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<title>More About Attributes of Compound Components</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="spirit_x3.abstracts.more_compound_attributes"></a><a class="link" href="more_compound_attributes.html" title="More About Attributes of Compound Components">More About
      Attributes of Compound Components</a>
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<p>
        While parsing input, it is often desirable to combine some constant elements
        with variable parts. For instance, let us look at the example of parsing
        or formatting a complex number, which is written as <code class="computeroutput"><span class="special">(</span><span class="identifier">real</span><span class="special">,</span> <span class="identifier">imag</span><span class="special">)</span></code>,
        where <code class="computeroutput"><span class="identifier">real</span></code> and <code class="computeroutput"><span class="identifier">imag</span></code> are the variables representing the
        real and imaginary parts of our complex number. This can be achieved by writing:
      </p>
<pre class="programlisting"><span class="char">'('</span> <span class="special">&gt;&gt;</span> <span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="string">", "</span> <span class="special">&gt;&gt;</span> <span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="char">')'</span>
</pre>
<p>
        Literals (such as <code class="computeroutput"><span class="char">'('</span></code> and <code class="computeroutput"><span class="string">", "</span></code>) do <span class="emphasis"><em>not</em></span>
        expose any attribute (well actually, they do expose the special type <code class="computeroutput"><span class="identifier">unused_type</span></code>, but in this context <code class="computeroutput"><span class="identifier">unused_type</span></code> is interpreted as if the component
        does not expose any attribute at all). It is very important to understand
        that the literals don't consume any of the elements of a fusion sequence
        passed to this component sequence. As said, they just don't expose any attribute
        and don't produce (consume) any data. The following example shows this:
      </p>
<pre class="programlisting"><span class="comment">// the following parses "(1.0, 2.0)" into a pair of double</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">input</span><span class="special">(</span><span class="string">"(1.0, 2.0)"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">strbegin</span> <span class="special">=</span> <span class="identifier">input</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">p</span><span class="special">;</span>
<span class="identifier">x3</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">strbegin</span><span class="special">,</span> <span class="identifier">input</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span>
    <span class="char">'('</span> <span class="special">&gt;&gt;</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="string">", "</span> <span class="special">&gt;&gt;</span> <span class="identifier">x3</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="char">')'</span><span class="special">,</span> <span class="comment">// parser grammar</span>
    <span class="identifier">p</span><span class="special">);</span>                                               <span class="comment">// attribute to fill while parsing</span>
</pre>
<p>
        where the first element of the pair passed in as the data to generate is
        still associated with the first <code class="computeroutput"><span class="identifier">double_</span></code>,
        and the second element is associated with the second <code class="computeroutput"><span class="identifier">double_</span></code>
        parser.
      </p>
<p>
        This behavior should be familiar as it conforms to the way other input and
        output formatting libraries such as <code class="computeroutput"><span class="identifier">scanf</span></code>,
        <code class="computeroutput"><span class="identifier">printf</span></code> or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">format</span></code>
        are handling their variable parts. In this context you can think about <span class="emphasis"><em>Spirit.X3</em></span>'s
        primitive components (such as the <code class="computeroutput"><span class="identifier">double_</span></code>
        above) as of being type safe placeholders for the attribute values.
      </p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
          <span class="bold"><strong>For sequences only:</strong></span> To keep it simple,
          unlike __Spirit.qi__, <span class="emphasis"><em>Spirit.X3</em></span> does not support more
          than one attribute anymore in the <code class="computeroutput"><span class="identifier">parse</span></code>
          and <code class="computeroutput"><span class="identifier">phrase_parse</span></code> function.
          Just use <code class="computeroutput"><span class="identifier">std</span><span class="special">:</span><span class="identifier">tuple</span><span class="error">'</span><span class="special">.</span> <span class="identifier">Be</span> <span class="identifier">sure</span> <span class="identifier">to</span>
          <span class="identifier">include</span> </code>boost/fusion/adapted/std_tuple.hpp'
          in this case.
        </p></td></tr>
</table></div>
<p>
        Let's take a look at this from a more formal perspective:
      </p>
<pre class="programlisting"><span class="identifier">a</span><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">Unused</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">&gt;&gt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">A</span>
</pre>
<p>
        which reads as:
      </p>
<div class="blockquote"><blockquote class="blockquote"><p>
          Given <code class="computeroutput"><span class="identifier">a</span></code> and <code class="computeroutput"><span class="identifier">b</span></code> are parsers, and <code class="computeroutput"><span class="identifier">A</span></code>
          is the attribute type of <code class="computeroutput"><span class="identifier">a</span></code>,
          and <code class="computeroutput"><span class="identifier">unused_type</span></code> is the
          attribute type of <code class="computeroutput"><span class="identifier">b</span></code>, then
          the attribute type of <code class="computeroutput"><span class="identifier">a</span> <span class="special">&gt;&gt;</span> <span class="identifier">b</span></code>
          (<code class="computeroutput"><span class="identifier">a</span> <span class="special">&lt;&lt;</span>
          <span class="identifier">b</span></code>) will be <code class="computeroutput"><span class="identifier">A</span></code>
          as well. This rule applies regardless of the position the element exposing
          the <code class="computeroutput"><span class="identifier">unused_type</span></code> is at.
        </p></blockquote></div>
<p>
        This rule is the key to the understanding of the attribute handling in sequences
        as soon as literals are involved. It is as if elements with <code class="computeroutput"><span class="identifier">unused_type</span></code> attributes 'disappeared' during
        attribute propagation. Notably, this is not only true for sequences but for
        any compound components. For instance, for alternative components the corresponding
        rule is:
      </p>
<pre class="programlisting"><span class="identifier">a</span><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">Unused</span> <span class="special">--&gt;</span> <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">A</span>
</pre>
<p>
        again, allowing to simplify the overall attribute type of an expression.
      </p>
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