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<title>Struct deduce_domain</title>
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<a name="boost.proto.deduce_domain"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Struct deduce_domain</span></h2>
<p>boost::proto::deduce_domain — A pseudo-domain for use in functions and metafunctions that require a domain parameter.
It indicates that the domain of the parent node should be inferred from the domains of the child nodes.</p>
</div>
<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../proto/reference.html#header.boost.proto.domain_hpp" title="Header <boost/proto/domain.hpp>">boost/proto/domain.hpp</a>>
</span>
<span class="keyword">struct</span> <a class="link" href="deduce_domain.html" title="Struct deduce_domain">deduce_domain</a> <span class="special">{</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idp176525560"></a><h2>Description</h2>
<p>
When <code class="computeroutput">proto::deduce_domain</code> is used as a domain — either
explicitly or implicitly by
<code class="computeroutput"><a class="link" href="make_expr_idp66958136.html" title="Function make_expr">proto::make_expr</a>()</code>,
<code class="computeroutput"><a class="link" href="unpack_expr_idp66974776.html" title="Function unpack_expr">proto::unpack_expr</a>()</code>,
or Proto's operator overloads — Proto will use the domains of the child expressions to
compute the domain of the parent. It is done in such a way that (A) expressions in domains
that share a common super-domain are interoperable, and (B) expressions that are in
the default domain (or a sub-domain thereof) are interoperable with <span class="emphasis"><em>all</em></span>
expressions. The rules are as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
A sub-domain is <span class="emphasis"><em>stronger</em></span> than its super-domain.
</li>
<li class="listitem">
<code class="computeroutput"><a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a></code>,
<code class="computeroutput"><a class="link" href="basic_default_domain.html" title="Struct basic_default_domain">proto::basic_default_domain</a></code>
and all their sub-domains are <span class="emphasis"><em>weaker</em></span> than all other domains.
</li>
<li class="listitem">
<code class="computeroutput"><a class="link" href="basic_default_domain.html" title="Struct basic_default_domain">proto::basic_default_domain</a></code>
is weaker than
<code class="computeroutput"><a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a></code>.
</li>
<li class="listitem">
For each child, define a set of domains <span class="emphasis"><em>S<sub>N</sub></em></span>
that includes the child's domain and all its super-domains.
</li>
<li class="listitem">
Define a set <span class="emphasis"><em>I<sub>S</sub></em></span> that is the intersection of
all the individual sets <span class="emphasis"><em>S<sub>N</sub></em></span> that don't contain
<code class="computeroutput"><a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a></code> or
<code class="computeroutput"><a class="link" href="basic_default_domain.html" title="Struct basic_default_domain">proto::basic_default_domain</a></code>.
</li>
<li class="listitem">
Define a set <span class="emphasis"><em>I<sub>W</sub></em></span> that is the intersection of
all the individual sets <span class="emphasis"><em>S<sub>N</sub></em></span> that contain
<code class="computeroutput"><a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a></code> or
<code class="computeroutput"><a class="link" href="basic_default_domain.html" title="Struct basic_default_domain">proto::basic_default_domain</a></code>.
</li>
<li class="listitem">
Define a set <span class="emphasis"><em>P</em></span> that is the union of
<span class="emphasis"><em>I<sub>S</sub></em></span> and
<span class="emphasis"><em>I<sub>W</sub></em></span>.
</li>
<li class="listitem">
The common domain is the strongest domain in set <span class="emphasis"><em>P</em></span>, with the
following caveats.
</li>
<li class="listitem">
Let <span class="emphasis"><em>U</em></span> be the union of all sets
<span class="emphasis"><em>S<sub>N</sub></em></span>. If the result is
<code class="computeroutput"><a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a></code> or
<code class="computeroutput"><a class="link" href="basic_default_domain.html" title="Struct basic_default_domain">proto::basic_default_domain</a></code>
and <span class="emphasis"><em>U</em></span> contains an element that is <span class="emphasis"><em>not </em></span><code class="computeroutput"><a class="link" href="default_domain.html" title="Struct default_domain">proto::default_domain</a></code> or
<code class="computeroutput"><a class="link" href="basic_default_domain.html" title="Struct basic_default_domain">proto::basic_default_domain</a></code>,
it is an error.
</li>
</ul></div>
<p>
</p>
<p>
Note: the above description sounds like it would be expensive to compute at compile time.
In fact, it can all be done using C++ function overloading.
</p>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2008 Eric Niebler<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>)
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