101 lines
5.3 KiB
HTML
101 lines
5.3 KiB
HTML
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
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<title>THE BOOST MPL LIBRARY: Placeholder Expression Definition</title>
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./placeholders.html" class="navigation-link">Prev</a> <a href="./lambda-and-non.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./placeholders.html" class="navigation-link">Back</a> <a href="./lambda-and-non.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.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>
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<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="./lambda-details.html" class="navigation-link">Lambda Details</a> / <a href="./placeholder-expression.html" class="navigation-link">Placeholder Expression Definition</a></td>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="placeholder-expression">
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<h1><a class="toc-backref" href="./lambda-details.html#id56" name="placeholder-expression">Placeholder Expression Definition</a></h1>
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<p>Now that you know just what <em>placeholder</em> means, we can define
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<em>placeholder expression</em>:</p>
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<div class="admonition-definition admonition">
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<p class="admonition-title first">Definition</p>
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<p>A placeholder expression is either:</p>
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<blockquote>
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<blockquote>
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<ul class="simple">
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<li>a placeholder</li>
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</ul>
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</blockquote>
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<p><em>or</em></p>
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<blockquote>
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<ul class="simple">
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<li>a template specialization with at least one argument that
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is a placeholder expression.</li>
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</ul>
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</blockquote>
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</blockquote>
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</div>
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<p>In other words, a placeholder expression always involves a
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placeholder.</p>
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<!-- DWA: I'm still not sure we shouldn't be at least mentioning the
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pitfall, but for now it's commented out.
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Lambda and Nullary Metafunctions
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- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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The definition of *placeholder expression* above has an interesting
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implication: an ordinary nullary metafunction is never a placeholder
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expression. In other words, even though ``add_pointer<int>`` is a
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nullary metafunction, it won't be invoked in the expression below;
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the assertion will always fail:
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.. parsed-literal::
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BOOST_STATIC_ASSERT((
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mpl::apply<
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boost::is_same<**boost::add_pointer<int>**,_1>
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, int\*
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>::type::value
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));
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In order to allow a nullary metafunction to be used as a lambda
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expression, MPL provides this definition of ``arg``:
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.. parsed-literal::
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// primary template definition (not a specialization)
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template <class F>
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struct arg
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{
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template <class A1 = void\_, class A2 = void\_, ... class *Am* = void\_>
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struct apply : F
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{
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};
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};
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When applied to a lambda expression's actual arguments, ``arg<F>``
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ignores them and simply returns ``F::type``. In other words, if
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``F`` is a nullary metafunction, ``arg<F>`` is a metafunction class
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that invokes ``F`` and returns the result. So we can transform
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add_pointer<int> into a placeholder and get the desired result
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with:
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.. parsed-literal::
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BOOST_STATIC_ASSERT((
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mpl::apply<
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boost::is_same<
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**mpl::arg<boost::add_pointer<int> >**
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, _1
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>
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, int\*
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>::type::value
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)); -->
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