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17<div class="section" id="apply-lambda-semantics">
18<h1><a class="toc-backref" href="./changes-in-boost-1-32-0.html#id64" name="apply-lambda-semantics"><tt class="literal"><span class="pre">apply</span></tt>/<tt class="literal"><span class="pre">lambda</span></tt> Semantics</a></h1>
19<ol class="arabic">
20<li><p class="first">Nullary metafunction classes are now consistent with all other
21metafunction classes, rather than being identical to nullary
22metafunctions:</p>
23<table border="1" class="table">
24<colgroup>
25<col width="37%" />
26<col width="22%" />
27<col width="40%" />
28</colgroup>
29<thead valign="bottom">
30<tr><th><p class="first last">Nullary...</p>
31</th>
32<th><p class="first last">Before</p>
33</th>
34<th><p class="first last">Now</p>
35</th>
36</tr>
37</thead>
38<tbody valign="top">
39<tr><td><p class="first last">Metafunction</p>
40</td>
41<td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
42</td>
43<td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
44</td>
45</tr>
46<tr><td><p class="first last">Metafunction class</p>
47</td>
48<td><p class="first last"><tt class="literal"><span class="pre">c::type</span></tt></p>
49</td>
50<td><p class="first last"><tt class="literal"><span class="pre">c::apply&lt;&gt;::type</span></tt> or
51<tt class="literal"><span class="pre">c::apply::type</span></tt> <a class="footnote-reference" href="#id21" id="id20" name="id20">[7]</a></p>
52</td>
53</tr>
54</tbody>
55</table>
56<p>This fixes a flaw in the metafunction class/metafunction algebra
57that long has been a major source of other inconsistencies throughout
58the library.</p>
59</li>
60<li><p class="first">The <tt class="literal"><span class="pre">apply</span></tt> metafunction now automatically invokes <tt class="literal"><span class="pre">lambda</span></tt>
61on its first argument.  For example, the following is well-formed now:</p>
62<pre class="literal-block">
63typedef apply&lt; plus&lt;_1,_1&gt;, int_&lt;2&gt; &gt;::type four;
64</pre>
65<p><tt class="literal"><span class="pre">apply_wrap</span></tt><em>n</em> auxiliary metafunctions are available for rare cases
66when it's crucial to eliminate the small overhead associated with lambda
67expressions while still avoiding various compiler bugs known to
68cause problems with
69the obvious <tt class="literal"><span class="pre">f::template</span> <span class="pre">apply&lt;...&gt;::type</span></tt> syntax.</p>
70</li>
71<li><p class="first">The <tt class="literal"><span class="pre">lambda</span></tt> implementation for broken compilers has been
72reworked to emulate the semantics of the full-fledged version
73as closely as possible; consequently, any code out there that
74was taking advantage of the semantic quirks of less complete
75emulation in the previous release may stop working with the new
76one.</p>
77</li>
78</ol>
79<p>To get a detailed picture of the new apply/lambda semantics, please
80see the test case at
81<a class="reference" href="http://cvs.sourceforge.net/viewcvs.py/boost/boost/libs/mpl/test/apply.cpp?view=markup" target="_top">http://cvs.sourceforge.net/viewcvs.py/boost/boost/libs/mpl/test/apply.cpp?view=markup</a>
82and the corresponding sections of the MPL <a class="reference" href="./reference-manual.html">reference manual</a>.</p>
83<table class="footnote" frame="void" id="id21" rules="none">
84<colgroup><col class="label" /><col /></colgroup>
85<tbody valign="top">
86<tr><td class="label"><a class="fn-backref" href="#id20" name="id21">[7]</a></td><td>The second form is available only on fully conforming compilers.</td></tr>
87</tbody>
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90
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