ANSI C unions - are they really useful?

Question

From a response to some question yesterday, I learned that it is nonportable and unsafe to write into one union member and read the value from another member of a different type, assuming underlying alignment of the members. So after some research I found a written source that repeats this claim and specifies a popular example - using union of int and float to find the binary representation of a float.

So, understanding that this assumption is not safe, I wonder - except for saving memory (duh...) what real use is there to unions?

Note: that is, under Standard C. Clearly, for a specific implementation, the rules are known in advance and can be taken advantage of.

EDIT: the word "unsafe", due to association of recent years, is probably a bad choice of wording, but I think the intention in clear.

EDIT 2: Since this point repeats in the answers - saving memory is a valid argument. I wanted to know if there was something beyond that.

Answer 1

Yes.

The provide a way of creating generic containers. Though, to get polymorphic behavior you must implement a vtable or type switching yourself...

There are, however, one of those features that you only use when you need them and need rather rarely.

Answer 2

Even if unions don't offer much in immediate usefulness (reduced memory usage aside), one advantage of using a union over dumping all of its members into a struct is that it makes the intended semantics clear: only one value (or set of values if it's a union of structs) is valid at any given time. It documents itself better.

The mutual exclusivity of members would be less obvious if you instead made all the union members separate members of a struct. Additionally, you'd still have the same problem of ill-defined behavior if you read a member that wasn't previously written to, but now you need to account for the application's semantics too (did it initialize all unused members to 0? did it leave them as garbage?), so in that sense, why wouldn't you use a union?