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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, your 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> |
| </div> |
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