reinterpret_cast
essentially tells the compiler to ignore all its type-safety and just accept what you are doing.
You are saying that your reference is not a reference to a 32-bit number but to a 64-bit number. That does of course mean (in 8-bits-per-byte system), 4 bytes that could potentially contain any data are read as part of your integer. You also have a "portability" issue related to big-endian systems in particular where the more significant bytes appear first and therefore will yield a different number even if the other bytes do happen to be zero.
Your correct way to perform this is static_cast
and not use references but "pass-by-value".
As it is, you can manage to write foo without any casting at all.
uint64_t foo( uint64_t a, uint64_t b ) { return a + b; }
and you can call it with 32-bit numbers and not worry if they overflow. (Try it with multiplying them).