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59 lines
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<title>THE BOOST MPL LIBRARY: The Importance of Being Lazy</title>
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./lambda-and-non.html" class="navigation-link">Prev</a> <a href="./details.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-and-non.html" class="navigation-link">Back</a> Along</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="./the-importance-of-being.html" class="navigation-link">The Importance of Being Lazy</a></td>
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<div class="section" id="the-importance-of-being">
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<h1><a class="toc-backref" href="./lambda-details.html#id58" name="the-importance-of-being">The Importance of Being Lazy</a></h1>
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<p>Recall the definition of <tt class="literal"><span class="pre">always_int</span></tt> from the previous chapter:</p>
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<pre class="literal-block">
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struct always_int
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{
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typedef int type;
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};
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</pre>
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<p>Nullary metafunctions might not seem very important at first, since
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something like <tt class="literal"><span class="pre">add_pointer<int></span></tt> could be replaced by <tt class="literal"><span class="pre">int*</span></tt> in
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any lambda expression where it appears. Not all nullary
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metafunctions are that simple, though:</p>
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<pre class="literal-block">
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typedef mpl::vector<int, char*, double&> seq;
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typedef <strong>mpl::transform<seq, boost::add_pointer<_> ></strong> calc_ptr_seq;
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</pre>
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<!-- @ example.prepend('''
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#include <boost/mpl/vector.hpp>
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#include <boost/mpl/transform.hpp>
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''')
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compile('all') -->
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<p>Note that <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> is a nullary metafunction, since it has
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<tt class="literal"><span class="pre">transform</span></tt>'s nested <tt class="literal"><span class="pre">::type</span></tt>. A C++ template is not
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instantiated until we actually "look inside it," though. Just
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naming <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> does not cause it to be evaluated, since we
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haven't accessed its <tt class="literal"><span class="pre">::type</span></tt> yet.</p>
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<p>Metafunctions can be invoked <em>lazily</em>, rather than immediately upon
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supplying all of their arguments. We can use <strong>lazy evaluation</strong> to
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improve compilation time when a metafunction result is only going
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to be used conditionally. We can sometimes also avoid contorting
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program structure by <em>naming</em> an invalid computation without
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actually performing it. That's what we've done with
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<tt class="literal"><span class="pre">calc_ptr_seq</span></tt> above, since you can't legally form <tt class="literal"><span class="pre">double&*</span></tt>.
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Laziness and all of its virtues will be a recurring theme
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throughout this book.</p>
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