The iterator type must be a model of .

A bidirectional iterator is an iterator that can read through a sequence of values. It can move in either direction through the sequence, and can be either mutable (data pointed to by it can be changed) or not mutable.

An iterator represents a position in a sequence. Therefore, the iterator can point into the sequence (returning a value when dereferenced and being incrementable), or be off-the-end (and not dereferenceable or incrementable).

The value type of the iterator

The category of the iterator

i
is incrementable (not off-the-end) and some dereferenceable iterator `j` exists such that `i == ++j`
Same as for predecrement
Equivalent to `{Iter j = i; --i; return j;}`
i
is dereferenceable or off-the-end
All iterator operations must take amortized constant time.
&i = &(--i)
i == j
implies `--i == --j`
++i; --i;
and `--i; ++i;` must end up with the value of `i` unmodified, if `i` both of the operations in the pair are valid.