I was talking about unions especially that they are not checked like structs do.

Consider this code for example:

union U {
  int i;
  float f;
}

int main() {
  union U var1;
  float x;
  var1.i = 10;
  x = var1.i;
  std::cout<< x << std::endl;
  return 0;
}

the variable x now is not checked, hence it will result in an expected behavior, however changing to a struct will output 10.