C++ union array and vars?

Since you are using C++ and not C, and since they are of the same types, why not just make x a reference to v[0] and y a reference to v[1]

I've used something like this before. I'm not sure its 100% OK by the standard, but it seems to be OK with any compilers I've needed to use it on.

struct Vec2
{
  float x;
  float y;
  float& operator[](int i) { return *(&x+i); }
};

You can add bounds checking etc to operator[] if you want ( you probably should want) and you can provide a const version of operator[] too.

If you're concerned about padding (and don't want to add the appropriate platform specific bits to force the struct to be unpadded) then you can use:

struct Vec2
{
  float x;
  float y;
  float& operator[](int i) {
    assert(i>=0);
    assert(i<2);
    return (i==0)?x:y;
  }
  const float& operator[](int i) const {
    assert(i>=0);
    assert(i<2);
    return (i==0)?x:y;
  }
};

Try this:

template<class T>
struct U1
{
    U1();
    T   v[2];
    T&  x;
    T&  y;
};

template<class T>
U1<T>::U1()
    :x(v[0])
    ,y(v[1])
{}

int main()
{
    U1<int>   data;

    data.x  = 1;
    data.y  = 2;
}

How about

union {
    struct {
        int x;
        int y;
    };
    int v[2];
};

edit:

union a {
    struct b { int first, second; } bee;
    int v[2];
};

Ugly, but that's more accurate