From ISO14882:2011(e) 5.6-4:
The binary / operator yields the quotient, and the binary % operator
yields the remainder from the division of the first expression by the
second. If the second operand of / or % is zero the behavior is
undefined. For integral operands the / operator yields the algebraic quotient with any fractional part discarded; if the quotient a/b is
representable in the type of the result, (a/b)*b + a%b is equal to a.
The rest is basic math:
(-7 / 3) => -2
-2 * 3 => -6
so a % b => -1
(7 / -3) => -2
-2 * -3 => 6
so a % b => 1
Note that
If both operands are nonnegative then the remainder is nonnegative; if
not, the sign of the remainder is implementation-defined.
from ISO14882:2003(e) is no longer present in ISO14882:2011(e)