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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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