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- <title>THE BOOST MPL LIBRARY: apply/lambda Semantics</title>
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- <div class="section" id="apply-lambda-semantics">
- <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>
- <ol class="arabic">
- <li><p class="first">Nullary metafunction classes are now consistent with all other
- metafunction classes, rather than being identical to nullary
- metafunctions:</p>
- <table border="1" class="table">
- <colgroup>
- <col width="37%" />
- <col width="22%" />
- <col width="40%" />
- </colgroup>
- <thead valign="bottom">
- <tr><th><p class="first last">Nullary...</p>
- </th>
- <th><p class="first last">Before</p>
- </th>
- <th><p class="first last">Now</p>
- </th>
- </tr>
- </thead>
- <tbody valign="top">
- <tr><td><p class="first last">Metafunction</p>
- </td>
- <td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
- </td>
- <td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
- </td>
- </tr>
- <tr><td><p class="first last">Metafunction class</p>
- </td>
- <td><p class="first last"><tt class="literal"><span class="pre">c::type</span></tt></p>
- </td>
- <td><p class="first last"><tt class="literal"><span class="pre">c::apply<>::type</span></tt> or
- <tt class="literal"><span class="pre">c::apply::type</span></tt> <a class="footnote-reference" href="#id21" id="id20" name="id20">[7]</a></p>
- </td>
- </tr>
- </tbody>
- </table>
- <p>This fixes a flaw in the metafunction class/metafunction algebra
- that long has been a major source of other inconsistencies throughout
- the library.</p>
- </li>
- <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>
- on its first argument. For example, the following is well-formed now:</p>
- <pre class="literal-block">
- typedef apply< plus<_1,_1>, int_<2> >::type four;
- </pre>
- <p><tt class="literal"><span class="pre">apply_wrap</span></tt><em>n</em> auxiliary metafunctions are available for rare cases
- when it's crucial to eliminate the small overhead associated with lambda
- expressions while still avoiding various compiler bugs known to
- cause problems with
- the obvious <tt class="literal"><span class="pre">f::template</span> <span class="pre">apply<...>::type</span></tt> syntax.</p>
- </li>
- <li><p class="first">The <tt class="literal"><span class="pre">lambda</span></tt> implementation for broken compilers has been
- reworked to emulate the semantics of the full-fledged version
- as closely as possible; consequently, any code out there that
- was taking advantage of the semantic quirks of less complete
- emulation in the previous release may stop working with the new
- one.</p>
- </li>
- </ol>
- <p>To get a detailed picture of the new apply/lambda semantics, please
- see the test case at
- <a class="reference" href="../../test/apply.cpp" target="_top">libs/mpl/test/apply.cpp</a>
- and the corresponding sections of the MPL <a class="reference" href="./reference-manual.html">reference manual</a>.</p>
- <table class="footnote" frame="void" id="id21" rules="none">
- <colgroup><col class="label" /><col /></colgroup>
- <tbody valign="top">
- <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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