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<title>Filter Iterator Adaptor Documentation</title>
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<h1>Filter Iterator Adaptor</h1>
Defined in header
<a href="../../boost/iterator_adaptors.hpp">boost/iterator_adaptors.hpp</a>
<p>
The filter iterator adaptor creates a view of an iterator range in
which some elements of the range are skipped over. A <a
href="http://www.sgi.com/tech/stl/Predicate.html">Predicate</a>
function object controls which elements are skipped. When the
predicate is applied to an element, if it returns <tt>true</tt> then
the element is retained and if it returns <tt>false</tt> then the
element is skipped over.
<h2>Synopsis</h2>
<pre>
namespace boost {
template <class Predicate, class BaseIterator, ...>
class filter_iterator_generator;
template <class Predicate, class BaseIterator>
typename filter_iterator_generator<Predicate, BaseIterator>::type
make_filter_iterator(BaseIterator first, BaseIterator last, const Predicate& p = Predicate());
}
</pre>
<hr>
<h2><a name="filter_iterator_generator">The Filter Iterator Type
Generator</a></h2>
The class <tt>filter_iterator_generator</tt> is a helper class whose
purpose is to construct a filter iterator type. The template
parameters for this class are the <tt>Predicate</tt> function object
type and the <tt>BaseIterator</tt> type that is being wrapped. In
most cases the associated types for the wrapped iterator can be
deduced from <tt>std::iterator_traits</tt>, but in some situations the
user may want to override these types, so there are also template
parameters for each of the iterator's associated types.
<pre>
template <class Predicate, class BaseIterator,
class Value, class Reference, class Pointer, class Category, class Distance>
class filter_iterator_generator
{
public:
typedef <tt><a href="./iterator_adaptors.htm#iterator_adaptor">iterator_adaptor</a><...></tt> type; // the resulting filter iterator type
}
</pre>
<h3>Example</h3>
The following example uses filter iterator to print out all the
positive integers in an array.
<pre>
struct is_positive_number {
bool operator()(int x) { return 0 < x; }
};
int main() {
int numbers[] = { 0, -1, 4, -3, 5, 8, -2 };
const int N = sizeof(numbers)/sizeof(int);
typedef boost::filter_iterator_generator<is_positive_number, int*, int>::type FilterIter;
is_positive_number predicate;
FilterIter::policies_type policies(predicate, numbers + N);
FilterIter filter_iter_first(numbers, policies);
FilterIter filter_iter_last(numbers + N, policies);
std::copy(filter_iter_first, filter_iter_last, std::ostream_iterator<int>(std::cout, " "));
std::cout << std::endl;
return 0;
}
</pre>
The output is:
<pre>
4 5 8
</pre>
<h3>Template Parameters</h3>
<Table border>
<TR>
<TH>Parameter</TH><TH>Description</TH>
</TR>
<TR>
<TD><a href="http://www.sgi.com/tech/stl/Predicate.html"><tt>Predicate</tt></a></TD>
<TD>The function object that determines which elements are retained and which elements are skipped.
</TR>
<TR>
<TD><tt>BaseIterator</tt></TD>
<TD>The iterator type being wrapped. This type must at least be a model
of the <a href="http://www.sgi.com/tech/stl/InputIterator">InputIterator</a> concept.</TD>
</TR>
<TR>
<TD><tt>Value</tt></TD>
<TD>The <tt>value_type</tt> of the resulting iterator,
unless const. If const, a conforming compiler strips constness for the
<tt>value_type</tt>. Typically the default for this parameter is the
appropriate type<a href="#1">[1]</a>.<br> <b>Default:</b>
<tt>std::iterator_traits<BaseIterator>::value_type</TD>
</TR>
<TR>
<TD><tt>Reference</tt></TD>
<TD>The <tt>reference</tt> type of the resulting iterator, and in
particular, the result type of <tt>operator*()</tt>. Typically the default for
this parameter is the appropriate type.<br> <b>Default:</b> If
<tt>Value</tt> is supplied, <tt>Value&</tt> is used. Otherwise
<tt>std::iterator_traits<BaseIterator>::reference</tt> is
used.</TD>
</TR>
<TR>
<TD><tt>Pointer</tt></TD>
<TD>The <tt>pointer</tt> type of the resulting iterator, and in
particular, the result type of <tt>operator->()</tt>.
Typically the default for
this parameter is the appropriate type.<br>
<b>Default:</b> If <tt>Value</tt> was supplied, then <tt>Value*</tt>,
otherwise <tt>std::iterator_traits<BaseIterator>::pointer</tt>.</TD>
</TR>
<TR>
<TD><tt>Category</tt></TD>
<TD>The <tt>iterator_category</tt> type for the resulting iterator.
Typically the
default for this parameter is the appropriate type. If you override
this parameter, do not use <tt>bidirectional_iterator_tag</tt>
because filter iterators can not go in reverse.<br>
<b>Default:</b> <tt>std::iterator_traits<BaseIterator>::iterator_category</tt></TD>
</TR>
<TR>
<TD><tt>Distance</tt></TD>
<TD>The <tt>difference_type</tt> for the resulting iterator. Typically the default for
this parameter is the appropriate type.<br>
<b>Default:</b> <tt>std::iterator_traits<BaseIterator>::difference_type</TD>
</TR>
</table>
<h3>Model of</h3>
The filter iterator adaptor (the type
<tt>filter_iterator_generator<...>::type</tt>) may be a model of <a
href="http://www.sgi.com/tech/stl/InputIterator.html">InputIterator</a> or <a
href="http://www.sgi.com/tech/stl/ForwardIterator.html">ForwardIterator</a>
depending on the adapted iterator type.
<h3>Members</h3>
The filter iterator type implements all of the member functions and
operators required of the <a
href="http://www.sgi.com/tech/stl/ForwardIterator.html">ForwardIterator</a>
concept. In addition it has the following constructor:
<pre>filter_iterator_generator::type(const BaseIterator& it, const Policies& p = Policies())</pre>
<p>
The policies type has only one public function, which is its constructor:
<pre>filter_iterator_generator::policies_type(const Predicate& p, const BaseIterator& end)</pre>
<p>
<hr>
<p>
<h2><a name="make_filter_iterator">The Make Filter Iterator Function</a></h2>
<pre>
template <class Predicate, class BaseIterator>
typename filter_generator<Predicate, BaseIterator>::type
make_filter_iterator(BaseIterator first, BaseIterator last, const Predicate& p = Predicate())
</pre>
This function provides a convenient way to create filter iterators.
<h3>Example</h3>
In this example we print out all numbers in the array that are
greater than negative two.
<pre>
int main()
{
int numbers[] = { 0, -1, 4, -3, 5, 8, -2 };
const int N = sizeof(numbers)/sizeof(int);
std::copy(boost::make_filter_iterator(numbers, numbers + N,
std::bind2nd(std::greater<int>(), -2)),
boost::make_filter_iterator(numbers + N, numbers + N,
std::bind2nd(std::greater<int>(), -2)),
std::ostream_iterator<int>(std::cout, " "));
std::cout << std::endl;
}
</pre>
The output is:
<pre>
0 -1 4 5 8
</pre>
<p>
In the next example we print the positive numbers using the
<tt>make_filter_iterator()</tt> function.
<pre>
struct is_positive_number {
bool operator()(int x) { return 0 < x; }
};
int main()
{
int numbers[] = { 0, -1, 4, -3, 5, 8, -2 };
const int N = sizeof(numbers)/sizeof(int);
std::copy(boost::make_filter_iterator<is_positive_number>(numbers, numbers + N),
boost::make_filter_iterator<is_positive_number>(numbers + N, numbers + N),
std::ostream_iterator<int>(std::cout, " "));
std::cout << std::endl;
return 0;
}
</pre>
The output is:
<pre>
4 5 8
</pre>
<h3>Notes</h3>
<a name="1">[1]</a> If the compiler does not support partial
specialization and the wrapped iterator type is a builtin pointer then
the <tt>Value</tt> type must be explicitly specified (don't use the
default).
<hr>
<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %b %Y" startspan -->09 Mar 2001<!--webbot bot="Timestamp" endspan i-checksum="14894" --></p>
<p> Copyright Jeremy Siek 2000. Permission to copy, use,
modify, sell and distribute this document is granted provided this copyright
notice appears in all copies. This document is provided "as is"
without express or implied warranty, and with no claim as to its suitability for
any purpose.</p>
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