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143 lines
6.7 KiB
HTML
143 lines
6.7 KiB
HTML
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<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="generator" content="Docutils 0.6: http://docutils.sourceforge.net/" />
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<title></title>
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<meta name="author" content="Dean Michael Berris" />
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<link rel="stylesheet" href="../../../rst.css" type="text/css" />
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</head>
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<body>
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<div class="document">
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<table class="docinfo" frame="void" rules="none">
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td><a class="first reference external" href="mailto:mikhailberis@gmail.com">Dean Michael Berris</a></td></tr>
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<tr class="field"><th class="docinfo-name">License:</th><td class="field-body">Distributed under the Boost Software License, Version 1.0
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(See accompanying file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</td>
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</tr>
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</tbody>
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</table>
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<div class="section" id="function-input-iterator">
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<h1>Function Input Iterator</h1>
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<p>The Function Input Iterator allows for creating iterators that encapsulate
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a nullary function object and a state object which tracks the number of times
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the iterator has been incremented. A Function Input Iterator models the
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<a class="reference external" href="http://www.sgi.com/tech/stl/InputIterator.html">InputIterator</a> concept and is useful for creating bounded input iterators.</p>
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<p>Like the Generator Iterator, the Function Input Iterator takes a function
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that models the <a class="reference external" href="http://www.sgi.com/tech/stl/Generator.html">Generator</a> concept (which is basically a nullary or 0-arity
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function object). Each increment of the function Function Input Iterator
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invokes the generator function and stores the value in the iterator. When
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the iterator is dereferenced the stored value is returned.</p>
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<p>The Function Input Iterator encapsulates a state object which models the
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<a class="reference internal" href="#incrementable-concept">Incrementable Concept</a> and the <a class="reference external" href="http://www.sgi.com/tech/stl/EqualityComparable.html">EqualityComparable</a> Concept. These concepts are
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described below as:</p>
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<div class="section" id="incrementable-concept">
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<h2>Incrementable Concept</h2>
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<p>A type models the Incrementable Concept when it supports the pre- and post-
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increment operators. For a given object <tt class="docutils literal"><span class="pre">i</span></tt> with type <tt class="docutils literal"><span class="pre">I</span></tt>, the following
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constructs should be valid:</p>
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<table border="1" class="docutils">
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<colgroup>
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<col width="24%" />
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<col width="46%" />
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<col width="30%" />
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</colgroup>
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<tbody valign="top">
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<tr><td colspan="3">Construct Description Return Type</td>
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</tr>
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<tr><td>i++</td>
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<td>Post-increment i.</td>
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<td>I</td>
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</tr>
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<tr><td>++i</td>
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<td>Pre-increment i.</td>
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<td>I&</td>
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</tr>
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</tbody>
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</table>
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<p>NOTE: An Incrementable type should also be <a class="reference external" href="http://www.sgi.com/tech/stl/DefaultConstructible.html">DefaultConstructible</a>.</p>
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</div>
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<div class="section" id="synopsis">
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<h2>Synopsis</h2>
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<pre class="literal-block">
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namespace {
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template <class Function, class State>
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class function_input_iterator;
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template <class Function, class State>
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typename function_input_iterator<Function, State>
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make_function_input_iterator(Function & f);
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struct infinite;
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}
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</pre>
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</div>
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<div class="section" id="function-input-iterator-class">
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<h2>Function Input Iterator Class</h2>
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<p>The class Function Input Iterator class takes two template parameters
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<tt class="docutils literal"><span class="pre">Function</span></tt> and <tt class="docutils literal"><span class="pre">State</span></tt>. These two template parameters tell the
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Function Input Iterator the type of the function to encapsulate and
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the type of the internal state value to hold.</p>
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<p>The <tt class="docutils literal"><span class="pre">State</span></tt> parameter is important in cases where you want to
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control the type of the counter which determines whether two iterators
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are at the same state. This allows for creating a pair of iterators which
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bound the range of the invocations of the encapsulated functions.</p>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<p>The following example shows how we use the function input iterator class
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in cases where we want to create bounded (lazy) generated ranges.</p>
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<pre class="literal-block">
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struct generator {
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typedef int result_type;
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generator() { srand(time(0)); }
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result_type operator() () const {
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return rand();
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}
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};
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int main(int argc, char * argv[]) {
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generator f;
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copy(
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make_function_input_iterator(f, 0),
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make_function_input_iterator(f, 10),
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ostream_iterator<int>(cout, " ")
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);
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return 0;
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}
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</pre>
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<p>Here we can see that we've bounded the number of invocations using an <tt class="docutils literal"><span class="pre">int</span></tt>
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that counts from <tt class="docutils literal"><span class="pre">0</span></tt> to <tt class="docutils literal"><span class="pre">10</span></tt>. Say we want to create an endless stream
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of random numbers and encapsulate that in a pair of integers, we can do
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it with the <tt class="docutils literal"><span class="pre">boost::infinite</span></tt> helper class.</p>
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<pre class="literal-block">
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copy(
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make_function_input_iterator(f,infinite()),
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make_function_input_iterator(f,infinite()),
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ostream_iterator<int>(count, " ")
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);
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</pre>
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<p>Above, instead of creating a huge vector we rely on the STL copy algorithm
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to traverse the function input iterator and call the function object f
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as it increments the iterator. The special property of <tt class="docutils literal"><span class="pre">boost::infinite</span></tt>
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is that equating two instances always yield false -- and that incrementing
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an instance of <tt class="docutils literal"><span class="pre">boost::infinite</span></tt> doesn't do anything. This is an efficient
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way of stating that the iterator range provided by two iterators with an
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encapsulated infinite state will definitely be infinite.</p>
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</div>
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</div>
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</div>
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<div class="footer">
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<hr class="footer" />
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<a class="reference external" href="function_input_iterator.rst">View document source</a>.
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Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source.
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</div>
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</body>
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</html>
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