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How to compile C code with anonymous structs / unions?

Asked 2009-12-28T23:05:17.960
73

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
c++ c unions anonymous-class anonymous-struct
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3 Answers

33

(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);
answered 2009-12-28T23:18:02.877
24

The new C11 standard will support anonymous structures and unions, see foreword paragraph 6 of the April 2011 draft.

http://en.wikipedia.org/wiki/C1X

The strange part is that both gcc and clang now support anonymous structures and unions in C89 and C99 mode. In my machine no warnings appear.

answered 2011-11-11T20:39:40.413
7

Anonymous unions is a feature of C++ language. C language has no anonymous unions.

Anonymous structs don't exist in neither C nor C++.

The declaration you presented in your question might compile with GCC C++ complier, but it would be just a compiler-specific extension, which has nothing to do with neither standard C nor standard C++.

On top of that, regardless of how you implement it, neither C nor C++ language guarantees that your assertions will hold.

answered 2009-12-28T23:14:21.030

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