Re: [Boost-users] iterators

From: Jeremy Maitin-Shepard (jbms_at_[hidden])
Date: 2003-10-05 01:33:04

Witz <witz_at_[hidden]> writes:
> Firstly, is it really necessary to have the predicate as part of the template
> specification for the filter_iterator? Surely it can be assumed that it will
> always be convertible to boost::function<bool (const value_type&)> ?

There is significant run-time overhead when using boost::function,
including a virtual function call.  You can, of course, manually specify
boost::function<bool (const value_type &)> as a the function type.  Even
if the template parameters for filter_iterator were reversed, so that it
would be possible to specify a default value for Predicate, setting it
to a boost::function type would require that boost.function be a
dependency of filter iterator, even if the default value is not used,
and would encourage (inefficient) use of boost::function even when the
actual function type is known.

> Secondly, by allowing the return type of the function used by the
> transform_iterator to be a reference can we not use it to implement a more
> generic form of indirect_iterator?

If the life-time of the value to which the returned reference refers is
longer than that of the reference itself, then you can use
projection_iterator, which is also a random access iterator.
transform_iterator is designed specifically for the case that the
returned value is not stored somewhere else, and consequently is only an
input iterator.  (This distinction is of little importance, however, in
the new iterator adaptors library, which David Abrahams has proposed for
standardization.)

-- 
Jeremy Maitin-Shepard