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<title>Types</title>
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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="variadic_macro_data.vmd_specific.vmd_type"></a><a class="link" href="vmd_type.html" title="Types">Types</a>
</h3></div></div></div>
<p>
        A subset of identifiers is VMD types, called a 'v-type'. These are identifiers
        which represent all of the preprocessor data types which VMD can parse. This
        subset of identifiers is automatically registered and pre-detected by VMD.
        Each identifier type begins with the unique prefix 'BOOST_VMD_TYPE_'.
      </p>
<p>
        The actual types are:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            BOOST_VMD_TYPE_EMPTY, represents emptiness, ie. "empty data"
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_ARRAY, a Boost PP array
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_LIST, a Boost PP list
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_SEQ, a Boost PP seq
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_TUPLE, a Boost PP tuple
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_IDENTIFIER, identifier
          </li>
<li class="listitem">
            BOOST_BMD_TYPE_NUMBER, a number
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_TYPE, a type itself
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_SEQUENCE, a sequence
          </li>
<li class="listitem">
            BOOST_VMD_TYPE_UNKNOWN, an unknown type
          </li>
</ul></div>
<p>
        Since a v-type is itself an identifier the particular constraint on the input
        to test is exactly the same as for identifiers.
      </p>
<p>
        The constraint is that the beginning input character, ignoring any whitespace,
        passed as the input to test must be either:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            an identifier character, ie. an alphanumeric or an underscore
          </li>
<li class="listitem">
            the left parenthesis of a tuple
          </li>
</ul></div>
<p>
        and if the first character is not the left parenthesis of a tuple the remaining
        characters must be alphanumeric or an underscore until a space character
        or end of input occurs.
      </p>
<p>
        If this is not the case the behavior is undefined, and most likely a preprocessing
        error will occur.
      </p>
<p>
        The macro used to test for a particular type in VMD is called BOOST_VMD_IS_TYPE.
        The macro takes a single parameter, the input to test against.
      </p>
<p>
        The macro returns 1 if the parameter is a v-type, otherwise the macro returns
        0.
      </p>
<p>
        A v-type is also an identifier, which has been registered and pre-detected,
        so you can also use the VMD functionality which works with identifiers to
        work with a v-type as an identifier if you like.
      </p>
<h5>
<a name="variadic_macro_data.vmd_specific.vmd_type.h0"></a>
        <span class="phrase"><a name="variadic_macro_data.vmd_specific.vmd_type.example"></a></span><a class="link" href="vmd_type.html#variadic_macro_data.vmd_specific.vmd_type.example">Example</a>
      </h5>
<p>
        Let us look at an example of how to use BOOST_VMD_IS_TYPE.
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="identifier">BOOST_VMD_IS_TYPE</span><span class="special">(</span><span class="identifier">input</span><span class="special">)</span>

<span class="identifier">returns</span><span class="special">:</span>

<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">BOOST_VMD_TYPE_SEQ</span><span class="special">,</span> <span class="number">1</span>
<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">BOOST_VMD_TYPE_NUMBER</span><span class="special">,</span> <span class="number">1</span>
<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">SQUARE</span><span class="special">,</span> <span class="number">0</span>
<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span> <span class="identifier">DATA</span><span class="special">,</span> <span class="number">0</span> <span class="identifier">since</span> <span class="identifier">there</span> <span class="identifier">are</span> <span class="identifier">tokens</span> <span class="identifier">after</span> <span class="identifier">the</span> <span class="identifier">type</span>
<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="special">%</span><span class="number">44</span><span class="special">,</span> <span class="identifier">does</span> <span class="keyword">not</span> <span class="identifier">meet</span> <span class="identifier">the</span> <span class="identifier">constraint</span> <span class="identifier">therefore</span> <span class="identifier">undefined</span> <span class="identifier">behavior</span>
<span class="keyword">if</span> <span class="identifier">input</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">BOOST_VMD_TYPE_EMPTY</span> <span class="special">),</span> <span class="number">0</span> <span class="identifier">since</span> <span class="identifier">the</span> <span class="identifier">macro</span> <span class="identifier">begins</span> <span class="identifier">with</span> <span class="identifier">a</span> <span class="identifier">tuple</span> <span class="keyword">and</span> <span class="keyword">this</span> <span class="identifier">can</span> <span class="identifier">be</span> <span class="identifier">tested</span> <span class="keyword">for</span>
</pre>
<h5>
<a name="variadic_macro_data.vmd_specific.vmd_type.h1"></a>
        <span class="phrase"><a name="variadic_macro_data.vmd_specific.vmd_type.usage"></a></span><a class="link" href="vmd_type.html#variadic_macro_data.vmd_specific.vmd_type.usage">Usage</a>
      </h5>
<p>
        To use the BOOST_VMD_IS_TYPE macro either include the general header:
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
        or include the specific header:
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">vmd</span><span class="special">/</span><span class="identifier">is_type</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
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