C-style 'union' in Ada?

["Brian Catlin" <briancatlin@mindspring.com>]

I'm still trying to get both lobes around the Ada paradigm.  I have a record that I want to hold a pointer to a buffer which is
either a String or Wide_String, but I don't want to statically declare which, so using a variant record won't work (at least given
my understanding).  Likewise, I don't want to have two record definitions, one for String, and one for Wide_String.  In the package
I'm designing, I don't care whether it is a String or Wide_String, I will just pass it to the file I/O packages (stream, in this
case).  In C, I would:

struct
    {
    ULONG    length;

    union
        {
        STRING_DESC                *ansi_string;
        WIDE_STRING_DESC    *wide_string;
        } strings;
    } string_desc;

and then:

struct
    {
    ...
    string_desc    file_name;
    ...
    } some_struct;

What is the idiom or method to use in Ada?

 -Brian