<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Struct template protect</title> <link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../../proto/reference.html#header.boost.proto.transform.make_hpp" title="Header <boost/proto/transform/make.hpp>"> <link rel="prev" href="noinvoke.html" title="Struct template noinvoke"> <link rel="next" href="protect/impl.html" title="Struct template impl"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> <td 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class="refentrytitle">Struct template protect</span></h2> <p>boost::proto::protect — A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> which prevents another <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> from being applied in an <a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a>.</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.make_hpp" title="Header <boost/proto/transform/make.hpp>">boost/proto/transform/make.hpp</a>> </span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a><span class="special">></span> <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"><</span> <span class="identifier">protect</span><span class="special"><</span><span class="identifier">PrimitiveTransform</span><span class="special">></span> <span class="special">></span> <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> <span class="special">,</span> <span class="keyword">typename</span> <span class="special">,</span> <span class="keyword">typename</span> <span class="special">></span> <span class="keyword">struct</span> <a class="link" href="protect/impl.html" title="Struct template impl">impl</a> <span class="special">{</span> <span class="comment">// types</span> <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> <span class="special">}</span><span class="special">;</span> <span class="special">}</span><span class="special">;</span></pre></div> <div class="refsect1"> <a name="idm45555170530656"></a><h2>Description</h2> <p> When building higher order transforms with <code class="computeroutput"> <a class="link" href="make.html" title="Struct template make">proto::make<></a> </code> or <code class="computeroutput"> <a class="link" href="lazy.html" title="Struct template lazy">proto::lazy<></a> </code>, you sometimes would like to build types that are parameterized with Proto transforms. In such lambda-style transforms, Proto will unhelpfully find all nested transforms and apply them, even if you don't want them to be applied. Consider the following transform, which will replace the <code class="computeroutput">proto::_</code> in <code class="computeroutput">Bar<proto::_>()</code> with <code class="computeroutput">proto::terminal<int>::type</code>: </p> <p> </p> <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">Bar</span> <span class="special">{</span><span class="special">}</span><span class="special">;</span> <span class="keyword">struct</span> <span class="identifier">Foo</span> <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="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Bar</span><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">></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> <a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special"><</span><span class="keyword">int</span><span class="special">></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> <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</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="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</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"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span> <span class="special">}</span></pre> <p> </p> <p> If you actually wanted to default-construct an object of type <code class="computeroutput">Bar<proto::_></code>, you would have to protect the <code class="computeroutput">_</code> to prevent it from being applied. You can use <code class="computeroutput">proto::protect<></code> as follows: </p> <p> </p> <pre class="programlisting"><span class="comment">// OK: replace anything with Bar<_>()</span> <span class="keyword">struct</span> <span class="identifier">Foo</span> <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="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Bar</span><span class="special"><</span><a class="link" href="protect.html" title="Struct template protect">proto::protect</a><span class="special"><</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">></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><span class="special">;</span></pre> <p> </p> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="noinvoke.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../proto/reference.html#header.boost.proto.transform.make_hpp"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="protect/impl.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>