Can you assign the value of one union member to another?

Question

Consider the following code snippet:

union
{
   int a;
   float b;
};

a = /* ... */;

b = a;               // is this UB?
b = b + something;

I

Answer 1

I would expect that unless the source and destination variables are the same type, such a thing would be Undefined Behavior in C, and I see no reason to expect C++ to handle it any differently. Given long long *x,*y;, some compilers might process a statement like *x = *y >>8; by generating code to read all of *y, compute the result, and store it to *x, but a compiler might perfectly legitimately write code that copied parts of *y to *x individually. The standard makes clear that if *x and *y are pointers to the same object of the same type, the compiler must ensure that no part of the value gets overwritten while that part is still needed in the computation, but compilers are not required to deal with aliasing in other situations.