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