Two arrays in a union in C++

Question

is it possible to share two arrays in a union like this:

struct
    {
        union
        {
            float m_V[Height * Length];
            float m_M[Height] [Length];
        } m_U;
    };

Do these two arrays share the same memory size or is one of them longer?

Answer 1

Both arrays are required to have the same size and layout. Of course, if you initialize anything using m_V, then all accesses to m_M are undefined behavior; a compiler might, for example, note that nothing in m_V has changed, and return an earlier value, even though you've modifed the element through m_M. I've actually used a compiler which did so, in the distant past. I would avoid accesses where the union isn't visible, say by passing a reference to m_V and a reference to m_M to the same function.