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:Author:
`Dean Michael Berris <mailto:me@deanberris.com>`_
:License:
Distributed under the Boost Software License, Version 1.0
(See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
:Copyright:
Copyright 2012 Google, Inc.
Function Input Iterator
=======================
The Function Input Iterator allows for creating iterators that encapsulate
a nullary function object and a state object which tracks the number of times
the iterator has been incremented. A Function Input Iterator models the
`InputIterator`_ concept and is useful for creating bounded input iterators.
.. _InputIterator: http://www.sgi.com/tech/stl/InputIterator.html
The Function Input Iterator takes a function that models the Generator_ concept
(which is basically a nullary or 0-arity function object). The first dereference
of the iterator at a given position invokes the generator function and stores
and returns the result; subsequent dereferences at the same position simply
return the same stored result. Incrementing the iterator places it at a new
position, hence a subsequent dereference will generate a new value via another
invokation of the generator function. This ensures the generator function is
invoked precisely when the iterator is requested to return a (new) value.
.. _Generator: http://www.sgi.com/tech/stl/Generator.html
The Function Input Iterator encapsulates a state object which models the
`Incrementable Concept`_ and the EqualityComparable_ Concept. These concepts are
described below as:
.. _EqualityComparable: http://www.sgi.com/tech/stl/EqualityComparable.html
Incrementable Concept
---------------------
A type models the Incrementable Concept when it supports the pre- and post-
increment operators. For a given object ``i`` with type ``I``, the following
constructs should be valid:
========= ================= ===========
Construct Description Return Type
-----------------------------------------
i++ Post-increment i. I
++i Pre-increment i. I&
========= ================= ===========
NOTE: An Incrementable type should also be DefaultConstructible_.
.. _DefaultConstructible: http://www.sgi.com/tech/stl/DefaultConstructible.html
Synopsis
--------
::
namespace {
template <class Function, class State>
class function_input_iterator;
template <class Function, class State>
typename function_input_iterator<Function, State>
make_function_input_iterator(Function & f, State s);
struct infinite;
}
Function Input Iterator Class
-----------------------------
The class Function Input Iterator class takes two template parameters
``Function`` and ``State``. These two template parameters tell the
Function Input Iterator the type of the function to encapsulate and
the type of the internal state value to hold.
The ``State`` parameter is important in cases where you want to
control the type of the counter which determines whether two iterators
are at the same state. This allows for creating a pair of iterators which
bound the range of the invocations of the encapsulated functions.
Examples
--------
The following example shows how we use the function input iterator class
in cases where we want to create bounded (lazy) generated ranges.
::
struct generator {
typedef int result_type;
generator() { srand(time(0)); }
result_type operator() () const {
return rand();
}
};
int main(int argc, char * argv[]) {
generator f;
copy(
make_function_input_iterator(f, 0),
make_function_input_iterator(f, 10),
ostream_iterator<int>(cout, " ")
);
return 0;
}
Here we can see that we've bounded the number of invocations using an ``int``
that counts from ``0`` to ``10``. Say we want to create an endless stream
of random numbers and encapsulate that in a pair of integers, we can do
it with the ``boost::infinite`` helper class.
::
copy(
make_function_input_iterator(f,infinite()),
make_function_input_iterator(f,infinite()),
ostream_iterator<int>(cout, " ")
);
Above, instead of creating a huge vector we rely on the STL copy algorithm
to traverse the function input iterator and call the function object f
as it increments the iterator. The special property of ``boost::infinite``
is that equating two instances always yield false -- and that incrementing
an instance of ``boost::infinite`` doesn't do anything. This is an efficient
way of stating that the iterator range provided by two iterators with an
encapsulated infinite state will definitely be infinite.
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