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| <td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./composition-and-argument-binding.html" class="navigation-link">Composition and Argument Binding</a> / <a href="./placeholders.html" class="navigation-link">Placeholders</a></td> |
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| <div class="section" id="placeholders"> |
| <h1><a class="toc-backref" href="./composition-and-argument-binding.html#id1529">Placeholders</a></h1> |
| <div class="section" id="id909"> |
| <span id="placeholder"></span><h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> |
| <pre class="literal-block"> |
| namespace <a href="./placeholders.html" class="identifier">placeholders</a> { |
| typedef <em>unspecified</em> _; |
| typedef <a href="./arg.html" class="identifier">arg</a><1> <a href="./placeholders.html" class="identifier">_1</a>; |
| typedef <a href="./arg.html" class="identifier">arg</a><2> <a href="./placeholders.html" class="identifier">_2</a>; |
| <em>...</em> |
| typedef <a href="./arg.html" class="identifier">arg</a><<em>n</em>> _<em>n</em>; |
| } |
| |
| using <a href="./placeholders.html" class="identifier">placeholders</a>::_; |
| using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_1</a>; |
| using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_2</a>; |
| <em>...</em> |
| using <a href="./placeholders.html" class="identifier">placeholders</a>::_<em>n</em>; |
| </pre> |
| </div> |
| <div class="section" id="id910"> |
| <h3><a class="subsection-title" href="#description" name="description">Description</a></h3> |
| <p>A placeholder in a form <tt class="literal"><span class="pre">_</span></tt><em>n</em> is simply a synonym for the corresponding |
| <tt class="literal"><span class="pre"><a href="./arg.html" class="identifier">arg</a><n></span></tt> specialization. The unnamed placeholder <tt class="literal"><span class="pre">_</span></tt> (underscore) carries |
| <a class="reference internal" href="./bind.html#bind-semantics">special meaning</a> in bind and lambda expressions, and does not have |
| defined semantics outside of these contexts.</p> |
| <p>Placeholder names can be made available in the user namespace through |
| <tt class="literal"><span class="pre">using</span> <span class="pre">namespace</span> <span class="pre">mpl::<a href="./placeholders.html" class="identifier">placeholders</a>;</span></tt> directive.</p> |
| </div> |
| <div class="section" id="id912"> |
| <h3><a class="subsection-title" href="#header" name="header">Header</a></h3> |
| <pre class="literal-block"> |
| #include <<a href="../../../../boost/mpl/placeholders.hpp" class="header">boost/mpl/placeholders.hpp</a>> |
| </pre> |
| <p>[<em>Note:</em> The include might be omitted when using placeholders to construct a <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a> for passing it to MPL's own algorithm or metafunction: any library |
| component that is documented to accept a lambda expression makes the placeholders |
| implicitly available for the user code — <em>end note</em>]</p> |
| </div> |
| <div class="section" id="id913"> |
| <h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3> |
| <p>None.</p> |
| </div> |
| <div class="section" id="id914"> |
| <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3> |
| <p>For any integral constant <tt class="literal"><span class="pre">n</span></tt> in the range [1, <a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a>] and |
| arbitrary types <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt>:</p> |
| <pre class="literal-block"> |
| typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><em>n</em><_<em>n</em>,a1,<em>...</em>a<em>n</em>>::type x; |
| </pre> |
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| <col class="field-name" /> |
| <col class="field-body" /> |
| <tbody valign="top"> |
| <tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p> |
| </td> |
| </tr> |
| <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p> |
| <pre class="last literal-block"> |
| typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><em>n</em>< <a href="./arg.html" class="identifier">arg</a><<em>n</em>>,a1,<em>...</em>a<em>n</em> >::type x; |
| </pre> |
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| <div class="section" id="id915"> |
| <h3><a class="subsection-title" href="#example" name="example">Example</a></h3> |
| <pre class="literal-block"> |
| typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>< <a href="./placeholders.html" class="identifier">_1</a>,bool,char,short,int,long >::type t1; |
| typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>< <a href="./placeholders.html" class="identifier">_3</a>,bool,char,short,int,long >::type t3; |
| |
| <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< t1, bool > )); |
| <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< t3, short > )); |
| </pre> |
| </div> |
| <div class="section" id="id916"> |
| <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> |
| <p><a class="reference internal" href="./composition-and-argument-binding.html">Composition and Argument Binding</a>, <a class="reference internal" href="./arg.html">arg</a>, <a class="reference internal" href="./lambda.html">lambda</a>, <a class="reference internal" href="./bind.html">bind</a>, <a class="reference internal" href="./apply.html">apply</a>, <a class="reference internal" href="./apply-wrap.html">apply_wrap</a></p> |
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