double x = 3.9
(double) 3.9000000

sizeof(x)
(unsigned long) 8

x & 0xFF
error: invalid operands to binary expression ('double' and 'int')
x & 0xFF
~ ^ ~~~~

static_cast<uint64_t>(x) & 0xFF
(unsigned long) 3

*reinterpret_cast<uint64_t*>(&x) 
(unsigned long) 4615964438073389875

*reinterpret_cast<uint64_t*>(&x) & 0xFF
(unsigned long) 51