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
1
64
9/15/2014 7:13:41 PM

Accepted Answer

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

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

51
7/4/2012 10:49:06 AM

(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);

Licensed under: CC-BY-SA with attribution
Not affiliated with: Stack Overflow
Icon