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<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./details.html" class="navigation-link">Details</a></td>
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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 &lt;boost/mpl/<em>component-name</em>.hpp&gt;
</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">&lt;boost/mpl/bool.hpp&gt;</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'
//      &amp;&amp; x is not 'char'
//      &amp;&amp; x is not a floating type
typedef mpl::find_if&lt;
      some_sequence
    , mpl::and_&lt;
          boost::is_convertible&lt;_1,int&gt;
        , mpl::not_&lt;boost::is_same&lt;_1,char&gt; &gt;
        , mpl::not_&lt;boost::is_float&lt;_1&gt; &gt;
      &gt;
    &gt;::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 &quot;manually.&quot;</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>
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