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<tr class="t-nv"><td colspan="5"> <a href="sfinae.html" title="cpp/language/sfinae"> SFINAE</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="concepts.html" title="cpp/language/constraints"> Constraints and concepts</a><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(concepts TS)</span> </td></tr>
</table></div>
</div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/language/templates/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>A template parameter pack is a template parameter that accepts zero or more template arguments (non-types, types, or templates). A function parameter pack is a function parameter that accepts zero or more function arguments.
</p><p>A template with at least one parameter pack is called a <i>variadic template</i>.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="parameter_pack.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="parameter_pack.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="parameter_pack.html#Pack_expansion"><span class="tocnumber">3</span> <span class="toctext">Pack expansion</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="parameter_pack.html#Expansion_loci"><span class="tocnumber">4</span> <span class="toctext">Expansion loci</span></a>
<ul>
<li class="toclevel-2 tocsection-5"><a href="parameter_pack.html#Function_argument_lists"><span class="tocnumber">4.1</span> <span class="toctext">Function argument lists</span></a></li>
<li class="toclevel-2 tocsection-6"><a href="parameter_pack.html#Template_argument_lists"><span class="tocnumber">4.2</span> <span class="toctext">Template argument lists</span></a></li>
<li class="toclevel-2 tocsection-7"><a href="parameter_pack.html#Function_parameter_list"><span class="tocnumber">4.3</span> <span class="toctext">Function parameter list</span></a></li>
<li class="toclevel-2 tocsection-8"><a href="parameter_pack.html#Template_parameter_list"><span class="tocnumber">4.4</span> <span class="toctext">Template parameter list</span></a></li>
<li class="toclevel-2 tocsection-9"><a href="parameter_pack.html#Base_specifiers_and_member_initializer_lists"><span class="tocnumber">4.5</span> <span class="toctext">Base specifiers and member initializer lists</span></a></li>
<li class="toclevel-2 tocsection-10"><a href="parameter_pack.html#Braced_init_lists"><span class="tocnumber">4.6</span> <span class="toctext">Braced init lists</span></a></li>
<li class="toclevel-2 tocsection-11"><a href="parameter_pack.html#Lambda_captures"><span class="tocnumber">4.7</span> <span class="toctext">Lambda captures</span></a></li>
<li class="toclevel-2 tocsection-12"><a href="parameter_pack.html#The_sizeof..._operator"><span class="tocnumber">4.8</span> <span class="toctext">The sizeof... operator</span></a></li>
<li class="toclevel-2 tocsection-13"><a href="parameter_pack.html#Dynamic_exception_specifications"><span class="tocnumber">4.9</span> <span class="toctext">Dynamic exception specifications</span></a></li>
<li class="toclevel-2 tocsection-14"><a href="parameter_pack.html#Alignment_specifier"><span class="tocnumber">4.10</span> <span class="toctext">Alignment specifier</span></a></li>
<li class="toclevel-2 tocsection-15"><a href="parameter_pack.html#Attribute_list"><span class="tocnumber">4.11</span> <span class="toctext">Attribute list</span></a></li>
<li class="toclevel-2"><a href="parameter_pack.html#Fold-expressions"><span class="tocnumber">4.12</span> <span class="toctext">Fold-expressions</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-17"><a href="parameter_pack.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-18"><a href="parameter_pack.html#See_also"><span class="tocnumber">6</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<p>Template parameter pack (appears in a <a href="class_template.html" title="cpp/language/class template">class template</a> and in a <a href="function_template.html" title="cpp/language/function template">function template</a> parameter list)
</p>
<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep">
</td></tr>
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<td> <span class="t-spar">type</span> <code><b>...</b></code> <span class="t-spar">Args</span><span class="t-mark">(optional)</span>
</td>
<td> (1)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
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<td> <code><b>typename|class</b></code> <code><b>...</b></code> <span class="t-spar">Args</span><span class="t-mark">(optional)</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
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<td> <code><b>template</b></code> <code><b><</b></code> <span class="t-spar">parameter-list</span> <code><b>></b></code> <code><b>typename<span class="t-mark-rev t-since-cxx17">(C++17)</span>|class</b></code> <code><b>...</b></code> <span class="t-spar">Args</span><span class="t-mark">(optional)</span>
</td>
<td> (3)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>
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<td colspan="10" class="t-sdsc-sep">
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<p>Function parameter pack (a form of <a href="declarations.html" title="cpp/language/declarations">declarator</a>, appears in a function parameter list of a variadic function template)
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">Args</span> <code><b>...</b></code> <span class="t-spar">args</span><span class="t-mark">(optional)</span>
</td>
<td> (4)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>Parameter pack expansion (appears in a body of a variadic template)
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">pattern</span> <code><b>...</b></code>
</td>
<td> (5)
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> A non-type template parameter pack with an optional name</div>
<div class="t-li1"><span class="t-li">2)</span> A type template parameter pack with an optional name</div>
<div class="t-li1"><span class="t-li">3)</span> A template template parameter pack with an optional name</div>
<div class="t-li1"><span class="t-li">4)</span> A function parameter pack with an optional name</div>
<div class="t-li1"><span class="t-li">5)</span> Parameter pack expansion: expands to comma-separated list of zero or more <code>pattern</code>s. Pattern must include at least one parameter pack.</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>A variadic class template can be instantiated with any number of template arguments:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ... <span class="me1">Types</span><span class="sy1">></span> <span class="kw1">struct</span> Tuple <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
Tuple<span class="sy1"><></span> t0<span class="sy4">;</span> <span class="co1">// Types contains no arguments</span>
Tuple<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> t1<span class="sy4">;</span> <span class="co1">// Types contains one argument: int</span>
Tuple<span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">></span> t2<span class="sy4">;</span> <span class="co1">// Types contains two arguments: int and float</span>
Tuple<span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span> error<span class="sy4">;</span> <span class="co1">// error: 0 is not a type</span></pre></div></div>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: mention that it must be last in class template, but not function </td></tr></table>
<p>A variadic function template can be called with any number of function arguments (the template arguments are deduced through <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a>):
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ... <span class="me1">Types</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Types ... <span class="me1">args</span><span class="br0">)</span><span class="sy4">;</span>
f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: args contains no arguments</span>
f<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: args contains one argument: int</span>
f<span class="br0">(</span><span class="nu0">2</span>, <span class="nu16">1.0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: args contains two arguments: int and double</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=3" title="Edit section: Pack expansion">edit</a>]</span> <span class="mw-headline" id="Pack_expansion">Pack expansion</span></h3>
<p>A pattern followed by an ellipsis, in which the name of at least one parameter pack appears at least once, is <i>expanded</i> into zero or more comma-separated instantiations of the pattern, where the name of the parameter pack is replaced by each of the types from the pack, in order.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ...<span class="me1">Us</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Us... <span class="me1">pargs</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ...<span class="me1">Ts</span><span class="sy1">></span> <span class="kw4">void</span> g<span class="br0">(</span>Ts... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span>
f<span class="br0">(</span><span class="sy3">&</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// “&args...” is a pack expansion</span>
<span class="co1">// “&args” is its pattern</span>
<span class="br0">}</span>
g<span class="br0">(</span><span class="nu0">1</span>, <span class="nu16">0.2</span>, <span class="st0">"a"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts... args expand to int E1, double E2, const char* E3</span>
<span class="co1">// &args... expands to &E1, &E2, &E3</span>
<span class="co1">// Us... pargs expand to int* E1, double* E2, const char** E3</span></pre></div></div>
<p>If the names of two parameter packs appear in the same pattern, they are expanded simultaneously, and they must have the same length:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>...<span class="sy1">></span> <span class="kw1">struct</span> Tuple <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">></span> <span class="kw1">struct</span> Pair <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ...<span class="me1">Args1</span><span class="sy1">></span> <span class="kw1">struct</span> zip <span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ...<span class="me1">Args2</span><span class="sy1">></span> <span class="kw1">struct</span> with <span class="br0">{</span>
<span class="kw1">typedef</span> Tuple<span class="sy1"><</span>Pair<span class="sy1"><</span>Args1, Args2<span class="sy1">></span>...<span class="sy1">></span> type<span class="sy4">;</span>
<span class="co1">// Pair<Args1, Args2>... is the pack expansion</span>
<span class="co1">// Pair<Args1, Args2> is the pattern</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">typedef</span> zip<span class="sy1"><</span><span class="kw4">short</span>, <span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">with</span><span class="sy1"><</span><span class="kw4">unsigned</span> <span class="kw4">short</span>, <span class="kw4">unsigned</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> T1<span class="sy4">;</span>
<span class="co1">// Pair<Args1, Args2>... expands to</span>
<span class="co1">// Pair<short, unsigned short>, Pair<int, unsigned int> </span>
<span class="co1">// T1 is Tuple<Pair<short, unsigned short>, Pair<int, unsigned>></span>
 
<span class="kw1">typedef</span> zip<span class="sy1"><</span><span class="kw4">short</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">with</span><span class="sy1"><</span><span class="kw4">unsigned</span> <span class="kw4">short</span>, <span class="kw4">unsigned</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> T2<span class="sy4">;</span>
<span class="co1">// error: pack expansion contains parameter packs of different lengths</span></pre></div></div>
<p>If a pack expansion is nested within another pack expansion, the parameter packs that appear inside the innermost pack expansion are expanded by it, and there must be another pack mentioned in the enclosing pack expansion, but not in the innermost one:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ...<span class="me1">Args</span><span class="sy1">></span>
<span class="kw4">void</span> g<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span>
f<span class="br0">(</span><span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">const</span> Args<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// const_cast<const Args*>(&args) is the pattern, it expands two packs</span>
<span class="co1">// (Args and args) simultaneously</span>
 
f<span class="br0">(</span>h<span class="br0">(</span>args...<span class="br0">)</span> <span class="sy2">+</span> args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Nested pack expansion:</span>
<span class="co1">// inner pack expansion is "args...", it is expanded first</span>
<span class="co1">// outer pack expansion is h(E1, E2, E3) + args..., it is expanded</span>
<span class="co1">// second (as h(E1,E2,E3) + E1, h(E1,E2,E3) + E2, h(E1,E2,E3) + E3</span>
<span class="br0">}</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=4" title="Edit section: Expansion loci">edit</a>]</span> <span class="mw-headline" id="Expansion_loci">Expansion loci</span></h3>
<p>Depending on where the expansion takes place, the resulting comma-separated list is a different kind of list: function parameter list, member initializer list, attribute list, etc. The following is the list of all allowed contexts
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=5" title="Edit section: Function argument lists">edit</a>]</span> <span class="mw-headline" id="Function_argument_lists">Function argument lists</span></h4>
<p>A pack expansion may appear inside the parantheses of a function call operator, in which case the largest expression to the left of the ellipsis is the pattern that is expanded.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1">f<span class="br0">(</span><span class="sy3">&</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// expands to f(&E1, &E2, &E3)</span>
f<span class="br0">(</span>n, <span class="sy2">++</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// expands to f(n, ++E1, ++E2, ++E3);</span>
f<span class="br0">(</span><span class="sy2">++</span>args..., n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// expands to f(++E1, ++E2, ++E3, n);</span>
f<span class="br0">(</span><span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">const</span> Args<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// f(const_cast<const E1*>(&X1), const_cast<const E2*>(&X2), const_cast<const E3*>(&X3)</span>
f<span class="br0">(</span>h<span class="br0">(</span>args...<span class="br0">)</span> <span class="sy2">+</span> args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// expands to </span>
<span class="co1">// f(h(E1,E2,E3) + E1, h(E1,E2,E3) + E2, h(E1,E2,E3) + E3)</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=6" title="Edit section: Template argument lists">edit</a>]</span> <span class="mw-headline" id="Template_argument_lists">Template argument lists</span></h4>
<p>Pack expansions can be used anywhere in a template argument list, provided the template has the parameters to match the expansion.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> A, <span class="kw1">class</span> B, <span class="kw1">class</span>...<span class="me1">C</span><span class="sy1">></span> <span class="kw4">void</span> func<span class="br0">(</span>A arg1, B arg2, C...<span class="me1">arg3</span><span class="br0">)</span>
<span class="br0">{</span>
container<span class="sy1"><</span>A,B,C...<span class="sy1">></span> t1<span class="sy4">;</span> <span class="co1">// expands to container<A,B,E1,E2,E3> </span>
container<span class="sy1"><</span>C...,A,B<span class="sy1">></span> t2<span class="sy4">;</span> <span class="co1">// expands to container<E1,E2,E3,A,B> </span>
container<span class="sy1"><</span>A,C...,B<span class="sy1">></span> t3<span class="sy4">;</span> <span class="co1">// expands to container<A,E1,E2,E3,B> </span>
<span class="br0">}</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=7" title="Edit section: Function parameter list">edit</a>]</span> <span class="mw-headline" id="Function_parameter_list">Function parameter list</span></h4>
<p>In a function parameter list, if an ellipsis appears in a parameter declaration (whether it names a function parameter pack (as in, <span class="t-spar">Args</span> <code><b>...</b></code> <span class="t-spar">args</span>) or not) the parameter declaration is the pattern:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> ...<span class="me1">Ts</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Ts...<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
f<span class="br0">(</span><span class="st0">'a'</span>, <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts... expands to void f(char, int)</span>
f<span class="br0">(</span><span class="nu16">0.1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts... expands to void f(double)</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> ...<span class="me1">Ts</span>, <span class="kw4">int</span>... <span class="me1">N</span><span class="sy1">></span> <span class="kw4">void</span> g<span class="br0">(</span>Ts <span class="br0">(</span><span class="sy3">&</span>...<span class="me1">arr</span><span class="br0">)</span><span class="br0">[</span>N<span class="br0">]</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="kw4">int</span> n<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span><span class="sy4">;</span>
g<span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">char</span>, <span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="st0">"a"</span>, n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts (&...arr)[N] expands to </span>
<span class="co1">// const char (&)[2], int(&)[1]</span></pre></div></div>
<p>Note: In the pattern <code>Ts (&...arr)[N]</code>, the ellipsis is the innermost element, not the last element as in all other pack expansions.
</p><p>Note: <code>Ts (&...)[N]</code> is not allowed because the C++11 grammar requires the parenthesized ellipsis to have a name: <a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1488">CWG #1488</a>.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=8" title="Edit section: Template parameter list">edit</a>]</span> <span class="mw-headline" id="Template_parameter_list">Template parameter list</span></h4>
<p>Pack expansion may appear in a template parameter list:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">T</span><span class="sy1">></span> <span class="kw1">struct</span> value_holder
<span class="br0">{</span>
<span class="kw1">template</span><span class="sy1"><</span>T... <span class="me1">Values</span><span class="sy1">></span> <span class="co1">// expands to a non-type template parameter </span>
<span class="kw1">struct</span> apply <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// list, such as <int, char, int(&)[5]></span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=9" title="Edit section: Base specifiers and member initializer lists">edit</a>]</span> <span class="mw-headline" id="Base_specifiers_and_member_initializer_lists">Base specifiers and member initializer lists</span></h4>
<p>A pack expansion may designate the list of base classes in a <a href="class.html" title="cpp/language/class">class declaration</a>. Typically, this also means that the constructor needs to use a pack expansion in the <a href="constructor.html" title="cpp/language/initializer list">member initializer list</a> to call the constructors of these bases:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Mixins</span><span class="sy1">></span>
<span class="kw1">class</span> X <span class="sy4">:</span> <span class="kw1">public</span> Mixins... <span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
X<span class="br0">(</span><span class="kw4">const</span> Mixins<span class="sy3">&</span>... <span class="me1">mixins</span><span class="br0">)</span> <span class="sy4">:</span> Mixins<span class="br0">(</span>mixins<span class="br0">)</span>... <span class="br0">{</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=10" title="Edit section: Braced init lists">edit</a>]</span> <span class="mw-headline" id="Braced_init_lists">Braced init lists</span></h4>
<p>In a braced-init-list (brace-enclosed list of initializers and other braced-init-lists, used in <a href="list_initialization.html" title="cpp/language/list initialization">list-initialization</a> and some other contexts), a pack expansion may appear as well:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">></span> <span class="kw4">void</span> func<span class="br0">(</span>Ts... <span class="me1">args</span><span class="br0">)</span><span class="br0">{</span>
<span class="kw4">const</span> <span class="kw4">int</span> size <span class="sy1">=</span> sizeof...<span class="br0">(</span>args<span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="kw4">int</span> res<span class="br0">[</span>size<span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,args...,<span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// since initializer lists guarantee sequencing, this can be used to</span>
<span class="co1">// call a function on each element of a pack, in order:</span>
<span class="kw4">int</span> dummy<span class="br0">[</span>sizeof...<span class="br0">(</span>Ts<span class="br0">)</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span> <span class="br0">(</span><a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> args, <span class="nu0">0</span><span class="br0">)</span>... <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=11" title="Edit section: Lambda captures">edit</a>]</span> <span class="mw-headline" id="Lambda_captures">Lambda captures</span></h4>
<p>A parameter pack may appear in the capture clause of a <a href="lambda.html" title="cpp/language/lambda">lambda</a> expression
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ...<span class="me1">Args</span><span class="sy1">></span>
<span class="kw4">void</span> f<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span>
<span class="kw4">auto</span> lm <span class="sy1">=</span> <span class="br0">[</span><span class="sy3">&</span>, args...<span class="br0">]</span> <span class="br0">{</span> <span class="kw1">return</span> g<span class="br0">(</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
lm<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=12" title="Edit section: The sizeof... operator">edit</a>]</span> <span class="mw-headline" id="The_sizeof..._operator">The sizeof... operator</span></h4>
<p>The <a href="sizeof....html" title="cpp/language/sizeof...">sizeof...</a> operator is classified as a pack expansion as well
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Types</span><span class="sy1">></span>
<span class="kw1">struct</span> count <span class="br0">{</span>
<span class="kw4">static</span> <span class="kw4">const</span> <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> value <span class="sy1">=</span> sizeof...<span class="br0">(</span>Types<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=13" title="Edit section: Dynamic exception specifications">edit</a>]</span> <span class="mw-headline" id="Dynamic_exception_specifications">Dynamic exception specifications</span></h4>
<p>The list of exceptions in a <a href="except_spec.html" title="cpp/language/except spec">dynamic exception specification</a> may also be a pack expansion
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>...<span class="me1">X</span><span class="sy1">></span> <span class="kw4">void</span> func<span class="br0">(</span><span class="kw4">int</span> arg<span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span>X...<span class="br0">)</span>
<span class="br0">{</span>
<span class="co1">// ... throw different Xs in different situations</span>
<span class="br0">}</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=14" title="Edit section: Alignment specifier">edit</a>]</span> <span class="mw-headline" id="Alignment_specifier">Alignment specifier</span></h4>
<p>Pack expansions are allowed in both the lists of types and the lists of expressions used by the keyword <a href="alignas.html" title="cpp/language/alignas">alignas</a>
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=15" title="Edit section: Attribute list">edit</a>]</span> <span class="mw-headline" id="Attribute_list">Attribute list</span></h4>
<p>Pack expansions are allowed in the lists of <a href="attributes.html" title="cpp/language/attributes">attributes</a>, as in <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>attributes...<span class="br0">]</span><span class="br0">]</span></span></span>. For example: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> <span class="br0">[</span><span class="br0">[</span>attributes...<span class="br0">]</span><span class="br0">]</span> function<span class="br0">(</span><span class="br0">)</span></span></span>
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<h4> <span class="mw-headline" id="Fold-expressions">Fold-expressions</span></h4>
<p>In <a href="fold.html" title="cpp/language/fold">fold-expressions</a>, the pattern is the entire subexpression that does not contain an unexpanded parameter pack.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: a few words about partial specializations and other ways to access individual elements? </td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=17" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include <iostream></span>
 
<span class="kw4">void</span> tprintf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> format<span class="br0">)</span> <span class="co1">// base function</span>
<span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> format<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T, <span class="kw1">typename</span>... <span class="me1">Targs</span><span class="sy1">></span>
<span class="kw4">void</span> tprintf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> format, T value, Targs... <span class="me1">Fargs</span><span class="br0">)</span> <span class="co1">// recursive variadic function</span>
<span class="br0">{</span>
<span class="kw1">for</span> <span class="br0">(</span> <span class="sy4">;</span> <span class="sy2">*</span>format <span class="sy3">!</span><span class="sy1">=</span> <span class="st0">'<span class="es5">\0</span>'</span><span class="sy4">;</span> format<span class="sy2">++</span> <span class="br0">)</span> <span class="br0">{</span>
<span class="kw1">if</span> <span class="br0">(</span> <span class="sy2">*</span>format <span class="sy1">==</span> <span class="st0">'%'</span> <span class="br0">)</span> <span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> value<span class="sy4">;</span>
tprintf<span class="br0">(</span>format<span class="sy2">+</span><span class="nu0">1</span>, Fargs...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// recursive call</span>
<span class="kw1">return</span><span class="sy4">;</span>
<span class="br0">}</span>
<a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="sy2">*</span>format<span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
tprintf<span class="br0">(</span><span class="st0">"% world% %<span class="es1">\n</span>"</span>,<span class="st0">"Hello"</span>,<span class="st0">'!'</span>,<span class="nu0">123</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Hello world! 123</pre></div></div>
</div>
<p>The above example defines a function similar to <span class="t-lc"><a href="../io/c/printf.html" title="cpp/io/c/fprintf">std::printf</a></span>, that replace each occurrence of the character % in the format string with a value.
</p><p>The first overload is called when only the format string is passed and there is no parameter expansion.
</p><p>The second overload contains a separate template parameter for the head of the arguments and a parameter pack, this allows the recursive call to pass only the tail of the parameters until it becomes empty.
</p><p><code><b>Targs</b></code> is the template parameter pack and <code><b>Fargs</b></code> is the function parameter pack
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/parameter_pack&action=edit&section=18" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <a href="function_template.html" title="cpp/language/function template">function template</a>
</td></tr>
<tr class="t-dsc">
<td colspan="2"> <a href="class_template.html" title="cpp/language/class template">class template</a>
</td></tr>
<tr class="t-dsc">
<td> <a href="sizeof....html" title="cpp/language/sizeof..."><tt>sizeof...</tt></a>
</td>
<td> Queries the number of elements in a parameter pack.
</td></tr>
<tr class="t-dsc">
<td colspan="2"> <a href="../utility/variadic.html" title="cpp/utility/variadic">C-style variadic functions</a>
</td></tr>
<tr class="t-dsc">
<td> <a href="../preprocessor/replace.html" title="cpp/preprocessor/replace"> Preprocessor macros</a>
</td>
<td> Can be variadic as well
</td></tr>
</table>
<!--
NewPP limit report
Preprocessor visited node count: 2096/1000000
Preprocessor generated node count: 8266/1000000
Post‐expand include size: 39100/2097152 bytes
Template argument size: 13209/2097152 bytes
Highest expansion depth: 17/40
Expensive parser function count: 0/100
-->
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