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