iterator/doc/iterator_adaptor_body.rst

The ``iterator_adaptor`` class template adapts some ``Base`` [#base]_
type to create a new iterator.  Instantiations of ``iterator_adaptor``
are derived from a corresponding instantiation of ``iterator_facade``
and implement the core behaviors in terms of the ``Base`` type. In
essence, ``iterator_adaptor`` merely forwards all operations to an
instance of the ``Base`` type, which it stores as a member.

.. [#base] The term "Base" here does not refer to a base class and is
   not meant to imply the use of derivation. We have followed the lead
   of the standard library, which provides a base() function to access
   the underlying iterator object of a ``reverse_iterator`` adaptor.

The user of ``iterator_adaptor`` creates a class derived from an
instantiation of ``iterator_adaptor`` and then selectively
redefines some of the core member functions described in the table
above. The ``Base`` type need not meet the full requirements for an
iterator. It need only support the operations used by the core
interface functions of ``iterator_adaptor`` that have not been
redefined in the user's derived class.

Several of the template parameters of ``iterator_adaptor`` default to
``use_default``. This allows the user to make use of a default
parameter even when the user wants to specify a parameter later in the
parameter list.  Also, the defaults for the corresponding associated
types are fairly complicated, so metaprogramming is required to
compute them, and ``use_default`` can help to simplify the
implementation.  Finally, ``use_default`` is not left unspecified
because specification helps to highlight that the ``Reference``
template parameter may not always be identical to the iterator's
``reference`` type, and will keep users making mistakes based on that
assumption.