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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&lt;int&gt;</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&lt;int, char*, double&amp;&gt; seq;
typedef <strong>mpl::transform&lt;seq, boost::add_pointer&lt;_&gt; &gt;</strong> calc_ptr_seq;
</pre>
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   #include <boost/mpl/vector.hpp>
   #include <boost/mpl/transform.hpp>
''')
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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
<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 &quot;look inside it,&quot; 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&amp;*</span></tt>.
Laziness and all of its virtues will be a recurring theme
throughout this book.</p>
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