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)
Comments
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
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)