As an update to the other answers:
The last draft of C++11, n3242 which is for most practical purposes identical to the actual C++11 standard, says this in 5.6 point 4 (page 118):
For integral operands the / operator yields the algebraic quotient
with any fractional part discarded; (see note 80)
Note 80 states (note that notes are non-normative):
80) This is often called truncation towards zero.
Point 4 goes on to state:
if the quotient a/b is representable in the type of the result,
(a/b)*b + a%b is equal to a.
which can be shown to require the sign of a%b
to be the same as the sign of a
(when not zero).