of using a union instead of a structure? One is a

Answer 1

You use a union when your "thing" can be one of many different things but only one at a time.

You use a structure when your "thing" should be a group of other things.

For example, a union can be used for representing widgets or gizmos (with a field allowing us to know what it is), something like:

struct {
    int isAGizmo;
    union {
        widget w;
        gizmo g;
    }
}

In this example, w and g will overlap in memory.

I've seen this used in compilers where a token can be a numeric constant, keyword, variable name, string, and many other lexical elements (but, of course, each token is one of those things, you cannot have a single token that is both a variable name and a numeric constant).

Alternately, it may be illegal for you to process gizmos without widgets, in which case you could use:

struct {
    widget w;
    gizmo g;
}

In this case, g would be at a distinct memory location, somewhere after w (no overlap).

Use cases for this abound, such as structures containing record layouts for your phone book application which will no doubt earn you gazillions of dollars from your preferred app store :-)