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  10 <title>THE BOOST MPL LIBRARY: The Importance of Being Lazy</title>
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  17 <div class="section" id="the-importance-of-being">
  18 <h1><a class="toc-backref" href="./lambda-details.html#id58" name="the-importance-of-being">The Importance of Being Lazy</a></h1>
  19 <p>Recall the definition of <tt class="literal"><span class="pre">always_int</span></tt> from the previous chapter:</p>
  20 <pre class="literal-block">
  21 struct always_int
  22 {
  23     typedef int type;
  24 };
  25 </pre>
  26 <p>Nullary metafunctions might not seem very important at first, since
  27 something like <tt class="literal"><span class="pre">add_pointer&lt;int&gt;</span></tt> could be replaced by <tt class="literal"><span class="pre">int*</span></tt> in
  28 any lambda expression where it appears.  Not all nullary
  29 metafunctions are that simple, though:</p>
  30 <pre class="literal-block">
  31 typedef mpl::vector&lt;int, char*, double&amp;&gt; seq;
  32 typedef <strong>mpl::transform&lt;seq, boost::add_pointer&lt;_&gt; &gt;</strong> calc_ptr_seq;
  33 </pre>
  34 <!-- @ example.prepend('''
  35    #include <boost/mpl/vector.hpp>
  36    #include <boost/mpl/transform.hpp>
  37 ''')
  38 compile('all') -->
  39 <p>Note that <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> is a nullary metafunction, since it has
  40 <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
  41 instantiated until we actually &quot;look inside it,&quot; though.  Just
  42 naming <tt class="literal"><span class="pre">calc_ptr_seq</span></tt> does not cause it to be evaluated, since we
  43 haven't accessed its <tt class="literal"><span class="pre">::type</span></tt> yet.</p>
  44 <p>Metafunctions can be invoked <em>lazily</em>, rather than immediately upon
  45 supplying all of their arguments.  We can use <strong>lazy evaluation</strong> to
  46 improve compilation time when a metafunction result is only going
  47 to be used conditionally.  We can sometimes also avoid contorting
  48 program structure by <em>naming</em> an invalid computation without
  49 actually performing it.  That's what we've done with
  50 <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&amp;*</span></tt>.
  51 Laziness and all of its virtues will be a recurring theme
  52 throughout this book.</p>
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