A Forward Container is a Container whose
elements are arranged in a definite order: the ordering will not
change spontaneously from iteration to iteration. The requirement of
a definite ordering allows the definition of element-by-element
equality (if the container's element type is
Equality Comparable) and of
lexicographical ordering (if the container's element type is
LessThan Comparable).
Iterators into a Forward Container satisfy the
forward iterator requirements;
consequently, Forward Containers support multipass algorithms and
allow multiple iterators into the same container to be active at the
same time.
No additional types beyond those defined in
Container. However, the requirements for
the iterator type are strengthened: the iterator type must be a model
of Forward Iterator.
The equality and inequality operations are linear in the
container's size.
Invariants
Ordering
Two different iterations through a forward container will access
its elements in the same order, providing that there have been
no intervening mutative operations.