C++ union array and vars?

Question

There\'s no way to do something like this, in C++ is there?

union {
    {
        Scalar x, y;
    }
    Scalar v[2];
};

Where x == v[0]<

Answer 1

Given your example:

union
{
    struct
    {
        Scalar x, y;
    };

    Scalar v[2];
};

As others have noted, in general, the standard does not guarantee that there will be no padding between x and y, and actually compilers inserting padding in structures is pretty common behavior.

On the other hand, with solutions like:

struct U
{
    int   v[2];
    int&  x;
    int&  y;
};

U::U()
    : x(v[0])
    , y(v[1])
{}

what I don't like mainly is the fact that I have to mention x, y twice. For cases where I have more than just a few elements (say 10), this becomes much less readable and harder to maintain. Additionally U can no longer be a POD since it needs a user-defined constructor. And finally, the x & y references consume additional memory.

Hence, the (acceptable for me) compromise I've come up with is:

struct Point
{
    enum CoordType
    {
        X,
        Y,
        COUNT
    };

    int coords[CoordType::COUNT];
};

typedef Point::CoordType PtCoord;

With this you can then do:

Point p;
for ( int i = 0; i < PtCoord::COUNT; i++ )
    p.coords[i] = 100;
std::cout << p.coords[PtCoord::X] << " " << p.coords[PtCoord::Y] << std::endl;

// 100 100

A bit sophisticated but I prefer this over the references suggestion.