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- <div class="section" id="placeholders">
- <h1><a class="toc-backref" href="./lambda-details.html#id55" name="placeholders">Placeholders</a></h1>
- <p>The definition of "placeholder" may surprise you:</p>
- <div class="admonition-definition admonition">
- <p class="admonition-title first">Definition</p>
- <p>A <strong>placeholder</strong> is a metafunction class of the form <tt class="literal"><span class="pre">mpl::arg<X></span></tt>.</p>
- </div>
- <div class="section" id="implementation">
- <h2><a name="implementation">Implementation</a></h2>
- <p>The convenient names <tt class="literal"><span class="pre">_1</span></tt>, <tt class="literal"><span class="pre">_2</span></tt>,... <tt class="literal"><span class="pre">_5</span></tt> are actually
- <tt class="literal"><span class="pre">typedef</span></tt>s for specializations of <tt class="literal"><span class="pre">mpl::arg</span></tt> that simply
- select the <em>N</em>th argument for any <em>N</em>. <a class="footnote-reference" href="#config" id="id12" name="id12">[6]</a> The
- implementation of placeholders looks something like this:</p>
- <table class="footnote" frame="void" id="config" rules="none">
- <colgroup><col class="label" /><col /></colgroup>
- <tbody valign="top">
- <tr><td class="label"><a class="fn-backref" href="#id12" name="config">[6]</a></td><td>MPL provides five placeholders by default. See
- the Configuration Macros section of <a class="reference" href="./reference-manual.html">the MPL reference manual</a> for a
- description of how to change the number of placeholders
- provided.</td></tr>
- </tbody>
- </table>
- <pre class="literal-block">
- namespace boost { namespace mpl { namespace placeholders {
- template <int N> struct arg; // forward declarations
- struct void_;
- template <>
- struct arg<<strong>1</strong>>
- {
- template <
- class <strong>A1</strong>, class A2 = void_, ... class A<em>m</em> = void_>
- struct apply
- {
- typedef <strong>A1</strong> type; // return the first argument
- };
- };
- typedef <strong>arg<1> _1</strong>;
- template <>
- struct arg<<strong>2</strong>>
- {
- template <
- class A1, class <strong>A2</strong>, class A3 = void_, ...class A<em>m</em> = void_
- >
- struct apply
- {
- typedef <strong>A2</strong> type; // return the second argument
- };
- };
- typedef <strong>arg<2> _2</strong>;
- <em>more specializations and typedefs...</em>
- }}}
- </pre>
- <!-- @example.replace('...','') -->
- <p>Remember that invoking a metafunction class is the same as invoking
- its nested <tt class="literal"><span class="pre">apply</span></tt> metafunction. When a placeholder in a lambda
- expression is evaluated, it is invoked on the expression's actual
- arguments, returning just one of them. The results are then
- substituted back into the lambda expression and the evaluation
- process continues.</p>
- </div>
- <div class="section" id="the-unnamed-placeholder">
- <h2><a name="the-unnamed-placeholder">The Unnamed Placeholder</a></h2>
- <p>There's one special placeholder, known as the <strong>unnamed
- placeholder</strong>, that we haven't yet defined:</p>
- <pre class="literal-block">
- namespace boost { namespace mpl { namespace placeholders {
- <strong>typedef arg<-1> _;</strong> // the unnamed placeholder
- }}}
- </pre>
- <!-- @ stack[-2].prepend('namespace shield {')
- example.append('}') # so we don't conflict with the prefix
- compile('all') -->
- <p>The details of its implementation aren't important; all you really
- need to know about the unnamed placeholder is that it gets special
- treatment. When a lambda expression is being transformed into a
- metafunction class by <tt class="literal"><span class="pre">mpl::lambda</span></tt>,</p>
- <blockquote>
- the <em>n</em>th appearance of the unnamed placeholder <em>in a given
- template specialization</em> is replaced with <tt class="literal"><span class="pre">_</span></tt><em>n</em>.</blockquote>
- <p>So, for example, every row of Table 1.1
- below contains two equivalent lambda expressions.</p>
- <table border="1" class="table">
- <caption>Unnamed Placeholder Semantics</caption>
- <colgroup>
- <col width="48%" />
- <col width="52%" />
- </colgroup>
- <tbody valign="top">
- <tr><td><pre class="first last literal-block">
- mpl::plus<_,_>
- </pre>
- </td>
- <td><pre class="first last literal-block">
- mpl::plus<_1,_2>
- </pre>
- </td>
- </tr>
- <tr><td><pre class="first last literal-block">
- boost::is_same<
- _
- , boost::add_pointer<_>
- >
- </pre>
- </td>
- <td><pre class="first last literal-block">
- boost::is_same<
- _1
- , boost::add_pointer<_1>
- >
- </pre>
- </td>
- </tr>
- <tr><td><pre class="first last literal-block">
- mpl::multiplies<
- mpl::plus<_,_>
- , mpl::minus<_,_>
- >
- </pre>
- </td>
- <td><pre class="first last literal-block">
- mpl::multiplies<
- mpl::plus<_1,_2>
- , mpl::minus<_1,_2>
- >
- </pre>
- </td>
- </tr>
- </tbody>
- </table>
- <!-- @ for n in range(len(stack)):
- stack[n].wrap('typedef ', 'type%d;' % n)
- compile('all') -->
- <p>Especially when used in simple lambda expressions, the unnamed
- placeholder often eliminates just enough syntactic "noise" to
- significantly improve readability.</p>
- </div>
- </div>
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