Why is X % 0 an invalid expression?
I always thought X % 0 should equal X. Since you can't divide by zero, shouldn't the answer naturally be the remainder, X (everything left over)?
The C++ Standard(2003) says in §5.6/4,
[...] If the second operand of / or % is zero the behavior is undefined; [...]
That is, following expressions invoke undefined-behavior(UB):
X / 0; //UB
X % 0; //UB
Note also that -5 % 2 is NOT equal to -(5 % 2) (as Petar seems to suggest in his comment to his answer). It's implementation-defined. The spec says (§5.6/4),
[...] If both operands are nonnegative then the remainder is nonnegative; if not, the sign of the remainder is implementation-defined.