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<div class="titlepage"><div><div><h3 class="title">
<a name="variadic_macro_data.vmd_modifiers.vmd_modifiers_index"></a><a class="link" href="vmd_modifiers_index.html" title="Index modifiers">Index
      modifiers</a>
</h3></div></div></div>
<p>
        Index modifiers can be used with the BOOST_VMD_ELEM macro when identifier
        modifiers are being used. Index modifiers take two values:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            BOOST_VMD_RETURN_INDEX, return an index as a number, starting with 0,
            of the particular identifier modifier which matched, as part of the output
            of the BOOST_VMD_ELEM macro. If no particular identifier modifier matches,
            return emptiness as part of the output. The index number is determined
            purely by the order in which identifier modifiers are specified as optional
            parameters to BOOST_VMD_ELEM, whether singly as individual optional parameters
            or as a tuple of identifier modifiers.
          </li>
<li class="listitem">
            BOOST_VMD_RETURN_NO_INDEX, do not return an index as part of the output.
            This is the default value and need only be used to override the BOOST_VMD_RETURN_INDEX
            value if it is specified.
          </li>
</ul></div>
<p>
        The BOOST_VMD_RETURN_INDEX tells the programmer which one of the identifier
        modifiers matched the element's data as an index. Some macro programmers
        find this more useful for the purposes of macro branching logic than branching
        using the actual name of the identifier itself.
      </p>
<p>
        When the index modifier BOOST_VMD_RETURN_INDEX is specified, and identifier
        modifiers are specified along with the BOOST_VMD_TYPE_IDENTIFIER filter modifier,
        the output of BOOST_VMD_ELEM becomes a tuple of two elements. The first tuple
        element is the element matched and the last tuple element is the index, starting
        with 0, of the identifier modifier which matched. If an element is not matched
        both tuple elements are empty.
      </p>
<p>
        If the splitting modifier BOOST_VMD_RETURN_AFTER is also specified then the
        output is a tuple of three elements. The first tuple element is the element
        matched, the second tuple element is the rest of the sequence after the matching
        element, and the last tuple element is the numeric index. If an element is
        not matched then all three tuple elements are empty.
      </p>
<p>
        If identifier modifiers and the BOOST_VMD_TYPE_IDENTIFIER filter modifier
        are not specified as optional parameters, then if BOOST_VMD_RETURN_INDEX
        is specified it is ignored. If the splitting modifier BOOST_VMD_RETURN_ONLY_AFTER
        is specified, if BOOST_VMD_RETURN_INDEX is also specified it is ignored.
      </p>
<p>
        Let's see how this works:
      </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">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ANAME</span> <span class="special">(</span><span class="identifier">ANAME</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_APLACE</span> <span class="special">(</span><span class="identifier">APLACE</span><span class="special">)</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_REGISTER_ACOUNTRY</span> <span class="special">(</span><span class="identifier">ACOUNTRY</span><span class="special">)</span>

<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_ANAME_ANAME</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_VMD_DETECT_APLACE_APLACE</span>

<span class="preprocessor">#define</span> <span class="identifier">A_SEQUENCE</span> <span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">)</span> <span class="identifier">ANAME</span> <span class="special">(</span><span class="number">1</span><span class="special">)(</span><span class="number">2</span><span class="special">)</span> <span class="number">46</span>

<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'ANAME'</span>
<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="identifier">emptiness</span>
<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_INDEX</span><span class="special">,</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="special">(,)</span>
<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_INDEX</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">,</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(ANAME,0)'</span>
<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_INDEX</span><span class="special">,(</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">))</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(ANAME,2)'</span>
</pre>
<p>
        Used with splitting modifiers:
      </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">elem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_INDEX</span><span class="special">,</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="special">(,,)</span>
<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_INDEX</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">,</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(ANAME,(1)(2) 46,0)'</span>
<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_INDEX</span><span class="special">,(</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">),</span><span class="identifier">BOOST_VMD_RETURN_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(ANAME,(1)(2) 46,2)'</span>

<span class="identifier">BOOST_VMD_ELEM</span><span class="special">(</span><span class="number">1</span><span class="special">,</span><span class="identifier">A_SEQUENCE</span><span class="special">,</span><span class="identifier">BOOST_VMD_TYPE_IDENTIFIER</span><span class="special">,</span><span class="identifier">BOOST_VMD_RETURN_INDEX</span><span class="special">,(</span><span class="identifier">APLACE</span><span class="special">,</span><span class="identifier">ACOUNTRY</span><span class="special">,</span><span class="identifier">ANAME</span><span class="special">),</span><span class="identifier">BOOST_VMD_RETURN_ONLY_AFTER</span><span class="special">)</span> <span class="identifier">will</span> <span class="keyword">return</span> <span class="char">'(1)(2) 46'</span>
</pre>
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