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.