As written, it is UB, yes, but certainly in C++, and, I think, also in C, using a union is undefined behavior, too. I think (assuming isn’t and float to be of the same size) the main risk is that, if you do

   union {
     float f;
     int i;
   } foo;
   foo.f = 3.14;
   printf(“%x”, foo.i);

that the compiler can think the assignment to foo.f isn’t used anywhere, and thus can chose not to do it.

In C++, you have to use memmove (compilers can and often do recognize that idiom)

Yes, it violates the standard, although in practice it should work because the alignment is the same.