Purpose of Unions in C and C++

Question

I have used unions earlier comfortably; today I was alarmed when I read this post and came to know that this code

union ARGB
{
    uint32_t colour;

    str         


        

Answer 1

The most common use of union I regularly come across is aliasing.

Consider the following:

union Vector3f
{
  struct{ float x,y,z ; } ;
  float elts[3];
}

What does this do? It allows clean, neat access of a Vector3f vec;'s members by either name:

vec.x=vec.y=vec.z=1.f ;

or by integer access into the array

for( int i = 0 ; i < 3 ; i++ )
  vec.elts[i]=1.f;

In some cases, accessing by name is the clearest thing you can do. In other cases, especially when the axis is chosen programmatically, the easier thing to do is to access the axis by numerical index - 0 for x, 1 for y, and 2 for z.