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<div class="refentry">
<a name="boost.proto.pass_through"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span class="refentrytitle">Struct template pass_through</span></h2>
<p>boost::proto::pass_through — A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> that transforms the child expressions of an expression
node according to the corresponding children of a Grammar.</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.transform.pass_through_hpp" title="Header <boost/proto/transform/pass_through.hpp>">boost/proto/transform/pass_through.hpp</a>>
</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> Grammar<span class="special">></span>
<span class="keyword">struct</span> <a class="link" href="pass_through.html" title="Struct template pass_through">pass_through</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a>< pass_through<Grammar> > <span class="special">{</span>
<span class="comment">// member classes/structs/unions</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="../../Expr.html" title="Concept Expr">Expr</a><span class="special">,</span> <span class="keyword">typename</span> State<span class="special">,</span> <span class="keyword">typename</span> Data<span class="special">></span>
<span class="keyword">struct</span> <a class="link" href="pass_through/impl.html" title="Struct template impl">impl</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform_impl.html" title="Struct template transform_impl">proto::transform_impl</a><Expr, State, Data> <span class="special">{</span>
<span class="comment">// types</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Grammar</span><span class="special">,</span> <span class="identifier">N</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.GN"><span class="identifier">GN</span></a><span class="special">;</span> <span class="comment">// For each N in [0,Expr arity), for exposition only</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">result_of</span><span class="special">::</span><span class="identifier">child_c</span><span class="special"><</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">N</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.EN"><span class="identifier">EN</span></a><span class="special">;</span> <span class="comment">// For each N in [0,Expr arity), for exposition only</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">GN</span><span class="special">(</span><span class="identifier">EN</span><span class="special">,</span><span class="identifier">State</span><span class="special">,</span><span class="identifier">Data</span><span class="special">)</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.RN"><span class="identifier">RN</span></a><span class="special">;</span> <span class="comment">// For each N in [0,Expr arity), for exposition only</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_tag</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.T"><span class="identifier">T</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">::</span><span class="identifier">proto_domain</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.D"><span class="identifier">D</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">D</span><span class="special">::</span><span class="identifier">proto_generator</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.G"><span class="identifier">G</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<span class="keyword">typedef</span> <a class="link" href="listN.html" title="Struct template listN">proto::listN</a><span class="special"><</span><span class="identifier">R0</span><span class="special">,</span><span class="special">...</span><span class="identifier">RN</span><span class="special">></span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.A"><span class="identifier">A</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<span class="keyword">typedef</span> <a class="link" href="expr.html" title="Struct template expr">proto::expr</a><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.E"><span class="identifier">E</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<span class="keyword">typedef</span> <a class="link" href="basic_expr.html" title="Struct template basic_expr">proto::basic_expr</a><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">A</span><span class="special">></span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.BE"><span class="identifier">BE</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">if_</span><span class="special"><</span><a class="link" href="wants_basic_expr.html" title="Struct template wants_basic_expr">proto::wants_basic_expr</a><span class="special"><</span><span class="identifier">G</span><span class="special">></span><span class="special">,</span> <span class="identifier">BE</span><span class="special">,</span> <span class="identifier">E</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.expr_type"><span class="identifier">expr_type</span></a><span class="special">;</span> <span class="comment">// For exposition only</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">D</span><span class="special">(</span><span class="identifier">expr_type</span><span class="special">)</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span> <a class="link" href="pass_through/impl.html#boost.proto.pass_through.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span>
<span class="comment">// <a class="link" href="pass_through/impl.html#id1535345-bb">public member functions</a></span>
<span class="identifier">result_type</span> <a class="link" href="pass_through/impl.html#id1535350-bb"><span class="keyword">operator</span><span class="special">(</span><span class="special">)</span></a><span class="special">(</span><span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">expr_param</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">state_param</span><span class="special">,</span>
<span class="keyword">typename</span> <span class="identifier">impl</span><span class="special">::</span><span class="identifier">data_param</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id2880148"></a><h2>Description</h2>
<p>
Given a Grammar such as <code class="computeroutput"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a><T0, T1></code>,
an expression type that matches the grammar such as
<code class="computeroutput"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a><E0, E1>::type</code>, a state
<code class="computeroutput">S</code> and a data <code class="computeroutput">D</code>, the result of applying
the <code class="computeroutput">proto::pass_through<<a class="link" href="plus.html" title="Struct template plus">proto::plus</a><T0, T1> ></code>
transform is: </p>
<pre class="programlisting"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a><span class="special"><</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">T0</span><span class="special">(</span><span class="identifier">E0</span><span class="special">,</span> <span class="identifier">S</span><span class="special">,</span> <span class="identifier">D</span><span class="special">)</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">result_of</span><span class="special"><</span><span class="identifier">T1</span><span class="special">(</span><span class="identifier">E1</span><span class="special">,</span> <span class="identifier">S</span><span class="special">,</span> <span class="identifier">D</span><span class="special">)</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span>
<span class="special">></span><span class="special">::</span><span class="identifier">type</span></pre>
<p>
</p>
<p>
The above demonstrates how child transforms and child expressions are applied pairwise, and how the
results are reassembled into a new expression node with the same tag type as the original.
</p>
<p>
The explicit use of <code class="computeroutput">proto::pass_through<></code> is not usually
needed, since the expression generator metafunctions such as
<code class="computeroutput"><a class="link" href="plus.html" title="Struct template plus">proto::plus</a><></code> have
<code class="computeroutput">proto::pass_through<></code> as their default transform. So,
for instance, these are equivalent:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><code class="computeroutput">
<a class="link" href="when.html" title="Struct template when">proto::when</a>< <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><X, Y>, proto::pass_through< <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><X, Y> > >
</code></li>
<li class="listitem"><code class="computeroutput">
<a class="link" href="when.html" title="Struct template when">proto::when</a>< <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><X, Y>, <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><X, Y> >
</code></li>
<li class="listitem"><code class="computeroutput">
<a class="link" href="when.html" title="Struct template when">proto::when</a>< <a class="link" href="plus.html" title="Struct template plus">proto::plus</a><X, Y> > // because of proto::when<class X, class Y=X>
</code></li>
<li class="listitem"><code class="computeroutput">
<a class="link" href="plus.html" title="Struct template plus">proto::plus</a><X, Y> // because plus<> is both a grammar and a transform
</code></li>
</ul></div>
<p>
</p>
<p>
For example, consider the following transform that promotes all
<code class="computeroutput">float</code> terminals in an expression to
<code class="computeroutput">double</code>.
</p>
<pre class="programlisting"><span class="comment">// This transform finds all float terminals in an expression and promotes</span>
<span class="comment">// them to doubles.</span>
<span class="keyword">struct</span> <span class="identifier">Promote</span> <span class="special">:</span>
<a class="link" href="or_.html" title="Struct template or_">proto::or_</a><span class="special"><</span>
<a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special"><</span><a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><span class="keyword">float</span><span class="special">></span><span class="special">,</span> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><span class="keyword">double</span><span class="special">></span><span class="special">::</span><span class="identifier">type</span><span class="special">(</span><a class="link" href="_value.html" title="Struct _value">proto::_value</a><span class="special">)</span> <span class="special">></span><span class="special">,</span>
<span class="comment">// terminal<>'s default transform is a no-op:</span>
<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">></span><span class="special">,</span>
<span class="comment">// nary_expr<> has a pass_through<> transform:</span>
<a class="link" href="nary_expr.html" title="Struct template nary_expr">proto::nary_expr</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <a class="link" href="vararg.html" title="Struct template vararg">proto::vararg</a><span class="special"><</span><span class="identifier">Promote</span><span class="special">></span> <span class="special">></span>
<span class="special">></span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span></pre>
<p>
</p>
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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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