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| <h1><a class="toc-backref" href="./tutorial-metafunctions.html#id59" name="details">Details</a></h1> |
| <p>By now you should have a fairly complete view of the fundamental |
| concepts and language of both template metaprogramming in general |
| and of the Boost Metaprogramming Library. This section |
| reviews the highlights.</p> |
| <dl> |
| <dt>Metafunction forwarding.</dt> |
| <dd>The technique of using public derivation to |
| supply the nested <tt class="literal"><span class="pre">type</span></tt> of a metafunction by accessing the one |
| provided by its base class.</dd> |
| <dt>Metafunction class.</dt> |
| <dd>The most basic way to formulate a compile-time |
| function so that it can be treated as polymorphic metadata; that |
| is, as a type. A metafunction class is a class with a nested |
| metafunction called <tt class="literal"><span class="pre">apply</span></tt>.</dd> |
| <dt>MPL.</dt> |
| <dd><p class="first">Most of this book's examples will use the Boost |
| Metaprogramming Library. Like the Boost type traits headers, |
| MPL |
| headers follow a simple convention:</p> |
| <pre class="literal-block"> |
| #include <boost/mpl/<em>component-name</em>.hpp> |
| </pre> |
| <p class="last">If the component's name ends in an underscore, however, the |
| corresponding MPL header name does not include the trailing |
| underscore. For example, <tt class="literal"><span class="pre">mpl::bool_</span></tt> can be found in |
| <tt class="literal"><span class="pre"><boost/mpl/bool.hpp></span></tt>. Where the library deviates from this |
| convention, we'll be sure to point it out to you.</p> |
| </dd> |
| </dl> |
| <!-- @ignore() --> |
| <dl> |
| <dt>Higher-order function.</dt> |
| <dd>A function that operates on or returns a function. Making |
| metafunctions polymorphic with other metadata is a key |
| ingredient in higher-order metaprogramming.</dd> |
| <dt>Lambda expression.</dt> |
| <dd>Simply put, a lambda expression is callable metadata. Without |
| some form of callable metadata, higher-order metafunctions |
| would be impossible. Lambda expressions have two basic forms: |
| <em>metafunction classes</em> and <em>placeholder expressions</em>.</dd> |
| <dt>Placeholder expression.</dt> |
| <dd><p class="first">A kind of lambda expression that, through the use of |
| placeholders, enables in-place <em>partial metafunction |
| application</em> and <em>metafunction composition</em>. As you will see |
| throughout this book, these features give us the truly amazing |
| ability to build up almost any kind of complex type computation |
| from more primitive metafunctions, right at its point of use:</p> |
| <pre class="literal-block"> |
| // find the position of a type x in some_sequence such that: |
| // x is convertible to 'int' |
| // && x is not 'char' |
| // && x is not a floating type |
| typedef mpl::find_if< |
| some_sequence |
| , mpl::and_< |
| boost::is_convertible<_1,int> |
| , mpl::not_<boost::is_same<_1,char> > |
| , mpl::not_<boost::is_float<_1> > |
| > |
| >::type iter; |
| </pre> |
| <p class="last">Placeholder expressions make good on the promise of algorithm reuse |
| without forcing us to write new metafunction classes. The |
| corresponding capability is often sorely missed in the runtime |
| world of the STL, since it is often much easier to write a loop |
| by hand than it is to use standard algorithms, despite their |
| correctness and efficiency advantages.</p> |
| </dd> |
| </dl> |
| <!-- @ example.prepend(''' |
| #include <boost/mpl/and.hpp> |
| #include <boost/mpl/not.hpp> |
| #include <boost/mpl/find_if.hpp> |
| #include <boost/type_traits/is_convertible.hpp> |
| #include <boost/type_traits/is_float.hpp> |
| typedef mpl::vector<char, double, short, long> some_sequence; |
| ''') |
| compile() --> |
| <dl> |
| <dt>The <tt class="literal"><span class="pre">lambda</span></tt> metafunction.</dt> |
| <dd>A metafunction that transforms a lambda expression into a |
| corresponding metafunction class. For detailed information on |
| <tt class="literal"><span class="pre">lambda</span></tt> and the lambda evaluation process, |
| please see the <a class="reference" href="./reference-manual.html">the MPL reference manual</a>.</dd> |
| <dt>The <tt class="literal"><span class="pre">apply</span></tt> metafunction.</dt> |
| <dd>A metafunction that invokes its first argument, which must be a |
| lambda expression, on its remaining arguments. In general, to |
| invoke a lambda expression, you should always pass it to |
| <tt class="literal"><span class="pre">mpl::apply</span></tt> along with the arguments you want to apply it |
| to in lieu of using <tt class="literal"><span class="pre">lambda</span></tt> and invoking the result "manually."</dd> |
| <dt>Lazy evaluation.</dt> |
| <dd>A strategy of delaying evaluation until a result is |
| required, thereby avoiding any unnecessary computation and any |
| associated unnecessary errors. Metafunctions are only invoked |
| when we access their nested <tt class="literal"><span class="pre">::type</span></tt>s, so we can supply all |
| of their arguments without performing any computation and |
| delay evaluation to the last possible moment.</dd> |
| </dl> |
| </div> |
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