How to use anonymous struct or class?

Question

So, it's possible to declalre anonymous class or struct but how to I make it useful?

int main() {
    class{
        int ClassVal;
    };
    struct{
        short StructVal;
    };
    StructVal = 5;   //StructVal is undefined
    ClassVal = 5;    //ClassVal is undefined too?
    return 0;
}

if you put both of them outside of main function they will be inaccessible as well. I'm asking this only because it's somehow intersting :)

EDIT: Why union outside of main function (at global scope) must be static declared for example:

static struct {
    int x;
};
int main() {
   //...
}

Answer 1

Anonymous classes and structures may be used to directly define a variable:

int main()
{
    class
    {
        int ClassVal;
    } classVar;

    struct
    {
        short StructVal;
    } structVar;

    structVar.StructVal = 5;
    classVar.ClassVal = 5;

    return 0;
}

The above is not very common like that, but very common when used in unions as described by Simon Richter in his answer.

Answer 2

These are most useful in the form of nested struct and union :

struct typed_data {
    enum type t;
    union {
        int i;                       // simple value
        struct {
            union {                  // any of these need a length as well
                char *cp;
                unsigned char *ucp;
                wchar_t *wcp;
            };
            unsigned int len;
        };
    };
 };

Now typed_data is declared as a type with the members t , i , cp , ucp , wcp and len , with minimal storage requirements for its intended use.