When are anonymous structs and unions useful in C11?

Question

C11 adds, among other things, 'Anonymous Structs and Unions'.

I poked around but could not find a clear explanation of when anonymous structs and unions would be useful. I ask because I don't completely understand what they are. I get that they are structs or unions without the name afterwards, but I have always (had to?) treat that as an error so I can only conceive a use for named structs.

Answer 1

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.

Answer 2

A typical and real world use of anonymous structs and unions are to provide an alternative view to data. For example when implementing a 3D point type:

typedef struct {     union{         struct{             double x;              double y;             double z;         };         double raw[3];     }; }vec3d_t;  vec3d_t v; v.x = 4.0; v.raw[1] = 3.0; // Equivalent to v.y = 3.0 v.z = 2.0; 

This is useful if you interface to code that expects a 3D vector as a pointer to three doubles. Instead of doing f(&v.x) which is ugly, you can do f(v.raw) which makes your intent clear.