[a1k0n]

A coworker of mine was just bit badly by Java's insistence that unsigned types are so evil that the language shouldn't have them. He calculated a 32-bit hash, but since the ints are all signed, he took the absolute value before the modulo with the hash table size. That's all well and good, but abs(-2147483648) is still -2147483648 in 32-bit two's complement arithmetic.

I'm sure I don't need to point out that this particular problem had nothing to do with unsigned types (they were signed!). A better rule of thumb is: never use "long" in C/C++ unless you really don't care whether it's 32 or 64 bits.