When are anonymous structs and unions useful in C11?

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Anonymous union inside structures are very useful in practice. Consider that you want to implement a discriminated sum type (or tagged union), an aggregate with a boolean and either a float or a char* (i.e. a string), depending upon the boolean flag. With C11 you should be able to code

typedef struct {
    bool is_float;
    union {
       float f;
       char* s;
    };
} mychoice_t;

double as_float(mychoice_t* ch) 
{ 
   if (ch->is_float) return ch->f;
   else return atof(ch->s);
}

With C99, you’ll have to name the union, and code ch->u.f and ch->u.s which is less readable and more verbose.

Another way to implement some tagged union type is to use casts. The Ocaml runtime gives a lot of examples.

The SBCL implementation of Common Lisp does use some union to implement tagged union types. And GNU make also uses them.

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