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  26 <a name="boost.proto.protect"></a><div class="titlepage"></div>
  27 <div class="refnamediv">
  28 <h2><span class="refentrytitle">Struct template protect</span></h2>
  29 <p>boost::proto::protect &#8212; A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> which prevents another
  30           <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> from being applied in an
  31           <a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a>.</p>
  32 </div>
  33 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  34 <div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.transform.make_hpp" title="Header &lt;boost/proto/transform/make.hpp&gt;">boost/proto/transform/make.hpp</a>&gt;
  35
  36 </span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a><span class="special">&gt;</span>
  37 <span class="keyword">struct</span> <a class="link" href="protect.html" title="Struct template protect">protect</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a><span class="special">&lt;</span> <span class="identifier">protect</span><span class="special">&lt;</span><span class="identifier">PrimitiveTransform</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{</span>
  38   <span class="comment">// member classes/structs/unions</span>
  39   <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="special">,</span> <span class="keyword">typename</span> <span class="special">,</span> <span class="keyword">typename</span> <span class="special">&gt;</span>
  40   <span class="keyword">struct</span> <a class="link" href="protect/impl.html" title="Struct template impl">impl</a> <span class="special">{</span>
  41     <span class="comment">// types</span>
  42     <span class="keyword">typedef</span> <span class="identifier">PrimitiveTransform</span> <a class="link" href="protect/impl.html#boost.proto.protect.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span>
  43   <span class="special">}</span><span class="special">;</span>
  44 <span class="special">}</span><span class="special">;</span></pre></div>
  45 <div class="refsect1">
  46 <a name="id2930333"></a><h2>Description</h2>
  47 <p>
  48             When building higher order transforms with
  49             <code class="computeroutput">
  50               <a class="link" href="make.html" title="Struct template make">proto::make&lt;&gt;</a>
  51             </code> or
  52             <code class="computeroutput">
  53               <a class="link" href="lazy.html" title="Struct template lazy">proto::lazy&lt;&gt;</a>
  54             </code>,
  55             you sometimes would like to build types that are parameterized with Proto transforms. In such
  56             lambda-style transforms, Proto will unhelpfully find all nested transforms and apply them, even
  57             if you don't want them to be applied. Consider the following transform, which will replace the
  58             <code class="computeroutput">proto::_</code> in
  59             <code class="computeroutput">Bar&lt;proto::_&gt;()</code>
  60             with <code class="computeroutput">proto::terminal&lt;int&gt;::type</code>:
  61           </p>
  62 <p>
  63             </p>
  64 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
  65 <span class="keyword">struct</span> <span class="identifier">Bar</span>
  66 <span class="special">{</span><span class="special">}</span><span class="special">;</span>
  67
  68 <span class="keyword">struct</span> <span class="identifier">Foo</span> <span class="special">:</span>
  69   <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Bar</span><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span> <span class="special">&gt;</span>
  70 <span class="special">{</span><span class="special">}</span><span class="special">;</span>
  71
  72 <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">}</span><span class="special">;</span>
  73
  74 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span> <span class="special">{</span>
  75   <span class="identifier">Foo</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span><span class="special">;</span>
  76   <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="keyword">typeid</span><span class="special">(</span><span class="identifier">Foo</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span><span class="special">)</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
  77 <span class="special">}</span></pre>
  78 <p>
  79           </p>
  80 <p>
  81             If you actually wanted to default-construct an object of type
  82             <code class="computeroutput">Bar&lt;proto::_&gt;</code>, you would have to protect the
  83             <code class="computeroutput">_</code> to prevent it from being applied. You can
  84             use <code class="computeroutput">proto::protect&lt;&gt;</code> as follows:
  85           </p>
  86 <p>
  87             </p>
  88 <pre class="programlisting"><span class="comment">// OK: replace anything with Bar&lt;_&gt;()</span>
  89 <span class="keyword">struct</span> <span class="identifier">Foo</span> <span class="special">:</span>
  90   <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Bar</span><span class="special">&lt;</span><a class="link" href="protect.html" title="Struct template protect">proto::protect</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">(</span><span class="special">)</span> <span class="special">&gt;</span>
  91 <span class="special">{</span><span class="special">}</span><span class="special">;</span></pre>
  92 <p>
  93           </p>
  94 </div>
  95 </div>
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  97 <td align="left"></td>
  98 <td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
  99         Distributed under the Boost Software License, Version 1.0. (See accompanying
 100         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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