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      <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Numerics</b></font>
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<p>Similar to <tt>chlit</tt>, <tt>strlit</tt> etc. numeric parsers are also primitives.
  Numeric parsers are placed on


 a section of their own

 to give this important building
  block better focus. The framework includes a couple of predefined objects for
  parsing signed and unsigned integers and real numbers. These parsers are fully
  parametric. Most of the important aspects of numeric parsing can be finely adjusted
  to suit. This includes the radix base, the minimum and maximum number of allowable
  digits, the exponent, the fraction etc. Policies control the real number parsers'
  behavior. There are some predefined policies covering the most common real number
  formats but the user can supply her own when needed. </p>
<h2>uint_parser</h2>
<p>This class is the simplest among the members of the numerics package. The <tt>uint_parser</tt>
  can parse unsigned integers of arbitrary length and size. The <tt>uint_parser</tt>
  parser can be used to parse ordinary primitive C/C++ integers or even user defined
  scalars such as bigints (unlimited precision integers). Like most of the classes
  in Spirit, the <tt>uint_parser</tt> is a template class. Template parameters
  fine tune its behavior. The uint_parser is so flexible that the other numeric
  parsers are implemented using it as the backbone.</p>
<pre><code><font color="#000000"><span class=identifier>    </span><span class=keyword>template </span><span class=special>&lt;
        </span><span class=keyword>typename </span><span class=identifier>T </span><span class=special>= </span><span class="keyword">unsigned</span><span class=special>,
        </span><span class=keyword>int </span><span class=identifier>Radix </span><span class=special>= </span><span class=number>10</span><span class=special>,
        </span><span class=keyword>unsigned </span><span class=identifier>MinDigits </span><span class=special>= </span><span class=number>1</span><span class=special>,
        </span><span class=keyword>int </span><span class=identifier>MaxDigits </span><span class=special>= -</span><span class=number>1</span><span class=special>&gt;
    </span><span class=keyword>struct </span><span class=identifier>uint_parser </span><span class=special>{ </span><span class=comment>/*...*/ </span><span class=special>};</span></font></code></pre>
<table width="90%" border="0" align="center">
  <tr>
    <td colspan="2" class="table_title">uint_parser template parameters</td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>T</b></td>
    <td class="table_cells" width="79%">The numeric base type of the numeric parser.
      Defaults to <tt>unsigned</tt></td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>Radix</b></td>
    <td class="table_cells" width="79%">The radix base. This can be either 2:
      binary, 8: octal, 10: decimal and 16: hexadecimal. Defaults to 10; decimal</td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>MinDigits</b></td>
    <td class="table_cells" width="79%">The minimum number of digits allowable</td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>MaxDigits</b></td>
    <td class="table_cells" width="79%">The maximum number of digits allowable.
      If this is -1, then the maximum limit becomes unbounded</td>
  </tr>
</table>
<br>
<table width="90%" border="0" align="center">
  <tr>
    <td colspan="2" class="table_title">Predefined uint_parsers</td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>bin_p</b></td>
    <td class="table_cells" width="79%"><code><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>,
      </span><span class=number>2</span><span class=special>, </span><span class=number>1</span><span class=special>,
      -</span><span class=number>1</span><span class=special>&gt; </span><span class=keyword>const</span></code></td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>oct_p</b></td>
    <td class="table_cells" width="79%"><code><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>,
      </span><span class=number>8</span><span class=special>, </span><span class=number>1</span><span class=special>,
      -</span><span class=number>1</span><span class=special>&gt; </span><span class=keyword>const</span></code></td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>uint_p</b></td>
    <td class="table_cells" width="79%"><code><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>,
      </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>,
      -</span><span class=number>1</span><span class=special>&gt; </span><span class=keyword>const</span></code></td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>hex_p</b></td>
    <td class="table_cells" width="79%"><code><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>,
      </span><span class=number>16</span><span class=special>, </span><span class=number>1</span><span class=special>,
      -</span><span class=number>1</span><span class=special>&gt; </span><span class=keyword>const</span></code></td>
  </tr>
</table>
<p>The following example shows how the uint_parser can be used to parse thousand
  separated numbers. The example can correctly parse numbers such as 1,234,567,890.</p>
<pre><span class=keyword>    </span><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>, </span><span class=number>3</span><span class=special>&gt; </span><span class=identifier>uint3_p</span><span class=special>;        </span><span class=comment>//  1..3 digits
    </span><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>3</span><span class=special>, </span><span class=number>3</span><span class=special>&gt; </span><span class=identifier>uint3_3_p</span><span class=special>;      </span><span class=comment>//  exactly 3 digits
    </span><span class=identifier>ts_num_p </span><span class=special>= </span><span class=special>(</span><span class=identifier>uint3_p </span><span class=special>&gt;&gt; </span><span class=special>*(</span><span class=literal>',' </span><span class=special>&gt;&gt; </span><span class=identifier>uint3_3_p</span><span class=special>));    </span><span class=comment>//  our thousand separated number parser</span></pre>
<p><tt>bin_p</tt>, <tt>oct_p</tt>, <tt>uint_p</tt> and <tt>hex_p</tt> are parser
  generator objects designed to be used within expressions. Here's an example
  of a rule that parses comma delimited list of numbers (We've seen this <a href="quick_start.html#list_of_numbers">before</a>):</p>
<pre><code><span class=identifier>    </span><span class=identifier>list_of_numbers </span><span class=special>=</span> <span class=identifier>real_p </span><span class=special>&gt;&gt; *(</span><span class=literal>','</span> <span class=special>&gt;&gt; </span><span class=identifier>real_p</span><span class=special>)</span></code>;
</pre>
<p></p>
<p>Later, we shall see how we can extract the actual numbers parsed by the numeric
  parsers. We shall deal with this when we get to the section on <a href="semantic_actions.html#specialized_actions">specialized
  actions</a>.</p>
<h2>int_parser</h2>
<p>The <tt>int_parser</tt> can parse signed integers of arbitrary length and size.
  This is almost the same as the <tt>uint_parser</tt>. The only difference is
  the additional task of parsing the <tt><span class="quotes">'+'</span></tt>
  or <tt class="quotes">'-'</tt> sign preceding the number. The class interface
  is the same as that of the uint_parser.<br>
</p>
<table width="90%" border="0" align="center">
  <tr>
    <td colspan="2" class="table_title">A predefined int_parser</td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>int_p</b></td>
    <td class="table_cells" width="79%"><span class=identifier><code>int_parser</code></span><code><span class=special>&lt;</span><span class=keyword>int</span><span class=special>,
      </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>,
      -</span><span class=number>1</span><span class=special>&gt; </span><span class=keyword>const</span></code></td>
  </tr>
</table>
<h2>real_parser</h2>
<p>The <tt>real_parser</tt> can parse real numbers of arbitrary length and size
  limited by its parametric type <tt>T</tt>. The <tt>real_parser</tt> is a template
  class with 2 template parameters. Here's the <tt>real_parser</tt> template interface:</p>
<pre><span class=keyword>    template</span><span class=special>&lt;
        </span><span class=keyword>typename </span><span class=identifier>T </span><span class=special>= </span><span class=keyword>double</span><span class=special>,
        </span><span class=keyword>typename </span><span class=identifier>RealPoliciesT </span><span class=special>= </span><span class=identifier>ureal_parser_policies</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt; </span><span class=special>&gt;
</span><span class=keyword>    struct </span><span class=identifier>real_parser</span><span class=special>;</span></pre>
<p>The first template parameter is its numeric base type <tt>T</tt>. This defaults
  to <tt>double</tt>. </p>
<table width="80%" border="0" align="center">
  <tr>
    <td class="note_box"><img src="theme/bulb.gif" width="13" height="18"><b>
      Parsing special numeric types</b><br>
      <br>
      Notice that the numeric base type <tt>T</tt> can be specified by the user.
      This means that we can use the numeric parsers to parse user defined numeric
      types such as <tt>fixed_point</tt> (fixed point reals) and <tt>bigint</tt>
      (unlimited precision integers).</td>
  </tr>
</table>
<p>The second template parameter is a class that groups all the policies and defaults
  to <tt>ureal_parser_policies&lt;T&gt;</tt>. Policies control the real number
  parsers' behavior. The default policies provided are designed to parse C/C++
  style real numbers of the form <b>nnn.fff.Eeee</b> where <b>nnn</b> is the whole
  number part, <b>fff</b> is the fractional part, <b>E</b> is <tt class="quotes">'e'</tt>
  or <tt class="quotes">'E'</tt> and <b>eee</b> is the exponent optionally preceded
  by <tt class="quotes">'-'</tt> or <tt><span class="quotes">'+'</span></tt>.
  This corresponds to the following grammar, with the exception that plain integers
  without the decimal point are also accepted by default.</p>
<pre><code><font color="#000000"><span class=keyword>    </span><span class=identifier>floatingliteral
        </span><span class=special>=   </span><span class=identifier>fractionalconstant </span><span class=special>&gt;&gt; </span><span class=special>!</span><span class=identifier>exponentpart
        </span><span class=special>|  </span><span class=special>+</span><span class=identifier>digit_p </span><span class=special>&gt;&gt; </span><span class=identifier>exponentpart
        </span><span class=special>;

    </span><span class=identifier>fractionalconstant
        </span><span class=special>=  </span><span class=special>*</span><span class=identifier>digit_p </span><span class=special>&gt;&gt; </span><span class=literal>'.' </span><span class=special>&gt;&gt; </span><span class=special>+</span><span class=identifier>digit_p
        </span><span class=special>|  </span><span class=special>+</span><span class=identifier>digit_p </span><span class=special>&gt;&gt; </span><span class=literal>'.'
        </span><span class=special>;

    </span><span class=identifier>exponentpart
        </span><span class=special>=   </span><span class=special>(</span><span class=literal>'e' </span><span class=special>| </span><span class=literal>'E'</span><span class=special>) </span><span class=special>&gt;&gt; </span><span class=special>!(</span><span class=literal>'+' </span><span class=special>| </span><span class=literal>'-'</span><span class=special>) </span><span class=special>&gt;&gt; </span><span class=special>+</span><span class=identifier>digit_p
        </span><span class=special>;</span></font></code></pre>
<p>The default policies are provided to take care of the most common case (there
  are many ways to represent, and hence parse, real numbers). In most cases, the
  default setting of the <tt>real_parser</tt> is sufficient and can be used straight
  out of the box. Actually, there are four <tt>real_parser</tt>s pre-defined for
  immediate use:</p>
<table width="90%" border="0" align="center">
  <tr>
    <td colspan="2" class="table_title">Predefined real_parsers</td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>ureal_p</b></td>
    <td class="table_cells" width="79%"><span class=identifier><code>real_parser</code></span><code><span class=special>&lt;</span><span class=keyword>double</span><span class=special>,
      </span><span class=identifier>ureal_parser_policies</span><span class=special>&lt;</span><span class=keyword>double</span><span class=special>&gt;
      &gt; </span><span class=keyword>const</span></code></td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>real_p</b></td>
    <td class="table_cells" width="79%"><span class=identifier><code>real_parser</code></span><code><span class=special>&lt;</span><span class=keyword>double</span><span class=special>,
      </span><span class=identifier>real_parser_policies</span><span class=special>&lt;</span><span class=keyword>double</span><span class=special>&gt;
      &gt; </span><span class=keyword>const</span></code></td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>strict_ureal_p</b></td>
    <td class="table_cells" width="79%"><span class=identifier><code>real_parser</code></span><code><span class=special>&lt;</span><span class=keyword>double</span><span class=special>,
      </span><span class=identifier>strict_ureal_parser_policies</span><span class=special>&lt;</span><span class=keyword>double</span><span class=special>&gt;
      &gt; </span><span class=keyword>const</span></code></td>
  </tr>
  <tr>
    <td class="table_cells" width="21%"><b>strict_real_p</b></td>
    <td class="table_cells" width="79%"><span class=identifier><code>real_parser</code></span><code><span class=special>&lt;</span><span class=keyword>double</span><span class=special>,
      </span><span class=identifier>strict_real_parser_policies</span><span class=special>&lt;</span><span class=keyword>double</span><span class=special>&gt;
      &gt; </span><span class=keyword>const</span></code></td>
  </tr>
</table>
<p>We've seen <tt>real_p</tt> before. <tt>ureal_p</tt> is its unsigned variant.</p>
<h3><a name="strict_reals"></a>Strict Reals </h3>
<p>Integer numbers are considered a subset of real numbers, so <tt>real_p</tt>
  and <tt>ureal_p</tt> recognize integer numbers (without a dot) as real numbers.
  <tt>strict_real_p</tt> and <tt>strict_ureal_p</tt> are the equivalent parsers
  that <strong>require</strong> a dot to be present for a number to be considered
  a successful match.</p>
<h2>Advanced: real_parser policies</h2>
<p>The parser policies break down real number parsing into 6 steps:</p>
<table width="90%" border="0" align="center">
  <tr>
    <td class="table_cells">1</td>
    <td class="table_cells"><b>parse_sign</b></td>
    <td class="table_cells">Parse the prefix sign</td>
  </tr>
  <tr>
    <td class="table_cells">2</td>
    <td class="table_cells"><b>parse_n</b></td>
    <td class="table_cells">Parse the integer at the left of the decimal point</td>
  </tr>
  <tr>
    <td class="table_cells">3</td>
    <td class="table_cells"><b>parse_dot</b></td>
    <td class="table_cells">Parse the decimal point</td>
  </tr>
  <tr>
    <td class="table_cells">4</td>
    <td class="table_cells"><b>parse_frac_n</b></td>
    <td class="table_cells">Parse the fraction after the decimal point</td>
  </tr>
  <tr>
    <td class="table_cells">5</td>
    <td class="table_cells"><b>parse_exp</b></td>
    <td class="table_cells">Parse the exponent prefix (e.g. 'e')</td>
  </tr>
  <tr>
    <td class="table_cells">6</td>
    <td class="table_cells"><b>parse_exp_n</b></td>
    <td class="table_cells">Parse the actual exponent</td>
  </tr>
</table>
<p>And the interaction of these sub-parsing tasks is further controlled by these
  3 policies:</p>
<table width="90%" border="0" align="center">
  <tr>
    <td class="table_cells">1</td>
    <td class="table_cells"><b>allow_leading_dot</b></td>
    <td class="table_cells">Allow a leading dot to be present (&quot;.1&quot; becomes
      equivalent to &quot;0.1&quot;)</td>
  </tr>
  <tr>
    <td class="table_cells">2</td>
    <td class="table_cells"><b>allow_trailing_dot</b></td>
    <td class="table_cells">Allow a trailing dot to be present (&quot;1.&quot; becomes
    equivalent to &quot;1.0&quot;)</td>
  </tr>
  <tr>
    <td class="table_cells">3</td>
    <td class="table_cells"><b>expect_dot</b></td>
    <td class="table_cells">Require a dot to be present (disallows &quot;1&quot; to
    be equivalent to &quot;1.0&quot;)</td>
  </tr>
</table>
<p>[ <img src="theme/lens.gif" width="15" height="16"> From here on, required
  reading: <a href="scanner.html">The Scanner</a>, <a href="indepth_the_parser.html">In-depth
  The Parser</a> and <a href="indepth_the_scanner.html">In-depth The Scanner</a>
  ]</p>
<h2>sign_parser and sign_p</h2>
<p>Before we move on, a small utility parser is included here to ease the parsing
  of the <span class="quotes">'-'</span> or <span class="quotes">'+'</span> sign.
  While it is easy to write one:</p>
<pre>    <span class=identifier>sign_p </span><span class=special>= </span><span class=special>(</span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'+'</span>) <span class=special>| </span><span class=literal>'-'</span><span class="special">)</span><span class=literal>;</span></pre>
<p>it is not possible to extract the actual sign (positive or negative) without
  resorting to semantic actions. The sign_p parser has a bool attribute returned
  to the caller through the match object which, after parsing, is set to <strong>true</strong>
  if the parsed sign is negative. This attribute detects if the negative sign
  has been parsed. Examples:</p>
<pre><span class=special>    </span><span class=keyword>bool </span><span class=identifier>is_negative</span><span class=special>;
    </span><span class=identifier>r </span><span class=special>= </span><span class=identifier>sign_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>is_negative</span><span class=special>)]</span><span class=special>;</span></pre>
<p><span class=special></span>or simply...</p>
<pre>    <span class=comment>// directly extract the result from the match result's value</span>
    <span class=keyword>bool </span><span class=identifier>is_negative </span><span class=special>= </span><span class=identifier>sign_p</span><span class=special>.</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>).</span><span class=identifier>value</span><span class=special>();</span><span class=comment> </span></pre>
<p>The sign_p parser expects attached semantic actions to have a signature (see
  <a href="semantic_actions.html#specialized_actions">Specialized Actions</a>
  for further detail) compatible with: </p>
<p><b>Signature for functions:</b></p>
<pre><code><font color="#000000"><span class=identifier>    </span><span class=keyword>void </span><span class=identifier>func</span><span class=special>(</span><span class="keyword">bool</span><span class=identifier> is_negative</span><span class=special>);</span></font></code></pre>
<p><b>Signature for functors:</b> </p>
<pre><code><font color="#000000"><span class=special>    </span><span class=keyword>struct </span><span class=identifier>ftor
    </span><span class=special>{
        </span><span class=keyword>void </span><span class=keyword>operator</span><span class=special>()(</span><span class="keyword">bool</span><span class=identifier> is_negative</span><span class=special>) </span><span class=keyword>const</span><span class=special>;
    </span><span class=special>};</span></font></code></pre>
<h2><span class=identifier>ureal_parser_policies</span></h2>
<pre><span class=comment>    </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=keyword>struct </span><span class=identifier>ureal_parser_policies
    </span><span class=special>{
        </span><span class=keyword>typedef </span><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>, -</span><span class=number>1</span><span class=special>&gt;   </span><span class=identifier>uint_parser_t</span><span class=special>;
        </span><span class=keyword>typedef </span><span class=identifier>int_parser</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>, -</span><span class=number>1</span><span class=special>&gt;    </span><span class=identifier>int_parser_t</span><span class=special>;

        </span><span class=keyword>static const bool</span> <span class=identifier>allow_leading_dot</span> <span class=special> =</span> <span class=literal>true</span><span class=special>;</span><span class=special>
        </span><span class=keyword>static const bool</span> <span class=identifier>allow_trailing_dot </span><span class=special>=</span> <span class=literal>true</span><span class=special>;</span><span class=special></span>
<span class=special>        </span><span class=keyword>static const bool</span> <span class=identifier>expect_dot</span> <span class=special>        =</span> <span class=literal>false</span><span class=special>;</span><span class=special></span><span class=special><br>
        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>match_result</span><span class=special>&lt;</span><span class=identifier>ScannerT</span><span class=special>, </span><span class="identifier">nil_t</span><span class=special>&gt;::</span><span class=identifier>type
        parse_sign</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>scan</span><span class=special>.</span><span class=identifier>no_match</span><span class=special>(); }

        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>uint_parser_t</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_n</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>uint_parser_t</span><span class=special>().</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>); }

        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>chlit</span><span class=special>&lt;&gt;, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_dot</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>ch_p</span><span class=special>(</span><span class=literal>'.'</span><span class=special>).</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>); }

        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>uint_parser_t</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_frac_n</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>uint_parser_t</span><span class=special>().</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>); }

        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>chlit</span><span class=special>&lt;&gt;, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_exp</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>as_lower_d</span><span class=special>[</span><span class=literal>'e'</span><span class=special>].</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>); }

        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=keyword>int</span><span class=identifier>_parser_t</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_exp_n</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return int</span><span class=identifier>_parser_t</span><span class=special>().</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>); }
    };
</span></pre>
<p><span class=special></span><span class=identifier>The default ureal_parser_policies
  uses the lower level integer numeric parsers to do its job. </span></p>
<h2><span class=identifier>real_parser_policies</span></h2>
<pre>    <span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=keyword>struct </span><span class=identifier>real_parser_policies </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>ureal_parser_policies</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=special>{
        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static </span><span class=keyword>typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>sign_parser</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        </span><span class=identifier>parse_sign</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>sign_p</span><span class=special>.</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>); </span><span class=special>}
    </span><span class=special>};</span></pre>
<p>Notice how the real_parser_policies replaced <b><tt>parse_sign</tt></b> of
  the <b>u</b>real_parser_policies from which it is subclassed. The default real_parser_policies
  simply uses a <tt>sign_p</tt> instead of <tt>scan.no_match()</tt> in the <tt>parse_sign
  </tt> step. </p>
<h2><span class=identifier>strict_ureal_parser_policies and strict_real_parser_policies</span></h2>
<pre>    <span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=keyword>struct </span><span class=identifier>strict_ureal_parser_policies </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>ureal_parser_policies</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=special>{</span>
<span class=special>        </span><span class=keyword>static const bool</span> <span class=identifier>expect_dot</span><span class=special> =</span> <span class=literal>true</span><span class=special>;</span><span class=special></span><span class=special></span>
    <span class=special>};</span>

    <span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=keyword>struct </span><span class=identifier>strict_real_parser_policies </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>real_parser_policies</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=special>{</span>
<span class=special>        </span><span class=keyword>static const bool</span> <span class=identifier>expect_dot</span><span class=special> =</span> <span class=literal>true</span><span class=special>;</span><span class=special></span><span class=special></span>
    <span class=special>};</span></pre>
<p>Again, these policies replaced just the policies they wanted different from
  their superclasses.</p>
<p><i>Specialized</i> real parser policies can reuse some of the defaults while
  replacing a few. For example, the following is a real number parser policy that
  parses thousands separated numbers with at most two decimal places and no exponent.
</p>
<p><img src="theme/lens.gif" width="15" height="16">The full source code can be
  viewed <a href="../example/fundamental/thousand_separated.cpp">here</a>. </p>
<pre>
<span class=identifier>    </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>T</span><span class=special>&gt;
    </span><span class=keyword>struct </span><span class=identifier>ts_real_parser_policies </span><span class=special>: </span><span class=keyword>public </span><span class=identifier>ureal_parser_policies</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>&gt;
    {
        </span><span class=comment>//  These policies can be used to parse thousand separated
        //  numbers with at most 2 decimal digits after the decimal
        //  point. e.g. 123,456,789.01

        </span><span class=keyword>typedef </span><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>, </span><span class=number>2</span><span class=special>&gt;  </span><span class=identifier>uint2_t</span><span class=special>;
        </span><span class=keyword>typedef </span><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=identifier>T</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>, -</span><span class=number>1</span><span class=special>&gt;   </span><span class=identifier>uint_parser_t</span><span class=special>;
        </span><span class=keyword>typedef </span><span class=identifier>int_parser</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>, -</span><span class=number>1</span><span class=special>&gt;  </span><span class=identifier>int_parser_t</span><span class=special>;

        </span><span class=comment>//////////////////////////////////  2 decimal places Max
        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>uint2_t</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_frac_n</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>uint2_t</span><span class=special>().</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>); }

</span><span class=special>        </span><span class=comment>//////////////////////////////////  No exponent<br></span>        <span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>chlit</span><span class=special>&lt;&gt;, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_exp</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>scan</span><span class=special>.</span><span class=identifier>no_match</span><span class=special>(); }

</span><span class=special>        </span><span class=comment>//////////////////////////////////  No exponent<br></span>        <span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>int_parser_t</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        parse_exp_n</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        { </span><span class=keyword>return </span><span class=identifier>scan</span><span class=special>.</span><span class=identifier>no_match</span><span class=special>(); }

        </span><span class=comment>//////////////////////////////////  Thousands separated numbers
        </span><span class=keyword>template </span><span class=special>&lt;</span><span class=keyword>typename </span><span class=identifier>ScannerT</span><span class=special>&gt;
        </span><span class=keyword>static typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>uint_parser_t</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type
        <a name="scanner_save"></a>parse_n</span><span class=special>(</span><span class=identifier>ScannerT</span><span class=special>&amp; </span><span class=identifier>scan</span><span class=special>)
        {
            </span><span class=keyword>typedef typename </span><span class=identifier>parser_result</span><span class=special>&lt;</span><span class=identifier>uint_parser_t</span><span class=special>, </span><span class=identifier>ScannerT</span><span class=special>&gt;::</span><span class=identifier>type RT</span><span class=special>;
            </span><span class="keyword">static </span><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>1</span><span class=special>, </span><span class=number>3</span><span class=special>&gt; </span><span class=identifier>uint3_p</span><span class=special>;
            </span><span class="keyword">static </span><span class=identifier>uint_parser</span><span class=special>&lt;</span><span class=keyword>unsigned</span><span class=special>, </span><span class=number>10</span><span class=special>, </span><span class=number>3</span><span class=special>, </span><span class=number>3</span><span class=special>&gt; </span><span class=identifier>uint3_3_p</span><span class=special>;

            </span><span class=keyword>if </span><span class=special>(</span><span class=identifier>RT hit </span><span class=special>= </span><span class=identifier>uint3_p</span><span class=special>.</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>))
            {
                </span><span class=identifier>T n</span><span class=special>;
                </span><span class="keyword">typedef typename </span>ScannerT<span class="special">::</span>iterator_t iterator_t;<span class=special>
                </span>iterator_t save<span class="special"> = </span>scan.first<span class="special">;
                </span><span class=keyword>while </span><span class=special>(</span><span class=identifier>match</span><span class=special>&lt;&gt; </span><span class=identifier>next </span><span class=special>= (</span><span class=literal>',' </span><span class=special>&gt;&gt; </span><span class=identifier>uint3_3_p</span><span class=special>[</span><span class=identifier>assign_a</span><span class=special>(</span><span class=identifier>n</span><span class=special>)]).</span><span class=identifier>parse</span><span class=special>(</span><span class=identifier>scan</span><span class=special>))
                {
                    </span><span class=identifier>hit</span><span class=special>.</span><span class=identifier>value</span><span class=special>() *= </span><span class=number>1000</span><span class=special>;
                    </span><span class=identifier>hit</span><span class=special>.</span><span class=identifier>value</span><span class=special>() += </span><span class=identifier>n</span><span class=special>;
                    </span><span class=identifier>scan</span><span class=special>.</span><span class=identifier>concat_match</span><span class=special>(</span><span class=identifier>hit</span><span class=special>, </span><span class=identifier>next</span><span class=special>);
                    </span><span class="identifier">save </span><span class=special><span class="special">= </span></span><span class="identifier">scan</span><span class="special">.</span><span class="identifier">first</span><span class=special><span class="special">;</span>
                }
                </span>scan<span class="special">.</span>first<span class="special"> = </span>save<span class="special">;
                </span><span class=keyword>return </span><span class=identifier>hit</span><span class=special>;

</span><span class=special>                // Note: On erroneous input such as &quot;123,45&quot;, the result should<br>                // be a partial match &quot;123&quot;. 'save' is used to makes sure that<br>                // the scanner position is placed at the last *valid* parse<br>                // position.<br>            }
            </span><span class=keyword>return </span><span class=identifier>scan</span><span class=special>.</span><span class=identifier>no_match</span><span class=special>();
        }
    };</span></pre>
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