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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="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> |
| 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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