reference member of a strange C++ struct

Question

I came across a library called PolyBoolean. This is essentially irrelevant to my question. In the library there is a strange struct type as follows:

struct VNODE2
{
    VNODE2 * next;
    VNODE2 * prev;
    UINT32   Flags;
    union
    {
        VECT2 p;
        GRID2 g;
    };
}; 

and GRID2 is defined as:

struct GRID2
{
    INT32 x, y;
};

The GRID2 defines a point, VECT2 is another representation of a point similar to GRID2 and VNODE2 is a node. Say I have a VNODE2 structure variable v2, how can I get the value of x and y in g?

I tried v2.g.x, the vs2008 compiler gave me an error: "error C2059: syntax error" Any help would be greatly appreciated.

Answer 1

this is anonymous union, so you refer to members just as they are, see:

be sure that you declared GRID2 before your struct. Here is working example. You can also name your union to create instance in your class.

typedef int UINT32;
typedef int VECT2;
typedef int INT32;

struct GRID2
{
    INT32 x, y;
};

struct VNODE2
{
    VNODE2 * next;
    VNODE2 * prev;
    UINT32   Flags;
    union
    {
        VECT2 p;
        GRID2 g;
    };
}; 

int main(int argc, char** argv) {

    VNODE2 v;
    v.g.x=1;
    return 0;
}

in case if you would like your union to have name do it as follows:

struct GRID2
{
    INT32 x, y;
};

struct VNODE2
{
    VNODE2 * next;
    VNODE2 * prev;
    UINT32   Flags;
    union
    {
        VECT2 p;
        GRID2 g;
    }myNamedUnion;
}; 

int main(int argc, char** argv) {

    VNODE2 v;
    v.myNamedUnion.g.x=1;
    return 0;
}