boost/libs/mpl/doc/tutorial/partial-metafunction.html

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  10 <title>THE BOOST MPL LIBRARY: Partial Metafunction Application</title>
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  18 <h1><a class="toc-backref" href="./more-lambda-capabilities.html#id52" name="partial-metafunction">Partial Metafunction Application</a></h1>
  19 <p>Consider the lambda expression <tt class="literal"><span class="pre">mpl::plus&lt;_1,_1&gt;</span></tt>.  A single
  20 argument is directed to both of <tt class="literal"><span class="pre">plus</span></tt>'s parameters, thereby
  21 adding a number to itself.  Thus, a <em>binary</em> metafunction,
  22 <tt class="literal"><span class="pre">plus</span></tt>, is used to build a <em>unary</em> lambda expression.  In other
  23 words, we've created a whole new computation!  We're not done yet,
  24 though:  By supplying a non-placeholder as one of the arguments, we
  25 can build a unary lambda expression that adds a fixed value, say
  26 42, to its argument:</p>
  27 <pre class="literal-block">
  28 mpl::plus&lt;_1, mpl::int_&lt;42&gt; &gt;
  29 </pre>
  30 <!-- @ apply_test = 'enum { value = mpl::apply<', ''' >::type::value };
  31               BOOST_STATIC_ASSERT(value == %d);'''
  32
  33 example.wrap(apply_test[0],
  34              ', mpl::int_<3>' + apply_test[1] % 45)
  35 compile() -->
  36 <p>The process of binding argument values to a subset of a function's
  37 parameters is known in the world of functional programming as
  38 <strong>partial function application</strong>.</p>
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