How to compile C code with anonymous structs / unions?

Question

I can do this in c++/g++:

struct vec3 {      union {         struct {             float x, y, z;         };          float xyz[3];     };  }; 

Then,

vec3 v; assert(&v.xyz[0] == &v.x); assert(&v.xyz[1] == &v.y); assert(&v.xyz[2] == &v.z); 

will work.

How does one do this in c with gcc? I have

typedef struct {     union {         struct {             float x, y, z;         };         float xyz[3];     }; } Vector3; 

But I get errors all around, specifically

line 5: warning: declaration does not declare anything line 7: warning: declaration does not declare anything 

Answer 1

according to http://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html#Unnamed-Fields

-fms-extensions will enable the feature you (and I) want.

Answer 2

(This answer applies to C99, not C11).

C99 does not have anonymous structures or unions. You have to name them:

typedef struct {     union {         struct {             float x, y, z;         } individual;         float xyz[3];     } data; } Vector3; 

And then you have to use the name when accessing them:

assert(&v.data.xyz[0] == &v.data.individual.x); 

In this case, because your top level structure has a single item of type union, you could simplify this:

typedef union {     struct {         float x, y, z;     } individual;     float xyz[3]; } Vector3; 

and accessing the data now becomes:

assert(&v.xyz[0] == &v.individual.x);