<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> <!-- Software License, Version 1.0. (See accompanying --> <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> <title>THE BOOST MPL LIBRARY: apply/lambda Semantics</title> <link rel="stylesheet" href="../style.css" type="text/css" /> </head> <body class="docframe"> <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./changes-in-boost-1-32-0.html" class="navigation-link">Prev</a> <a href="./iterator-protocol.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./iterator-protocol.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./changes-in-boost-1-32-0.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> <td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./changelog-history.html" class="navigation-link">Changelog & History</a> / <a href="./changes-in-boost-1-32-0.html" class="navigation-link">Changes in Boost 1.32.0 Release</a> / <a href="./apply-lambda-semantics.html" class="navigation-link">apply/lambda Semantics</a></td> </tr></table><div class="header-separator"></div> <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> </tbody> </table> </div> <div class="footer-separator"></div> <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./changes-in-boost-1-32-0.html" class="navigation-link">Prev</a> <a href="./iterator-protocol.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./iterator-protocol.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./changes-in-boost-1-32-0.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> </tr></table></body> </html>