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130 lines
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<title>THE BOOST MPL LIBRARY: Incomplete Support for Lambda Expressions</title>
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<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Prev</a> <a href="./eti.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Back</a> <a href="./eti.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./portability.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>
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<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./technical-details.html" class="navigation-link">Technical Details</a> / <a href="./portability.html" class="navigation-link">Portability</a> / <a href="./incomplete-support-for.html" class="navigation-link">Incomplete Support for Lambda Expressions</a></td>
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<div class="section" id="incomplete-support-for">
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<h1><a class="toc-backref" href="./portability.html#id75" name="incomplete-support-for">Incomplete Support for Lambda Expressions</a></h1>
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<p>Seasoned MPL users will agree with us that if there is
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anything in the MPL that is seemingly magical both in power and
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its nature, then it's MPL lambda expressions. In reality, the
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mechanism that bring this to life is very straightforward and
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probably can be explained to anyone generally familiar with C++
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templates in less than 10 minutes.</p>
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<!-- People are going to hate you for saying so and not actually -->
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<!-- explaining it. -->
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<p>Unfortunately, this mechanism also happens to rely on support for
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partial template specialization and template template parameters.
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Among the so-called deficient compilers — basically, most of the
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compilers released before the year 2000 — the chances are poor
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that you'll find <em>complete</em> support for <em>both</em> of these features.
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Please see our <a class="reference" href="./portability.html#compatibility-table">compatibility table</a> for the list of the products
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which fall into this category.</p>
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<p>Although it's not possible to implement <em>fully transparent</em> lambda
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expressions without these two features, a slightly more limited
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implementation that requires some manual assistance from the
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metafunction author is possible. This section describes the manual
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work required and the limitations of the result.</p>
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<div class="section" id="incomplete-the-problem">
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<h2><a name="incomplete-the-problem">The Problem</a></h2>
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<p>If your compiler falls into the "deficient" category, the following
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valid MPL metaprogram will fail to compile for you:</p>
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<pre class="literal-block">
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#include <boost/mpl/apply.hpp>
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using namespace boost::mpl;
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template< typename T > struct add_const
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{
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typedef T const type;
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};
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typedef apply1< add_const<_1>,int >::type t; // t == int const
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</pre>
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<p>Worse yet, chances are it wil fail with a diagnostic backtrace
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leading you into the inside of the library and possibly creating an
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impression that there's something's wrong there. The fact is, both
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the program and the library are defect free (for the
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purpose of this particular demonstraction), and it's your compiler
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that is to blame.</p>
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</div>
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<div class="section" id="incomplete-the-solution">
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<h2><a name="incomplete-the-solution">The Solution</a></h2>
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<p>As previously mentioned, the solution requires some work from
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metafunction authors, but for the users of those metafunctions, the
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result is relatively transparent. Here's what we have to do to our
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earlier example:</p>
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<pre class="literal-block">
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#include <boost/mpl/apply.hpp>
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<strong>#include <boost/mpl/aux_/lambda_support.hpp></strong>
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using namespace boost::mpl;
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template< typename T > struct add_const
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{
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typedef T const type;
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<strong>BOOST_MPL_AUX_LAMBDA_SUPPORT(1, add_const, (T))</strong>
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};
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typedef apply1< add_const<_1>,int >::type t; // t == int const
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</pre>
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<p>With these two modifications, now the compiler that has been barking at us now
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happily accepts it. "Hey, that's not that bad at all!" you might say. Just put a
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little macro inside and be happy again.</p>
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</div>
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<div class="section" id="limitations">
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<h2><a name="limitations">Limitations</a></h2>
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<p>Unfortunately, that's not quite the end of the story. There are
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still cases where the above approach will fail and we will have to
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resort to writing out-of-line metafunction class. Here are the
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details:</p>
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<blockquote>
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<p>To make the lambda expression work without partial template
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specialization and
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template template parameters, the MPL has to implement some other way of
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pulling apart the template instantiations' expression tree, and the only way
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to do it is through an intrusive metafunction introspection
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mechanism. That's what hidden behind the <tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> macro
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we've seen above.</p>
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<p>But then, after we've got the information we need (the metafunction's arity
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and its exact template arguments) stored inside the metafunction itself,
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the only way for the library to access it is to look inside the metafunction.
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The latter, in its turn, means instantiating the metafunction, prematurely,
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before the actuall call, <em>with one or more placeholder arguments</em>. This last
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part is a potential problem.</p>
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</blockquote>
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<p>In other words, the mechanism works as long as your metafunction is
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"placeholder-safe" (can be safely instantiated on placeholder
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arguments), which comes down to the follwing two criteria:</p>
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<ol class="arabic simple">
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<li>The metafunction doesn't access its arguments' nested members, or</li>
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<li>The only accessed members are types named <tt class="literal"><span class="pre">::tag</span></tt> or <tt class="literal"><span class="pre">::type</span></tt> (the
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placeholders do contain these).</li>
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</ol>
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<p>If these two hold, you can safely put <tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> inside
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your metafunction and forget about the issue. If not, you are out of luck and
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probably have to write a metafunction class instead.</p>
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<p>The good news are that most of the MPL's own metafunctions and <a class="reference" href="../../../type_traits/index.html" target="_top">Boost.Type Traits</a>
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templates are "placeholder-safe" and have the workaround applied to them, so
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even on broken compilers things "just work" in about 90% of use cases.</p>
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<p>Please refer to the MPL <a class="reference" href="./reference-manual.html">reference manual</a> for the details on the
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<tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> macro.</p>
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</div>
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