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<title>Struct template protect</title>
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<div class="refentry" title="Struct template protect">
<a name="boost.proto.protect"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2><span 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="emphasis"><em>// 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>>
</em></span><span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a>>
<span class="bold"><strong>struct</strong></span> <a class="link" href="protect.html" title="Struct template protect">protect</a> : <span class="bold"><strong></strong></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a>< protect<PrimitiveTransform> > {
<span class="emphasis"><em>// member classes/structs/unions</em></span>
<span class="bold"><strong>template</strong></span><<span class="bold"><strong>typename</strong></span> , <span class="bold"><strong>typename</strong></span> , <span class="bold"><strong>typename</strong></span> >
<span class="bold"><strong>struct</strong></span> <a class="link" href="protect/impl.html" title="Struct template impl">impl</a> {
<span class="emphasis"><em>// types</em></span>
<span class="bold"><strong>typedef</strong></span> PrimitiveTransform <a class="link" href="protect/impl.html#boost.proto.protect.impl.result_type">result_type</a>;
};
};</pre></div>
<div class="refsect1" title="Description">
<a name="id1674234"></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">template<typename T>
struct Bar
{};
struct Foo :
<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, Bar<<a class="link" href="_.html" title="Struct _">proto::_</a>>() >
{};
<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><int>::type i = {0};
int main() {
Foo()(i);
std::cout << typeid(Foo()(i)).name() << std::endl;
}</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">// OK: replace anything with Bar<_>()
struct Foo :
<a class="link" href="when.html" title="Struct template when">proto::when</a><<a class="link" href="_.html" title="Struct _">proto::_</a>, Bar<<a class="link" href="protect.html" title="Struct template protect">proto::protect</a><<a class="link" href="_.html" title="Struct _">proto::_</a>> >() >
{};</pre>
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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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