64 lines
5.5 KiB
HTML
64 lines
5.5 KiB
HTML
<?xml version="1.0" encoding="utf-8" ?>
|
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
|
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
|
|
<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
|
|
<!-- Software License, Version 1.0. (See accompanying -->
|
|
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
|
|
<title>THE BOOST MPL LIBRARY: Handling Placeholders</title>
|
|
<link rel="stylesheet" href="../style.css" type="text/css" />
|
|
</head>
|
|
<body class="docframe">
|
|
<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./higher-order.html" class="navigation-link">Prev</a> <a href="./the-lambda-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./higher-order.html" class="navigation-link">Back</a> <a href="./more-lambda-capabilities.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial-metafunctions.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
|
|
<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./handling-placeholders.html" class="navigation-link">Handling Placeholders</a></td>
|
|
</tr></table><div class="header-separator"></div>
|
|
<div class="section" id="handling-placeholders">
|
|
<h1><a class="toc-backref" href="./tutorial-metafunctions.html#id48" name="handling-placeholders">Handling Placeholders</a></h1>
|
|
<p>Our implementation of <tt class="literal"><span class="pre">twice</span></tt> already works with metafunction
|
|
classes. Ideally, we would
|
|
like it to work with placeholder expressions too, much the same as
|
|
<tt class="literal"><span class="pre">mpl::transform</span></tt> allows us to pass either form. For example, we
|
|
would like to be able to write:</p>
|
|
<pre class="literal-block">
|
|
template <class X>
|
|
struct two_pointers
|
|
: twice<<strong>boost::add_pointer<_1></strong>, X>
|
|
{};
|
|
</pre>
|
|
<!-- @ example.append('typedef two_pointers<int>::type intstar2;')
|
|
compile('all', pop = 1, expect_error = True) -->
|
|
<p>But when we look at the implementation of <tt class="literal"><span class="pre">boost::add_pointer</span></tt>,
|
|
it becomes clear that the current definition of <tt class="literal"><span class="pre">twice</span></tt> can't
|
|
work that way.</p>
|
|
<pre class="literal-block">
|
|
template <class T>
|
|
struct add_pointer
|
|
{
|
|
typedef T* type;
|
|
};
|
|
</pre>
|
|
<!-- @ compile() -->
|
|
<p>To be invokable by <tt class="literal"><span class="pre">twice</span></tt>, <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt> would have
|
|
to be a metafunction class, along the lines of <tt class="literal"><span class="pre">add_pointer_f</span></tt>.
|
|
Instead, it's just a nullary metafunction returning the almost
|
|
senseless type <tt class="literal"><span class="pre">_1*</span></tt>. Any attempt to use <tt class="literal"><span class="pre">two_pointers</span></tt> will
|
|
fail when <tt class="literal"><span class="pre">apply1</span></tt> reaches for a nested <tt class="literal"><span class="pre">::apply</span></tt>
|
|
metafunction in <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt> and finds that it
|
|
doesn't exist.</p>
|
|
<p>We've determined that we don't get the behavior we want
|
|
automatically, so what next? Since <tt class="literal"><span class="pre">mpl::transform</span></tt> can do this
|
|
sort of thing, there ought to be a way for us to do it too — and
|
|
so there is.</p>
|
|
<ul class="toc simple" id="outline">
|
|
<li><a class="reference" href="./the-lambda-metafunction.html" id="id49" name="id49">The <tt class="literal"><span class="pre">lambda</span></tt> Metafunction</a></li>
|
|
<li><a class="reference" href="./the-apply-metafunction.html" id="id50" name="id50">The <tt class="literal"><span class="pre">apply</span></tt> Metafunction</a></li>
|
|
</ul>
|
|
</div>
|
|
|
|
<div class="footer-separator"></div>
|
|
<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./higher-order.html" class="navigation-link">Prev</a> <a href="./the-lambda-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./higher-order.html" class="navigation-link">Back</a> <a href="./more-lambda-capabilities.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial-metafunctions.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
|
|
</tr></table></body>
|
|
</html>
|