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95 lines
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<title>THE BOOST MPL LIBRARY: apply/lambda Semantics</title>
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<div class="section" id="apply-lambda-semantics">
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<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>
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<ol class="arabic">
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<li><p class="first">Nullary metafunction classes are now consistent with all other
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metafunction classes, rather than being identical to nullary
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metafunctions:</p>
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<col width="40%" />
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</colgroup>
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<thead valign="bottom">
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<tr><th><p class="first last">Nullary...</p>
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</th>
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<th><p class="first last">Before</p>
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</th>
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<th><p class="first last">Now</p>
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</th>
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</tr>
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</thead>
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<tbody valign="top">
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<tr><td><p class="first last">Metafunction</p>
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</td>
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<td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
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</td>
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<td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
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</td>
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</tr>
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<tr><td><p class="first last">Metafunction class</p>
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</td>
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<td><p class="first last"><tt class="literal"><span class="pre">c::type</span></tt></p>
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</td>
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<td><p class="first last"><tt class="literal"><span class="pre">c::apply<>::type</span></tt> or
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<tt class="literal"><span class="pre">c::apply::type</span></tt> <a class="footnote-reference" href="#id21" id="id20" name="id20">[7]</a></p>
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</td>
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</tr>
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</tbody>
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</table>
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<p>This fixes a flaw in the metafunction class/metafunction algebra
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that long has been a major source of other inconsistencies throughout
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the library.</p>
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</li>
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<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>
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on its first argument. For example, the following is well-formed now:</p>
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<pre class="literal-block">
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typedef apply< plus<_1,_1>, int_<2> >::type four;
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</pre>
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<p><tt class="literal"><span class="pre">apply_wrap</span></tt><em>n</em> auxiliary metafunctions are available for rare cases
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when it's crucial to eliminate the small overhead associated with lambda
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expressions while still avoiding various compiler bugs known to
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cause problems with
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the obvious <tt class="literal"><span class="pre">f::template</span> <span class="pre">apply<...>::type</span></tt> syntax.</p>
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</li>
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<li><p class="first">The <tt class="literal"><span class="pre">lambda</span></tt> implementation for broken compilers has been
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reworked to emulate the semantics of the full-fledged version
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as closely as possible; consequently, any code out there that
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was taking advantage of the semantic quirks of less complete
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emulation in the previous release may stop working with the new
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one.</p>
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</li>
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</ol>
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<p>To get a detailed picture of the new apply/lambda semantics, please
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see the test case at
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<a class="reference" href="../../test/apply.cpp" target="_top">libs/mpl/test/apply.cpp</a>
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and the corresponding sections of the MPL <a class="reference" href="./reference-manual.html">reference manual</a>.</p>
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<table class="footnote" frame="void" id="id21" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tbody valign="top">
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<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>
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