libs/spirit/doc/html/spirit/support/multi_pass.html


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<html>
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<title>The multi pass iterator</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="spirit.support.multi_pass"></a><a class="link" href="multi_pass.html" title="The multi pass iterator">The multi pass iterator</a>
</h3></div></div></div>
<p>
        Backtracking in <span class="emphasis"><em>Spirit.Qi</em></span> requires the use of the following
        types of iterator: forward, bidirectional, or random access. Because of backtracking,
        input iterators cannot be used. Therefore, the standard library classes
        <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istreambuf_iterator</span></code> and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span></code>,
        that fall under the category of input iterators, cannot be used. Another
        input iterator that is of interest is one that wraps a lexer, such as LEX.
      </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
          In general, <span class="emphasis"><em>Spirit.Qi</em></span> generates recursive descent
          parser which require backtracking parsers by design. For this reason we
          need to provide at least forward iterators to any of <span class="emphasis"><em>Spirit.Qi</em></span>'s
          API functions. This is not an absolute requirement though. In the future,
          we shall see more deterministic parsers that require no more than 1 character
          (token) of lookahead. Such parsers allow us to use input iterators such
          as the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span></code> as is.
        </p></td></tr>
</table></div>
<p>
        Backtracking can be implemented only if we are allowed to save an iterator
        position, i.e. making a copy of the current iterator. Unfortunately, with
        an input iterator, there is no way to do so, and thus input iterators will
        not work with backtracking in <span class="emphasis"><em>Spirit.Qi</em></span>. One solution
        to this problem is to simply load all the data to be parsed into a container,
        such as a vector or deque, and then pass the begin and end of the container
        to <span class="emphasis"><em>Spirit.Qi</em></span>. This method can be too memory intensive
        for certain applications, which is why the <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        iterator was created.
      </p>
<h5>
<a name="spirit.support.multi_pass.h0"></a>
        <span><a name="spirit.support.multi_pass.using_the_multi_pass"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.using_the_multi_pass">Using
        the multi_pass</a>
      </h5>
<p>
        The <code class="computeroutput"><span class="identifier">multi_pass</span></code> iterator will
        convert any input iterator into a forward iterator suitable for use with
        <span class="emphasis"><em>Spirit.Qi</em></span>. <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        will buffer data when needed and will discard the buffer when its contents
        is not needed anymore. This happens either if only one copy of the iterator
        exists or if no backtracking can occur.
      </p>
<p>
        A grammar must be designed with care if the <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        iterator is used. Any rule that may need to backtrack, such as one that contains
        an alternative, will cause data to be buffered. The rules that are optimal
        to use are repetition constructs (as kleene and plus).
      </p>
<p>
        Sequences of the form <code class="computeroutput"><span class="identifier">a</span> <span class="special">&gt;&gt;</span> <span class="identifier">b</span></code>
        will buffer data as well. This is different from the behavior of <a href="../../../../../../libs/spirit/classic/index.html" target="_top"><span class="emphasis"><em>Spirit.Classic</em></span></a>
        but for a good reason. Sequences need to reset the current iterator to its
        initial state if one of the components of a sequence fails to match. To compensate
        for this behavior we added functionality to the <code class="computeroutput"><span class="identifier">expect</span></code>
        parsers (i.e. constructs like <code class="computeroutput"><span class="identifier">a</span>
        <span class="special">&gt;</span> <span class="identifier">b</span></code>).
        Expectation points introduce deterministic points into the grammar ensuring
        no backtracking can occur if they match. For this reason we clear the buffers
        of any multi_pass iterator on each expectation point, ensuring minimal buffer
        content even for large grammars.
      </p>
<div class="important"><table border="0" summary="Important">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../images/important.png"></td>
<th align="left">Important</th>
</tr>
<tr><td align="left" valign="top">
<p>
          If you use an error handler in conjunction with the <code class="computeroutput"><span class="identifier">expect</span></code>
          parser while utilizing a <code class="computeroutput"><span class="identifier">multi_pass</span></code>
          iterator and you intend to use the error handler to force a <code class="computeroutput"><span class="identifier">retry</span></code> or a <code class="computeroutput"><span class="identifier">fail</span></code>
          (see the description of error handlers - <span class="bold"><strong>FIXME</strong></span>:
          insert link), then you need to instantiate the error handler using <code class="computeroutput"><span class="identifier">retry</span></code> or <code class="computeroutput"><span class="identifier">fail</span></code>,
          for instance:
</p>
<pre class="programlisting"><span class="identifier">rule</span> <span class="identifier">r</span><span class="special">&lt;</span><span class="identifier">iterator_type</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">;</span>
<span class="identifier">on_error</span><span class="special">&lt;</span><span class="identifier">retry</span><span class="special">&gt;(</span><span class="identifier">r</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">val</span><span class="special">(</span><span class="string">"Error!"</span><span class="special">));</span>
</pre>
<p>
          If you fail to do so the resulting code will trigger an assert statement
          at runtime.
        </p>
</td></tr>
</table></div>
<p>
        Any rule that repeats, such as kleene_star (<code class="computeroutput"><span class="special">*</span><span class="identifier">a</span></code>) or positive such as (<code class="computeroutput"><span class="special">+</span><span class="identifier">a</span></code>), will only buffer the data for the current
        repetition.
      </p>
<p>
        In typical grammars, ambiguity and therefore lookahead is often localized.
        In fact, many well designed languages are fully deterministic and require
        no lookahead at all. Peeking at the first character from the input will immediately
        determine the alternative branch to take. Yet, even with highly ambiguous
        grammars, alternatives are often of the form <code class="computeroutput"><span class="special">*(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span>
        <span class="special">|</span> <span class="identifier">c</span> <span class="special">|</span> <span class="identifier">d</span><span class="special">)</span></code>.
        The input iterator moves on and is never stuck at the beginning. Let's look
        at a Pascal snippet for example:
      </p>
<pre class="programlisting"><span class="identifier">program</span> <span class="special">=</span>
        <span class="identifier">programHeading</span> <span class="special">&gt;&gt;</span> <span class="identifier">block</span> <span class="special">&gt;&gt;</span> <span class="char">'.'</span>
    <span class="special">;</span>

<span class="identifier">block</span> <span class="special">=</span>
       <span class="special">*(</span>   <span class="identifier">labelDeclarationPart</span>
        <span class="special">|</span>   <span class="identifier">constantDefinitionPart</span>
        <span class="special">|</span>   <span class="identifier">typeDefinitionPart</span>
        <span class="special">|</span>   <span class="identifier">variableDeclarationPart</span>
        <span class="special">|</span>   <span class="identifier">procedureAndFunctionDeclarationPart</span>
        <span class="special">)</span>
    <span class="special">&gt;&gt;</span>  <span class="identifier">statementPart</span>
    <span class="special">;</span>
</pre>
<p>
        Notice the alternatives inside the Kleene star in the rule block . The rule
        gobbles the input in a linear manner and throws away the past history with
        each iteration. As this is fully deterministic LL(1) grammar, each failed
        alternative only has to peek 1 character (token). The alternative that consumes
        more than 1 character (token) is definitely a winner. After which, the Kleene
        star moves on to the next.
      </p>
<p>
        Now, after the lecture on the features to be careful with when using <code class="computeroutput"><span class="identifier">multi_pass</span></code>, you may think that <code class="computeroutput"><span class="identifier">multi_pass</span></code> is way too restrictive to use.
        That's not the case. If your grammar is deterministic, you can make use of
        the <code class="computeroutput"><span class="identifier">flush_multi_pass</span></code> pseudo
        parser in your grammar to ensure that data is not buffered when unnecessary
        (<code class="computeroutput"><span class="identifier">flush_multi_pass</span></code> is available
        from the <span class="emphasis"><em>Spirit.Qi</em></span> parser <a href="../../../../repository/doc/html/index.html" target="_top">Repository</a>).
      </p>
<p>
        Here we present a minimal example showing a minimal use case. The <code class="computeroutput"><span class="identifier">multi_pass</span></code> iterator is highly configurable,
        but the default policies have been chosen so that its easily usable with
        input iterators such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istreambuf_iterator</span></code>.
        For the complete source code of this example please refer to <a href="../../../../example/support/multi_pass.cpp" target="_top">multi_pass.cpp</a>.
      </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
    <span class="keyword">namespace</span> <span class="identifier">spirit</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">;</span>
    <span class="keyword">using</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">space</span><span class="special">;</span>
    <span class="keyword">using</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</span><span class="special">;</span>
    <span class="keyword">using</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">double_</span><span class="special">;</span>
    <span class="keyword">using</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">eol</span><span class="special">;</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">ifstream</span> <span class="identifier">in</span><span class="special">(</span><span class="string">"multi_pass.txt"</span><span class="special">);</span>    <span class="comment">// we get our input from this file</span>
    <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">in</span><span class="special">.</span><span class="identifier">is_open</span><span class="special">())</span> <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Could not open input file: 'multi_pass.txt'"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">istreambuf_iterator</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="identifier">base_iterator_type</span><span class="special">;</span>
    <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">multi_pass</span><span class="special">&lt;</span><span class="identifier">base_iterator_type</span><span class="special">&gt;</span> <span class="identifier">first</span> <span class="special">=</span>
        <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">make_default_multi_pass</span><span class="special">(</span><span class="identifier">base_iterator_type</span><span class="special">(</span><span class="identifier">in</span><span class="special">));</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">v</span><span class="special">;</span>
    <span class="keyword">bool</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">first</span>
      <span class="special">,</span> <span class="identifier">spirit</span><span class="special">::</span><span class="identifier">make_default_multi_pass</span><span class="special">(</span><span class="identifier">base_iterator_type</span><span class="special">())</span>
      <span class="special">,</span> <span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="special">*(</span><span class="char">','</span> <span class="special">&gt;&gt;</span> <span class="identifier">double_</span><span class="special">)</span>              <span class="comment">// recognize list of doubles</span>
      <span class="special">,</span> <span class="identifier">space</span> <span class="special">|</span> <span class="char">'#'</span> <span class="special">&gt;&gt;</span> <span class="special">*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="identifier">eol</span><span class="special">)</span> <span class="special">&gt;&gt;</span> <span class="identifier">eol</span>      <span class="comment">// comment skipper</span>
      <span class="special">,</span> <span class="identifier">v</span><span class="special">);</span>                                       <span class="comment">// data read from file</span>

    <span class="keyword">if</span> <span class="special">(!</span><span class="identifier">result</span><span class="special">)</span> <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Failed parsing input file!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="special">-</span><span class="number">2</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Successfully parsed input file!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
      </p>
<h5>
<a name="spirit.support.multi_pass.h1"></a>
        <span><a name="spirit.support.multi_pass.using_the_flush_multi_pass_parser"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.using_the_flush_multi_pass_parser">Using
        the flush_multi_pass parser</a>
      </h5>
<p>
        The <a href="http://boost-spirit.com" target="_top">Spirit</a> <a href="../../../../repository/doc/html/index.html" target="_top">Repository</a>
        contains the <code class="computeroutput"><span class="identifier">flush_multi_pass</span></code>
        parser component. This is usable in conjunction with the <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        iterator to minimize the buffering. It allows to insert explicit synchronization
        points into your grammar where it is safe to clear any stored input as it
        is ensured that no backtracking can occur at this point anymore.
      </p>
<p>
        When the <code class="computeroutput"><span class="identifier">flush_multi_pass</span></code>
        parser is used with <code class="computeroutput"><span class="identifier">multi_pass</span></code>,
        it will call <code class="computeroutput"><span class="identifier">multi_pass</span><span class="special">::</span><span class="identifier">clear_queue</span><span class="special">()</span></code>.
        This will cause any buffered data to be erased. This also will invalidate
        all other copies of multi_pass and they should not be used. If they are,
        an <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">illegal_backtracking</span></code> exception will be
        thrown.
      </p>
<h5>
<a name="spirit.support.multi_pass.h2"></a>
        <span><a name="spirit.support.multi_pass.the_multi_pass_policies"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.the_multi_pass_policies">The
        multi_pass Policies</a>
      </h5>
<p>
        The <code class="computeroutput"><span class="identifier">multi_pass</span></code> iterator is
        a templated class configurable using policies. The description of <code class="computeroutput"><span class="identifier">multi_pass</span></code> above is how it was originally
        implemented (before it used policies), and is the default configuration now.
        But, <code class="computeroutput"><span class="identifier">multi_pass</span></code> is capable
        of much more. Because of the open-ended nature of policies, you can write
        your own policy to make <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        behave in a way that we never before imagined.
      </p>
<p>
        The multi_pass class has two template parameters:
      </p>
<div class="variablelist">
<p class="title"><b>The multi_pass template parameters</b></p>
<dl>
<dt><span class="term">Input</span></dt>
<dd><p>
              The type multi_pass uses to acquire it's input. This is typically an
              input iterator, or functor.
            </p></dd>
<dt><span class="term">Policies</span></dt>
<dd><p>
              The combined policies to use to create an instance of a multi_pass
              iterator. This combined policy type is described below
            </p></dd>
</dl>
</div>
<p>
        It is possible to implement all of the required functionality of the combined
        policy in a single class. But it has shown to be more convenient to split
        this into four different groups of functions, i.e. four separate, but well
        coordinated policies. For this reason the <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        library implements a template <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">default_policy</span></code>
        allowing to combine several different policies, each implementing one of
        the functionality groups:
      </p>
<div class="table">
<a name="spirit.support.multi_pass.policies_needed_for_default_policy_template"></a><p class="title"><b>Table&#160;12.&#160;Policies needed for default_policy template</b></p>
<div class="table-contents"><table class="table" summary="Policies needed for default_policy template">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Template Parameter
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">OwnershipPolicy</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy determines how <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  deals with it's shared components.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">CheckingPolicy</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy determines how checking for invalid iterators is done.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">InputPolicy</span></code>
                </p>
              </td>
<td>
                <p>
                  A class that defines how <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  acquires its input. The <code class="computeroutput"><span class="identifier">InputPolicy</span></code>
                  is parameterized by the <code class="computeroutput"><span class="identifier">Input</span></code>
                  template parameter to the <code class="computeroutput"><span class="identifier">multi_pass</span></code>.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">StoragePolicy</span></code>
                </p>
              </td>
<td>
                <p>
                  The buffering scheme used by <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  is determined and managed by the StoragePolicy.
                </p>
              </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
        The <code class="computeroutput"><span class="identifier">multi_pass</span></code> library contains
        several predefined policy implementations for each of the policy types as
        described above. First we will describe those predefined types. Afterwards
        we will give some guidelines how you can write your own policy implementations.
      </p>
<h5>
<a name="spirit.support.multi_pass.h3"></a>
        <span><a name="spirit.support.multi_pass.predefined_policies"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.predefined_policies">Predefined
        policies</a>
      </h5>
<p>
        All predefined <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        policies are defined in the namespace <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">iterator_policies</span></code>.
      </p>
<div class="table">
<a name="spirit.support.multi_pass.predefined_policy_classes"></a><p class="title"><b>Table&#160;13.&#160;Predefined policy classes</b></p>
<div class="table-contents"><table class="table" summary="Predefined policy classes">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Class name
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <span class="bold"><strong>InputPolicy</strong></span> classes
                </p>
              </td>
<td class="auto-generated">&#160;</td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">input_iterator</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy directs <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  to read from an input iterator of type <code class="computeroutput"><span class="identifier">Input</span></code>.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">buffering_input_iterator</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy directs <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  to read from an input iterator of type <code class="computeroutput"><span class="identifier">Input</span></code>.
                  Additionally it buffers the last character received from the underlying
                  iterator. This allows to wrap iterators not buffering the last
                  character on their own (as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istreambuf_iterator</span></code>).
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">istream</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy directs <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  to read from an input stream of type <code class="computeroutput"><span class="identifier">Input</span></code>
                  (usually a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span></code>).
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">lex_input</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy obtains it's input by calling yylex(), which would
                  typically be provided by a scanner generated by <a href="http://flex.sourceforge.net/" target="_top">Flex</a>.
                  If you use this policy your code must link against a <a href="http://flex.sourceforge.net/" target="_top">Flex</a>
                  generated scanner.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">functor_input</span></code>
                </p>
              </td>
<td>
                <p>
                  This input policy obtains it's data by calling a functor of type
                  <code class="computeroutput"><span class="identifier">Input</span></code>. The functor
                  must meet certain requirements. It must have a typedef called
                  <code class="computeroutput"><span class="identifier">result_type</span></code> which
                  should be the type returned from <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. Also, since an input policy
                  needs a way to determine when the end of input has been reached,
                  the functor must contain a static variable named <code class="computeroutput"><span class="identifier">eof</span></code> which is comparable to a
                  variable of <code class="computeroutput"><span class="identifier">result_type</span></code>.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">split_functor_input</span></code>
                </p>
              </td>
<td>
                <p>
                  This is essentially the same as the <code class="computeroutput"><span class="identifier">functor_input</span></code>
                  policy except that the (user supplied) function object exposes
                  separate <code class="computeroutput"><span class="identifier">unique</span></code>
                  and <code class="computeroutput"><span class="identifier">shared</span></code> sub
                  classes, allowing to integrate the functors <span class="emphasis"><em>unique</em></span>
                  data members with the <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  data items held by each instance and its <span class="emphasis"><em>shared</em></span>
                  data members will be integrated with the <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  members shared by all copies.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <span class="bold"><strong>OwnershipPolicy</strong></span> classes
                </p>
              </td>
<td class="auto-generated">&#160;</td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">ref_counted</span></code>
                </p>
              </td>
<td>
                <p>
                  This class uses a reference counting scheme. The <code class="computeroutput"><span class="identifier">multi_pass</span></code> will delete it's shared
                  components when the count reaches zero.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">first_owner</span></code>
                </p>
              </td>
<td>
                <p>
                  When this policy is used, the first <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  created will be the one that deletes the shared data. Each copy
                  will not take ownership of the shared data. This works well for
                  <a href="http://boost-spirit.com" target="_top">Spirit</a>, since no dynamic
                  allocation of iterators is done. All copies are made on the stack,
                  so the original iterator has the longest lifespan.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <span class="bold"><strong>CheckingPolicy</strong></span> classes
                </p>
              </td>
<td class="auto-generated">&#160;</td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">no_check</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy does no checking at all.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">buf_id_check</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy keeps around a buffer id, or a buffer age. Every time
                  <code class="computeroutput"><span class="identifier">clear_queue</span><span class="special">()</span></code>
                  is called on a <code class="computeroutput"><span class="identifier">multi_pass</span></code>
                  iterator, it is possible that all other iterators become invalid.
                  When <code class="computeroutput"><span class="identifier">clear_queue</span><span class="special">()</span></code> is called, <code class="computeroutput"><span class="identifier">buf_id_check</span></code>
                  increments the buffer id. When an iterator is dereferenced, this
                  policy checks that the buffer id of the iterator matches the shared
                  buffer id. This policy is most effective when used together with
                  the <code class="computeroutput"><span class="identifier">split_std_deque</span></code>
                  StoragePolicy. It should not be used with the <code class="computeroutput"><span class="identifier">fixed_size_queue</span></code>
                  StoragePolicy, because it will not detect iterator dereferences
                  that are out of range.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  full_check
                </p>
              </td>
<td>
                <p>
                  This policy has not been implemented yet. When it is, it will keep
                  track of all iterators and make sure that they are all valid. This
                  will be mostly useful for debugging purposes as it will incur significant
                  overhead.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <span class="bold"><strong>StoragePolicy</strong></span> classes
                </p>
              </td>
<td class="auto-generated">&#160;</td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">split_std_deque</span></code>
                </p>
              </td>
<td>
                <p>
                  Despite its name this policy keeps all buffered data in a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>. All data is stored as
                  long as there is more than one iterator. Once the iterator count
                  goes down to one, and the queue is no longer needed, it is cleared,
                  freeing up memory. The queue can also be forcibly cleared by calling
                  <code class="computeroutput"><span class="identifier">multi_pass</span><span class="special">::</span><span class="identifier">clear_queue</span><span class="special">()</span></code>.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">fixed_size_queue</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;</span></code>
                </p>
              </td>
<td>
                <p>
                  This policy keeps a circular buffer that is size <code class="computeroutput"><span class="identifier">N</span><span class="special">+</span><span class="number">1</span></code> and stores <code class="computeroutput"><span class="identifier">N</span></code>
                  elements. <code class="computeroutput"><span class="identifier">fixed_size_queue</span></code>
                  is a template with a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>
                  parameter that specified the queue size. It is your responsibility
                  to ensure that <code class="computeroutput"><span class="identifier">N</span></code>
                  is big enough for your parser. Whenever the foremost iterator is
                  incremented, the last character of the buffer is automatically
                  erased. Currently there is no way to tell if an iterator is trailing
                  too far behind and has become invalid. No dynamic allocation is
                  done by this policy during normal iterator operation, only on initial
                  construction. The memory usage of this <code class="computeroutput"><span class="identifier">StoragePolicy</span></code>
                  is set at <code class="computeroutput"><span class="identifier">N</span><span class="special">+</span><span class="number">1</span></code> bytes, unlike <code class="computeroutput"><span class="identifier">split_std_deque</span></code>,
                  which is unbounded.
                </p>
              </td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h5>
<a name="spirit.support.multi_pass.h4"></a>
        <span><a name="spirit.support.multi_pass.combinations__how_to_specify_your_own_custom_multi_pass"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.combinations__how_to_specify_your_own_custom_multi_pass">Combinations:
        How to specify your own custom multi_pass</a>
      </h5>
<p>
        The beauty of policy based designs is that you can mix and match policies
        to create your own custom iterator by selecting the policies you want. Here's
        an example of how to specify a custom <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        that wraps an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span></code>,
        and is slightly more efficient than the default <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        (as generated by the <code class="computeroutput"><span class="identifier">make_default_multi_pass</span><span class="special">()</span></code> API function) because it uses the <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">first_owner</span></code> OwnershipPolicy and the <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">no_check</span></code> CheckingPolicy:
      </p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">multi_pass</span><span class="special">&lt;</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">default_policy</span><span class="special">&lt;</span>
        <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">first_owner</span>
      <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">no_check</span>
      <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">buffering_input_iterator</span>
      <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">split_std_deque</span>
    <span class="special">&gt;</span>
<span class="special">&gt;</span> <span class="identifier">first_owner_multi_pass_type</span><span class="special">;</span>
</pre>
<p>
        The default template parameters for <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">default_policy</span></code>
        are:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
            <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">ref_counted</span></code> OwnershipPolicy
          </li>
<li class="listitem">
            <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">no_check</span></code> CheckingPolicy, if <code class="computeroutput"><span class="identifier">BOOST_SPIRIT_DEBUG</span></code> is defined: <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">buf_id_check</span></code> CheckingPolicy
          </li>
<li class="listitem">
            <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">buffering_input_iterator</span></code> InputPolicy,
            and
          </li>
<li class="listitem">
            <code class="computeroutput"><span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">split_std_deque</span></code> StoragePolicy.
          </li>
</ul></div>
<p>
        So if you use <code class="computeroutput"><span class="identifier">multi_pass</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span>
        <span class="special">&gt;</span></code> you will get those pre-defined
        behaviors while wrapping an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span></code>.
      </p>
<h5>
<a name="spirit.support.multi_pass.h5"></a>
        <span><a name="spirit.support.multi_pass.dealing_with_constant_look_ahead"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.dealing_with_constant_look_ahead">Dealing
        with constant look ahead</a>
      </h5>
<p>
        There is one other pre-defined class called <code class="computeroutput"><span class="identifier">look_ahead</span></code>.
        The class <code class="computeroutput"><span class="identifier">look_ahead</span></code> is another
        predefine <code class="computeroutput"><span class="identifier">multi_pass</span></code> iterator
        type. It has two template parameters: <code class="computeroutput"><span class="identifier">Input</span></code>,
        the type of the input iterator to wrap, and a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">N</span></code>, which specifies the size of the buffer
        to the <code class="computeroutput"><span class="identifier">fixed_size_queue</span></code> policy.
        While the default multi_pass configuration is designed for safety, <code class="computeroutput"><span class="identifier">look_ahead</span></code> is designed for speed. <code class="computeroutput"><span class="identifier">look_ahead</span></code> is derived from a multi_pass
        with the following policies: <code class="computeroutput"><span class="identifier">input_iterator</span></code>
        InputPolicy, <code class="computeroutput"><span class="identifier">first_owner</span></code>
        OwnershipPolicy, <code class="computeroutput"><span class="identifier">no_check</span></code>
        CheckingPolicy, and <code class="computeroutput"><span class="identifier">fixed_size_queue</span><span class="special">&lt;</span><span class="identifier">N</span><span class="special">&gt;</span></code>
        StoragePolicy.
      </p>
<p>
        This iterator is defined by including the files:
      </p>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/support/look_ahead.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">support_look_ahead</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
        Also, see <a class="link" href="../structure/include.html" title="Include">Include Structure</a>.
      </p>
<h5>
<a name="spirit.support.multi_pass.h6"></a>
        <span><a name="spirit.support.multi_pass.reading_from_standard_input_streams"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.reading_from_standard_input_streams">Reading
        from standard input streams</a>
      </h5>
<p>
        Yet another predefined iterator for wrapping standard input streams (usually
        a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special">&lt;&gt;</span></code>)
        is called <code class="computeroutput"><span class="identifier">basic_istream_iterator</span><span class="special">&lt;</span><span class="identifier">Char</span><span class="special">,</span> <span class="identifier">Traits</span><span class="special">&gt;</span></code>. This class is usable as a drop in replacement
        for <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span><span class="special">&lt;</span><span class="identifier">Char</span><span class="special">,</span> <span class="identifier">Traits</span><span class="special">&gt;</span></code>.
        Its only difference is that it is a forward iterator (instead of the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">istream_iterator</span></code>,
        which is an input iterator). <code class="computeroutput"><span class="identifier">basic_istream_iterator</span></code>
        is derived from a multi_pass with the following policies: <code class="computeroutput"><span class="identifier">istream</span></code>
        InputPolicy, <code class="computeroutput"><span class="identifier">ref_counted</span></code>
        OwnershipPolicy, <code class="computeroutput"><span class="identifier">no_check</span></code>
        CheckingPolicy, and <code class="computeroutput"><span class="identifier">split_std_deque</span></code>
        StoragePolicy.
      </p>
<p>
        There exists an additional predefined typedef:
      </p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">basic_istream_iterator</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">istream_iterator</span><span class="special">;</span>
</pre>
<p>
        This iterator is defined by including the files:
      </p>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/support/istream_iterator.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">support_istream_iterator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
        Also, see <a class="link" href="../structure/include.html" title="Include">Include Structure</a>.
      </p>
<h5>
<a name="spirit.support.multi_pass.h7"></a>
        <span><a name="spirit.support.multi_pass.how_to_write_a_functor_for_use_with_the__code__phrase_role__identifier__functor_input__phrase___code__inputpolicy"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.how_to_write_a_functor_for_use_with_the__code__phrase_role__identifier__functor_input__phrase___code__inputpolicy">How
        to write a functor for use with the <code class="computeroutput"><span class="identifier">functor_input</span></code>
        InputPolicy</a>
      </h5>
<p>
        If you want to use the <code class="computeroutput"><span class="identifier">functor_input</span></code>
        InputPolicy, you can write your own function object that will supply the
        input to <code class="computeroutput"><span class="identifier">multi_pass</span></code>. The
        function object must satisfy several requirements. It must have a typedef
        <code class="computeroutput"><span class="identifier">result_type</span></code> which specifies
        the return type of its <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>. This is standard practice in the STL.
        Also, it must supply a static variable called eof which is compared against
        to know whether the input has reached the end. Last but not least the function
        object must be default constructible. Here is an example:
      </p>
<pre class="programlisting"><span class="comment">// define the function object</span>
<span class="keyword">class</span> <span class="identifier">iterate_a2m</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">typedef</span> <span class="keyword">char</span> <span class="identifier">result_type</span><span class="special">;</span>

    <span class="identifier">iterate_a2m</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">c</span><span class="special">(</span><span class="char">'A'</span><span class="special">)</span> <span class="special">{}</span>
    <span class="identifier">iterate_a2m</span><span class="special">(</span><span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">c_</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span> <span class="special">{}</span>

    <span class="identifier">result_type</span> <span class="keyword">operator</span><span class="special">()()</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">if</span> <span class="special">(</span><span class="identifier">c_</span> <span class="special">==</span> <span class="char">'M'</span><span class="special">)</span>
            <span class="keyword">return</span> <span class="identifier">eof</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="identifier">c_</span><span class="special">++;</span>
    <span class="special">}</span>

    <span class="keyword">static</span> <span class="identifier">result_type</span> <span class="identifier">eof</span><span class="special">;</span>

<span class="keyword">private</span><span class="special">:</span>
    <span class="keyword">char</span> <span class="identifier">c_</span><span class="special">;</span>
<span class="special">};</span>

<span class="identifier">iterate_a2m</span><span class="special">::</span><span class="identifier">result_type</span> <span class="identifier">iterate_a2m</span><span class="special">::</span><span class="identifier">eof</span> <span class="special">=</span> <span class="identifier">iterate_a2m</span><span class="special">::</span><span class="identifier">result_type</span><span class="special">(</span><span class="char">'\0'</span><span class="special">);</span>

<span class="comment">// create two iterators using the define function object, one of which is </span>
<span class="comment">// an end iterator</span>
<span class="keyword">typedef</span> <span class="identifier">multi_pass</span><span class="special">&lt;</span><span class="identifier">iterate_a2m</span>
  <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">functor_input</span>
  <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">first_owner</span>
  <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">no_check</span>
  <span class="special">,</span> <span class="identifier">iterator_policies</span><span class="special">::</span><span class="identifier">split_std_deque</span><span class="special">&gt;</span>
<span class="identifier">functor_multi_pass_type</span><span class="special">;</span>

<span class="identifier">functor_multi_pass_type</span> <span class="identifier">first</span> <span class="special">=</span> <span class="identifier">functor_multi_pass_t</span><span class="special">(</span><span class="identifier">iterate_a2m</span><span class="special">());</span>
<span class="identifier">functor_multi_pass_type</span> <span class="identifier">last</span><span class="special">;</span>

<span class="comment">// use the iterators: this will print "ABCDEFGHIJKL"</span>
<span class="keyword">while</span> <span class="special">(</span><span class="identifier">first</span> <span class="special">!=</span> <span class="identifier">last</span><span class="special">)</span> <span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">first</span><span class="special">;</span>
    <span class="special">++</span><span class="identifier">first</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<h5>
<a name="spirit.support.multi_pass.h8"></a>
        <span><a name="spirit.support.multi_pass.how_to_write_policies_for_use_with_multi_pass"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.how_to_write_policies_for_use_with_multi_pass">How
        to write policies for use with multi_pass</a>
      </h5>
<p>
        All policies to be used with the <code class="computeroutput"><span class="identifier">default_policy</span></code>
        template need to have two embedded classes: <code class="computeroutput"><span class="identifier">unique</span></code>
        and <code class="computeroutput"><span class="identifier">shared</span></code>. The <code class="computeroutput"><span class="identifier">unique</span></code> class needs to implement all required
        functions for a particular policy type. In addition it may hold all member
        data items being <span class="emphasis"><em>unique</em></span> for a particular instance of
        a <code class="computeroutput"><span class="identifier">multi_pass</span></code> (hence the name).
        The <code class="computeroutput"><span class="identifier">shared</span></code> class does not
        expose any member functions (except sometimes a constructor), but it may
        hold all member data items to be <span class="emphasis"><em>shared</em></span> between all
        copies of a particular <code class="computeroutput"><span class="identifier">multi_pass</span></code>.
      </p>
<h5>
<a name="spirit.support.multi_pass.h9"></a>
        <span><a name="spirit.support.multi_pass.inputpolicy"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.inputpolicy">InputPolicy</a>
      </h5>
<p>
        An <code class="computeroutput"><span class="identifier">InputPolicy</span></code> must have
        the following interface:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">input_policy</span>
<span class="special">{</span>
    <span class="comment">// Input is the same type used as the first template parameter</span>
    <span class="comment">// while instantiating the multi_pass</span>
    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Input</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">unique</span>
    <span class="special">{</span>
        <span class="comment">// these typedef's will be exposed as the multi_pass iterator</span>
        <span class="comment">// properties</span>
        <span class="keyword">typedef</span> <span class="identifier">__unspecified_type__</span> <span class="identifier">value_type</span><span class="special">;</span>
        <span class="keyword">typedef</span> <span class="identifier">__unspecified_type__</span> <span class="identifier">difference_type</span><span class="special">;</span>
        <span class="keyword">typedef</span> <span class="identifier">__unspecified_type__</span> <span class="identifier">distance_type</span><span class="special">;</span>
        <span class="keyword">typedef</span> <span class="identifier">__unspecified_type__</span> <span class="identifier">pointer</span><span class="special">;</span>
        <span class="keyword">typedef</span> <span class="identifier">__unspecified_type__</span> <span class="identifier">reference</span><span class="special">;</span>

        <span class="identifier">unique</span><span class="special">()</span> <span class="special">{}</span>
        <span class="keyword">explicit</span> <span class="identifier">unique</span><span class="special">(</span><span class="identifier">Input</span><span class="special">)</span> <span class="special">{}</span>

        <span class="comment">// destroy is called whenever the last copy of a multi_pass is</span>
        <span class="comment">// destructed (ownership_policy::release() returned true)</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">destroy</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// swap is called by multi_pass::swap()</span>
        <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">unique</span><span class="special">&amp;);</span>

        <span class="comment">// get_input is called whenever the next input character/token</span>
        <span class="comment">// should be fetched. </span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// This method is expected to return a reference to the next </span>
        <span class="comment">// character/token</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">::</span><span class="identifier">reference</span> <span class="identifier">get_input</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// advance_input is called whenever the underlying input stream </span>
        <span class="comment">// should be advanced so that the next call to get_input will be </span>
        <span class="comment">// able to return the next input character/token</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">advance_input</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// input_at_eof is called to test whether this instance is a </span>
        <span class="comment">// end of input iterator.</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// This method is expected to return true if the end of input is </span>
        <span class="comment">// reached. It is often used in the implementation of the function</span>
        <span class="comment">// storage_policy::is_eof.</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">input_at_eof</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// input_is_valid is called to verify if the parameter t represents </span>
        <span class="comment">// a valid input character/token</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//   t:     is the character/token to test for validity</span>
        <span class="comment">// </span>
        <span class="comment">// This method is expected to return true if the parameter t </span>
        <span class="comment">// represents a valid character/token.</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">input_is_valid</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">,</span> <span class="identifier">value_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">t</span><span class="special">);</span>
    <span class="special">};</span>

    <span class="comment">// Input is the same type used as the first template parameter passed</span>
    <span class="comment">// while instantiating the multi_pass</span>
    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Input</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">shared</span>
    <span class="special">{</span>
        <span class="keyword">explicit</span> <span class="identifier">shared</span><span class="special">(</span><span class="identifier">Input</span><span class="special">)</span> <span class="special">{}</span>
    <span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
        It is possible to derive the struct <code class="computeroutput"><span class="identifier">unique</span></code>
        from the type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">default_input_policy</span></code>. This type implements
        a minimal sufficient interface for some of the required functions, simplifying
        the task of writing a new input policy.
      </p>
<p>
        This class may implement a function <code class="computeroutput"><span class="identifier">destroy</span><span class="special">()</span></code> being called during destruction of the
        last copy of a <code class="computeroutput"><span class="identifier">multi_pass</span></code>.
        This function should be used to free any of the shared data items the policy
        might have allocated during construction of its <code class="computeroutput"><span class="identifier">shared</span></code>
        part. Because of the way <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        is implemented any allocated data members in <code class="computeroutput"><span class="identifier">shared</span></code>
        should <span class="underline">not</span> be deep copied in a copy
        constructor of <code class="computeroutput"><span class="identifier">shared</span></code>.
      </p>
<h5>
<a name="spirit.support.multi_pass.h10"></a>
        <span><a name="spirit.support.multi_pass.ownershippolicy"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.ownershippolicy">OwnershipPolicy</a>
      </h5>
<p>
        The <code class="computeroutput"><span class="identifier">OwnershipPolicy</span></code> must
        have the following interface:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">ownership_policy</span>
<span class="special">{</span>
    <span class="keyword">struct</span> <span class="identifier">unique</span>
    <span class="special">{</span>
        <span class="comment">// destroy is called whenever the last copy of a multi_pass is</span>
        <span class="comment">// destructed (ownership_policy::release() returned true)</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">destroy</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// swap is called by multi_pass::swap()</span>
        <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">unique</span><span class="special">&amp;);</span>

        <span class="comment">// clone is called whenever a multi_pass is copied</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">clone</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// release is called whenever a multi_pass is destroyed</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// The method is expected to return true if the destructed </span>
        <span class="comment">// instance is the last copy of a particular multi_pass. </span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">release</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// is_unique is called to test whether this instance is the only </span>
        <span class="comment">// existing copy of a particular multi_pass</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// The method is expected to return true if this instance is unique</span>
        <span class="comment">// (no other copies of this multi_pass exist).</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">is_unique</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>
    <span class="special">};</span>

    <span class="keyword">struct</span> <span class="identifier">shared</span> <span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
        It is possible to derive the struct <code class="computeroutput"><span class="identifier">unique</span></code>
        from the type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">default_ownership_policy</span></code>. This type implements
        a minimal sufficient interface for some of the required functions, simplifying
        the task of writing a new ownership policy.
      </p>
<p>
        This class may implement a function <code class="computeroutput"><span class="identifier">destroy</span><span class="special">()</span></code> being called during destruction of the
        last copy of a <code class="computeroutput"><span class="identifier">multi_pass</span></code>.
        This function should be used to free any of the shared data items the policy
        might have allocated during construction of its <code class="computeroutput"><span class="identifier">shared</span></code>
        part. Because of the way <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        is implemented any allocated data members in <code class="computeroutput"><span class="identifier">shared</span></code>
        should <span class="underline">not</span> be deep copied in a copy
        constructor of <code class="computeroutput"><span class="identifier">shared</span></code>.
      </p>
<h5>
<a name="spirit.support.multi_pass.h11"></a>
        <span><a name="spirit.support.multi_pass.checkingpolicy"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.checkingpolicy">CheckingPolicy</a>
      </h5>
<p>
        The <code class="computeroutput"><span class="identifier">CheckingPolicy</span></code> must have
        the following interface:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">checking_policy</span>
<span class="special">{</span>
    <span class="keyword">struct</span> <span class="identifier">unique</span>
    <span class="special">{</span>
        <span class="comment">// swap is called by multi_pass::swap()</span>
        <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">unique</span><span class="special">&amp;);</span>

        <span class="comment">// destroy is called whenever the last copy of a multi_pass is</span>
        <span class="comment">// destructed (ownership_policy::release() returned true)</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">destroy</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// docheck is called before the multi_pass is dereferenced or </span>
        <span class="comment">// incremented. </span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// This method is expected to make sure the multi_pass instance is</span>
        <span class="comment">// still valid. If it is invalid an exception should be thrown.</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">docheck</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// clear_queue is called whenever the function </span>
        <span class="comment">// multi_pass::clear_queue is called on this instance</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">clear_queue</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>
    <span class="special">};</span>

    <span class="keyword">struct</span> <span class="identifier">shared</span> <span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
        It is possible to derive the struct <code class="computeroutput"><span class="identifier">unique</span></code>
        from the type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">default_checking_policy</span></code>. This type implements
        a minimal sufficient interface for some of the required functions, simplifying
        the task of writing a new checking policy.
      </p>
<p>
        This class may implement a function <code class="computeroutput"><span class="identifier">destroy</span><span class="special">()</span></code> being called during destruction of the
        last copy of a <code class="computeroutput"><span class="identifier">multi_pass</span></code>.
        This function should be used to free any of the shared data items the policy
        might have allocated during construction of its <code class="computeroutput"><span class="identifier">shared</span></code>
        part. Because of the way <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        is implemented any allocated data members in <code class="computeroutput"><span class="identifier">shared</span></code>
        should <span class="underline">not</span> be deep copied in a copy
        constructor of <code class="computeroutput"><span class="identifier">shared</span></code>.
      </p>
<h5>
<a name="spirit.support.multi_pass.h12"></a>
        <span><a name="spirit.support.multi_pass.storagepolicy"></a></span><a class="link" href="multi_pass.html#spirit.support.multi_pass.storagepolicy">StoragePolicy</a>
      </h5>
<p>
        A <code class="computeroutput"><span class="identifier">StoragePolicy</span></code> must have
        the following interface:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">storage_policy</span>
<span class="special">{</span>
    <span class="comment">// Value is the same type as typename MultiPass::value_type</span>
    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">unique</span>
    <span class="special">{</span>
        <span class="comment">// destroy is called whenever the last copy of a multi_pass is</span>
        <span class="comment">// destructed (ownership_policy::release() returned true)</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">destroy</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// swap is called by multi_pass::swap()</span>
        <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span><span class="identifier">unique</span><span class="special">&amp;);</span>

        <span class="comment">// dereference is called whenever multi_pass::operator*() is invoked</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// This function is expected to return a reference to the current</span>
        <span class="comment">// character/token.</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">::</span><span class="identifier">reference</span> <span class="identifier">dereference</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// increment is called whenever multi_pass::operator++ is invoked</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">increment</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">clear_queue</span><span class="special">(</span><span class="identifier">MultiPass</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// is_eof is called to test whether this instance is a end of input </span>
        <span class="comment">// iterator.</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//</span>
        <span class="comment">// This method is expected to return true if the end of input is </span>
        <span class="comment">// reached. </span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">is_eof</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">);</span>

        <span class="comment">// less_than is called whenever multi_pass::operator==() is invoked</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//   rhs:   is the multi_pass reference this instance is compared </span>
        <span class="comment">//          to</span>
        <span class="comment">//</span>
        <span class="comment">// This function is expected to return true if the current instance</span>
        <span class="comment">// is equal to the right hand side multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">equal_to</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">,</span> <span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span><span class="special">);</span>

        <span class="comment">// less_than is called whenever multi_pass::operator&lt;() is invoked</span>
        <span class="comment">//</span>
        <span class="comment">//   mp:    is a reference to the whole multi_pass instance</span>
        <span class="comment">//   rhs:   is the multi_pass reference this instance is compared </span>
        <span class="comment">//          to</span>
        <span class="comment">//</span>
        <span class="comment">// This function is expected to return true if the current instance</span>
        <span class="comment">// is less than the right hand side multi_pass instance</span>
        <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">MultiPass</span><span class="special">&gt;</span>
        <span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">less_than</span><span class="special">(</span><span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">mp</span><span class="special">,</span> <span class="identifier">MultiPass</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span><span class="special">);</span>
    <span class="special">};</span>

    <span class="comment">// Value is the same type as typename MultiPass::value_type</span>
    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Value</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">shared</span> <span class="special">{};</span>
<span class="special">};</span>
</pre>
<p>
        It is possible to derive the struct <code class="computeroutput"><span class="identifier">unique</span></code>
        from the type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">default_storage_policy</span></code>. This type implements
        a minimal sufficient interface for some of the required functions, simplifying
        the task of writing a new storage policy.
      </p>
<p>
        This class may implement a function <code class="computeroutput"><span class="identifier">destroy</span><span class="special">()</span></code> being called during destruction of the
        last copy of a <code class="computeroutput"><span class="identifier">multi_pass</span></code>.
        This function should be used to free any of the shared data items the policy
        might have allocated during construction of its <code class="computeroutput"><span class="identifier">shared</span></code>
        part. Because of the way <code class="computeroutput"><span class="identifier">multi_pass</span></code>
        is implemented any allocated data members in <code class="computeroutput"><span class="identifier">shared</span></code>
        should <span class="underline">not</span> be deep copied in a copy
        constructor of <code class="computeroutput"><span class="identifier">shared</span></code>.
      </p>
<p>
        Generally, a <code class="computeroutput"><span class="identifier">StoragePolicy</span></code>
        is the trickiest policy to implement. You should study and understand the
        existing <code class="computeroutput"><span class="identifier">StoragePolicy</span></code> classes
        before you try and write your own.
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
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