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<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread"> Thread support library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../experimental.html" title="cpp/experimental"> Technical Specifications</a> </td></tr>
</table></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/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../language.1.html" title="cpp/language"> C++ language</a></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="functions.html" title="cpp/language/functions"> Functions</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> Declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> function declaration</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html#Parameter_list" title="cpp/language/function"> function parameter list</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html#Function_definition" title="cpp/language/function"> function definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_arguments.html" title="cpp/language/default arguments"> default arguments</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="variadic_arguments.html" title="cpp/language/variadic arguments"> variadic arguments</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="inline.html" title="cpp/language/inline"> inline specifier</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lambda.html" title="cpp/language/lambda"> lambda expression</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Function calls </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="adl.html" title="cpp/language/adl"> argument-dependent lookup (ADL)</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_other.html#Built-in_function_call_operator" title="cpp/language/operator other"> function-call operator</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concept/FunctionObject.html" title="cpp/concept/FunctionObject"> function objects</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Overloading </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="overload_resolution.html" title="cpp/language/overload resolution"> overload resolution</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operators.html" title="cpp/language/operators"> operator overloading</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="overloaded_address.html" title="cpp/language/overloaded address"> address of an overload set</a> </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/functions/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>A function declaration introduces the function name and its type. A function definition associates the function name/type with the function body.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="function.html#Function_declaration"><span class="tocnumber">1</span> <span class="toctext">Function declaration</span></a></li>
<li class="toclevel-1"><a href="function.html#Return_type_deduction_.28since_C.2B.2B14.29"><span class="tocnumber">2</span> <span class="toctext">Return type deduction <span>(since C++14)</span></span></a></li>
<li class="toclevel-1 tocsection-3"><a href="function.html#Parameter_list"><span class="tocnumber">3</span> <span class="toctext">Parameter list</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="function.html#Function_definition"><span class="tocnumber">4</span> <span class="toctext">Function definition</span></a></li>
<li class="toclevel-1"><a href="function.html#Deleted_functions"><span class="tocnumber">5</span> <span class="toctext">Deleted functions</span></a></li>
<li class="toclevel-1"><a href="function.html#func"><span class="tocnumber">6</span> <span class="toctext">__func__</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="function.html#Example_1:_non-member_functions"><span class="tocnumber">7</span> <span class="toctext">Example 1: non-member functions</span></a></li>
<li class="toclevel-1 tocsection-8"><a href="function.html#Defect_reports"><span class="tocnumber">8</span> <span class="toctext">Defect reports</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/function&action=edit&section=1" title="Edit section: Function declaration">edit</a>]</span> <span class="mw-headline" id="Function_declaration">Function declaration</span></h3>
<p>Function declarations may appear in any scope. A function declaration at class scope introduces a class member function (unless the friend specifier is used), see <a href="member_functions.html" title="cpp/language/member functions">member functions</a> and <a href="friend.html" title="cpp/language/friend">friend functions</a> for details.
</p><p>The type of the function being declared is composed from the <i>return type</i> (provided by the <span class="t-spar">decl-specifier-seq</span> of the <a href="declarations.html" title="cpp/language/declarations">declaration syntax</a>) and the function <span class="t-spar">declarator</span>
</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">noptr-declarator</span> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <span class="t-spar">cv</span><span class="t-mark">(optional)</span> <span class="t-spar">ref</span><span class="t-mark">(optional)</span> <span class="t-spar">except</span><span class="t-mark">(optional)</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">requires</span><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(concepts TS)</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">noptr-declarator</span> <code><b>(</b></code> <span class="t-spar">parameter-list</span> <code><b>)</b></code> <span class="t-spar">cv</span><span class="t-mark">(optional)</span> <span class="t-spar">ref</span><span class="t-mark">(optional)</span> <span class="t-spar">except</span><span class="t-mark">(optional)</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>-></b></code> <span class="t-spar">trailing</span> <span class="t-spar">requires</span><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(concepts TS)</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></table>
<p>(see <a href="declarations.html" title="cpp/language/declarations">Declarations</a> for the other forms of the <span class="t-spar">declarator</span> syntax)
</p>
<div class="t-li1"><span class="t-li">1)</span> Regular function declarator syntax</div>
<div class="t-li1"><span class="t-li">2)</span> Trailing return type declaration: trailing return type is only allowed on the outermost function declarator. The decl-specifier-seq in this case must contain the keyword <code>auto</code> </div>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">noptr-declarator</span>
</td>
<td> -
</td>
<td> any valid <span class="t-spar">declarator</span>, but if it begins with *, &, or &&, it has to be surrounded by parentheses.
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">parameter-list</span>
</td>
<td> -
</td>
<td> possibly empty, comma-separated list of the function parameters (see below for details)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> optional list of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">cv</span>
</td>
<td> -
</td>
<td> const/volatile qualification, only allowed in non-static member function declarations
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">ref</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> ref-qualification, only allowed in non-static member function declarations
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">except</span>
</td>
<td> -
</td>
<td> either <a href="except_spec.html" title="cpp/language/except spec">dynamic exception specification</a><span class="t-mark">(deprecated)</span> or <a href="noexcept_spec.html" title="cpp/language/noexcept spec">noexcept specification</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> Note that the exception specification is not part of the function type <span class="t-rev-inl t-since-cxx17"><span>except that non-throwing exception specification declares a function of noexcept function type</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">trailing</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> Trailing return type, useful if the return type depends on argument names, such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">auto</span> add<span class="br0">(</span>T t, U u<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> decltype<span class="br0">(</span>t <span class="sy2">+</span> u<span class="br0">)</span><span class="sy4">;</span></span></span> or is complicated, such as in <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span> fpif<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span><span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span></span></span>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">requires</span><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(concepts TS)</span>
</td>
<td> -
</td>
<td> The <i>requires</i> clause, which declares the associated <a href="concepts.html" title="cpp/language/constraints">constraints</a> for the function, which must be satisfied in order for the function to be selected by <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a>. Note that the associated constraint is part of function signature, but not part of function type.
</td></tr></table>
<p>Function declarators can be mixed with other declarators, where decl-specifier-seq allows:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// declares an int, an int*, a function, and a pointer to a function</span>
<span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span>, <span class="sy2">*</span>p <span class="sy1">=</span> <a href="../types/NULL.html"><span class="kw103">NULL</span></a>, f<span class="br0">(</span><span class="br0">)</span>, <span class="br0">(</span><span class="sy2">*</span>pf<span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// decl-specifier-seq is int</span>
<span class="co1">// declarator f() declares (but doesn't define)</span>
<span class="co1">// a function taking no arguments and returning int</span>
 
<span class="kw1">struct</span> S
<span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span> <span class="kw4">const</span>, g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span> <span class="co1">// declares two non-static member functions</span>
<span class="kw1">virtual</span> <span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span>, x<span class="sy4">;</span> <span class="co1">// compile-time error: virtual (in decl-specifier-seq)</span>
<span class="co1">// is only allowed in declarations of non-static</span>
<span class="co1">// member functions</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>The return type of a function cannot be a function type or an array type (but can be a pointer or reference to those).
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<h3> <span class="mw-headline" id="Return_type_deduction_.28since_C.2B.2B14.29">Return type deduction <span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></h3>
<p>If the <i>decl-specifier-seq</i> of the function declaration contains the keyword <code>auto</code>, trailing return type may be omitted, and will be deduced by the compiler from the type of the expression used in the <a href="return.html" title="cpp/language/return">return</a> statement. If the return type does not use <code>decltype(auto)</code>, the deduction follows the rules of <a href="template_argument_deduction.html#Other_contexts" 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="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
<span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// return type is int</span>
<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy3">&</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// return type is const int&</span></pre></div></div>
<p>If the return type is <code>decltype(auto)</code>, the return type is as what would be obtained if the expression used in the return statement were wrapped in <a href="decltype.html" title="cpp/language/decltype">decltype</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> x<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// return type is int, same as decltype(x)</span>
decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// return type is int&, same as decltype((x))</span></pre></div></div>
<p>(note: "<code>const decltype(auto)&</code>" is an error, <code>decltype(auto)</code> must be used on its own)
</p><p>If there are multiple return statements, they must all deduce to the same type
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> f<span class="br0">(</span><span class="kw4">bool</span> val<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">if</span><span class="br0">(</span>val<span class="br0">)</span> <span class="kw1">return</span> <span class="nu0">123</span><span class="sy4">;</span> <span class="co1">// deduces return type int</span>
<span class="kw1">else</span> <span class="kw1">return</span> <span class="nu17">3.14f</span><span class="sy4">;</span> <span class="co1">// deduces return type float: error</span>
<span class="br0">}</span></pre></div></div>
<p>If there is no return statement or if the argument of the return statement is a void expression, the declared return type must be <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> (not <code>decltype(auto)</code>), and the deduced return type is then <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span>
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// returns void</span>
<span class="kw4">auto</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// returns void</span>
<span class="kw4">auto</span><span class="sy2">*</span> x<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// error: cannot deduce auto* from void</span></pre></div></div>
<p>Once a return statement has been seen in a function, the return type deduced from that statement can be used in the rest of the function, including in other return statements.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> sum<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">if</span><span class="br0">(</span>i <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span>
<span class="kw1">return</span> i<span class="sy4">;</span> <span class="co1">// sum’s return type is int</span>
<span class="kw1">else</span>
<span class="kw1">return</span> sum<span class="br0">(</span>i <span class="sy2">-</span> <span class="nu0">1</span><span class="br0">)</span> <span class="sy2">+</span> i<span class="sy4">;</span> <span class="co1">// OK, sum’s return type is already known</span>
<span class="br0">}</span></pre></div></div>
<p>If the return statement uses a brace-init-list, deduction is not allowed:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> func <span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// error</span></pre></div></div>
<p><a href="virtual.html" title="cpp/language/virtual">Virtual functions</a> cannot use return type deduction.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> F
<span class="br0">{</span>
<span class="kw1">virtual</span> <span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// error</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>If a function uses return type deduction, it cannot be redeclared using the type that it deduces to, or another kind of return type deduction even if it deduces to the same type
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declared, not yet defined</span>
<span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// defined, return type is int</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error, cannot use the deduced type</span>
decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error, different kind of deduction</span>
<span class="kw4">auto</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// re-declared, OK</span>
 
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span>
<span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw1">friend</span> T frf<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">auto</span> frf<span class="br0">(</span><span class="kw4">int</span> i<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> i<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// not a friend of A<int></span></pre></div></div>
<p><a href="function_template.html" title="cpp/language/function template">Function templates</a> can use return type deduction. The deduction takes place at instantiation even if the expression in the return statement is not <a href="dependent_name.html" title="cpp/language/dependent name">dependent</a>. This instantiation is not in an immediate context for the purposes of <a href="sfinae.html" title="cpp/language/sfinae">SFINAE</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> T<span class="sy1">></span> <span class="kw4">auto</span> f<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw1">typedef</span> decltype<span class="br0">(</span>f<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span> fint_t<span class="sy4">;</span> <span class="co1">// instantiates f<int> to deduce return type</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw4">auto</span> f<span class="br0">(</span>T<span class="sy2">*</span> t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="sy2">*</span>t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span> <span class="sy1">=</span> <span class="sy3">&</span>f<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// instantiates both fs to determine return types,</span>
<span class="co1">// chooses second template overload</span></pre></div></div>
<p>Specializations of function templates that use return type deduction must use the same return type placeholders
</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> T<span class="sy1">></span> <span class="kw4">auto</span> g<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// #1</span>
<span class="kw1">template</span> <span class="kw4">auto</span> g<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, return type is int</span>
<span class="co1">//template char g(char); // error, no matching template</span>
 
<span class="kw1">template</span><span class="sy1"><></span> <span class="kw4">auto</span> g<span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, forward declaration with unknown return type</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy1">></span> T g<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// OK, not equivalent to #1</span>
<span class="kw1">template</span> <span class="kw4">char</span> g<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, now there is a matching template</span>
<span class="kw1">template</span> <span class="kw4">auto</span> g<span class="br0">(</span><span class="kw4">float</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// still matches #1</span>
<span class="co1">// void h() { return g(42); } // error, ambiguous</span></pre></div></div>
<p><a href="function_template.html#Explicit_instantiation" title="cpp/language/function template">Explicit instantiation declarations</a> do not themselves instantiate function templates that use return type deduction
</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> T<span class="sy1">></span> <span class="kw4">auto</span> f<span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> t<span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">extern</span> <span class="kw1">template</span> <span class="kw4">auto</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// does not instantiate f<int></span>
<span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>p<span class="br0">)</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="sy1">=</span> f<span class="sy4">;</span> <span class="co1">// instantiates f<int> to determine its return type,</span>
<span class="co1">// but an explicit instantiation definition </span>
<span class="co1">// is still required somewhere in the program</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/function&action=edit&section=3" title="Edit section: Parameter list">edit</a>]</span> <span class="mw-headline" id="Parameter_list">Parameter list</span></h3>
<p>Parameter list determines the arguments that can be specified when the function is called. It is a comma-separated list of <i>parameter declarations</i>, each of which has the following syntax
</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">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">declarator</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">declarator</span> <code><b>= </b></code> <span class="t-spar">initializer</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">abstract-declarator</span><span class="t-mark">(optional)</span>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span> <span class="t-spar">abstract-declarator</span><span class="t-mark">(optional)</span> <code><b>= </b></code> <span class="t-spar">initializer</span>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>...</b></code>
</td>
<td> (5)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>void</b></code>
</td>
<td> (6)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> Declares a named (formal) parameter. For the meanings of <span class="t-spar">decl-specifier-seq</span> and <span class="t-spar">declarator</span>, see <a href="declarations.html" title="cpp/language/declarations">declarations</a>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> <span class="sy2">*</span>p, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span>x<span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">2)</span> Declares a named (formal) parameter with a <a href="default_arguments.html" title="cpp/language/default arguments">default value</a>.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">7</span>, <span class="kw4">int</span> <span class="sy2">*</span>p <span class="sy1">=</span> nullptr, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span>x<span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> nullptr<span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">3)</span> Declares an unnamed parameter
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">int</span> <span class="sy2">*</span>, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">4)</span> Declares an unnamed parameter with a <a href="default_arguments.html" title="cpp/language/default arguments">default value</a>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="nu0">7</span>, <span class="kw4">int</span> <span class="sy2">*</span> <span class="sy1">=</span> nullptr, <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> nullptr<span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">5)</span> Declares a <a href="variadic_arguments.html" title="cpp/language/variadic arguments">variadic function</a>, may only appear as the last parameter in a parameter list.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> printf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt, ...<span class="br0">)</span><span class="sy4">;</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">6)</span> Indicates that the function takes no parameters, it is the exact synonym for an empty parameter list: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> declare the same function. Note that the type <code>void</code> (possibly cv-qualified) cannot be used in a parameter list otherwise: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span>, <span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span></span></span> are errors (although derived types, such as <code>void*</code> can be used). <span class="t-rev-inl t-since-cxx11"><span>In a template, only non-dependent void type can be used (a function taking a single parameter of type <code>T</code> does not become a no-parameter function if instantiated with <code>T = void</code>)</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span></div>
<p>Although <span class="t-spar">decl-specifier-seq</span> implies there can exist <a href="declarations.html#Specifiers" title="cpp/language/declarations">specifiers</a> other than type specifiers, the only other specifier allowed is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">register</span></span></span> <span class="t-rev-inl t-until-cxx11"><span>as well as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">auto</span></span></span> </span> <span><span class="t-mark-rev t-until-cxx11">(until C++11)</span></span></span>, and it has virtually no effect.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-concepts_ts"><td>
<p>If any of the function parameters uses a <i>placeholder</i> (either <code>auto</code> or a <a href="concepts.html" title="cpp/language/constraints">constrained type</a>), the function declaration is instead an abbreviated <a href="function_template.html" title="cpp/language/function template">function template</a> declaration:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">auto</span> <span class="br0">(</span>auto<span class="sy4">::</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #1</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T, <span class="kw1">typename</span> U, <span class="kw1">typename</span> V<span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>T <span class="br0">(</span>U<span class="sy4">::</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span>V<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same as #1</span>
 
<span class="kw4">void</span> g1<span class="br0">(</span><span class="kw4">const</span> C1<span class="sy2">*</span>, C2<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// #2 (assuming C1 and C2 are concepts</span>
<span class="kw1">template</span><span class="sy1"><</span>C1 T, C2 U<span class="sy1">></span> <span class="kw4">void</span> g1<span class="br0">(</span><span class="kw4">const</span> T<span class="sy2">*</span>, U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// same as #2</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-concepts-ts t-mark-ts">(concepts TS)</span></td></tr>
</table>
<p>Parameter names declared in function declarations are usually for only self-documenting purposes. They are used (but remain optional) in function definitions.
</p><p>The type of each function parameter in the parameter list is determined according to the following rules:
</p>
<div class="t-li1"><span class="t-li">1)</span> First, <span class="t-spar">decl-specifier-seq</span> and the declarator are combined as in any <a href="declarations.html" title="cpp/language/declarations">declaration</a> to determine the type.</div>
<div class="t-li1"><span class="t-li">2)</span> If the type is "array of T" or "array of unknown bound of T", it is replaced by the type "pointer to T"</div>
<div class="t-li1"><span class="t-li">3)</span> If the type is a function type F, it is replaced by the type "pointer to F"</div>
<div class="t-li1"><span class="t-li">4)</span> Top-level cv-qualifiers are dropped from the parameter type (This adjustment only affects the function type, but doesn't modify the property of the parameter: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> p, decltype<span class="br0">(</span>p<span class="br0">)</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">const</span> <span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span></span></span> declare the same function)</div>
<p>Because of these rules, the following function declarations declare exactly the same function:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span> s<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="br0">[</span><span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span> s<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span> <span class="kw4">const</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span> <span class="kw4">volatile</span> s<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>The following declarations also declare exactly the same function
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>g<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-until-cxx14"><td>Parameter type cannot be a type that includes a reference or a pointer to array of unknown bound, including a multi-level pointers/arrays of such types, or a pointer to functions whose parameters are such types</td>
<td><span class="t-mark-rev t-until-cxx14">(until C++14)</span></td></tr>
</table>
<p>The comma before the ellipsis parameter that indicates <a href="variadic_arguments.html" title="cpp/language/variadic arguments">variadic arguments</a> is optional, even if it follows the ellipsis that indicates a <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a> expansion, so the following function templates are exactly the same:
</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">Args</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Args..., ...<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> ...<span class="me1">Args</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Args... ...<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> ...<span class="me1">Args</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Args......<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>An example of when such declaration is used is the implementation of <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/is_function.html"><span class="kw455">std::<span class="me2">is_function</span></span></a></span></span>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/function&action=edit&section=4" title="Edit section: Function definition">edit</a>]</span> <span class="mw-headline" id="Function_definition">Function definition</span></h3>
<p>A non-member function definition may appear at namespace scope only (there are no nested functions). A <a href="member_functions.html" title="cpp/language/member functions">member function</a> definition may also appear in the body of a <a href="class.html" title="cpp/language/class">class definition</a>. They have the following syntax:
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">decl-specifier-seq</span><span class="t-mark">(optional)</span> <span class="t-spar">declarator</span> <span class="t-spar">virt-specifier-seq</span><span class="t-mark">(optional)</span> <span class="t-spar">function-body</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>where <span class="t-spar">function-body</span> is one of the following
</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">ctor-initializer</span><span class="t-mark">(optional)</span> <span class="t-spar">compound-statement</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">function-try-block</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>=</b></code> <code><b>delete</b></code> <code><b>;</b></code>
</td>
<td> (3)
</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>
<tr class="t-sdsc">
<td> <code><b>=</b></code> <code><b>default</b></code> <code><b>;</b></code>
</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>
<div class="t-li1"><span class="t-li">1)</span> regular function body</div>
<div class="t-li1"><span class="t-li">2)</span> <a href="function-try-block.html" title="cpp/language/function-try-block">function-try-block</a> (which is a regular function body wrapped in a try/catch block)</div>
<div class="t-li1"><span class="t-li">3)</span> explicitly deleted function definition</div>
<div class="t-li1"><span class="t-li">4)</span> explicitly defaulted function definition, only allowed for <a href="member_functions.html#Special_member_functions" title="cpp/language/member functions">special member functions</a>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> optional list of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">decl-specifier-seq</span>
</td>
<td> -
</td>
<td> the return type with specifiers, as in the <a href="declarations.html" title="cpp/language/declarations">declaration grammar</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">declarator</span>
</td>
<td> -
</td>
<td> function declarator, same as in the function declaration grammar above
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">virt-specifier-seq</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> <a href="override.html" title="cpp/language/override">override</a>, <a href="final.html" title="cpp/language/final">final</a>, or their combination in any order (only allowed for member functions)
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">ctor-initializer</span>
</td>
<td> -
</td>
<td> <a href="constructor.html" title="cpp/language/initializer list">member initializer list</a>, only allowed in constructors
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">compound-statement</span>
</td>
<td> -
</td>
<td> the brace-enclosed <a href="statements.html#Compound_statement" title="cpp/language/statements">sequence of statements</a> that constututes the body of a function
</td></tr></table></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> max<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span> b, <span class="kw4">int</span> c<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">int</span> m <span class="sy1">=</span> <span class="br0">(</span>a <span class="sy1">></span> b<span class="br0">)</span><span class="sy4">?</span> a <span class="sy4">:</span> b<span class="sy4">;</span>
<span class="kw1">return</span> <span class="br0">(</span>m <span class="sy1">></span> c<span class="br0">)</span><span class="sy4">?</span> m <span class="sy4">:</span> c<span class="sy4">;</span>
<span class="br0">}</span>
<span class="co1">// decl-specifier-seq is "int"</span>
<span class="co1">// declarator is "max(int a, int b, int c)"</span>
<span class="co1">// body is { ... }</span></pre></div></div>
<p>The function body is a <a href="statements.html#Compound_statement" title="cpp/language/statements">compound statement</a> (sequence of zero or more statements surrounded by a pair of curly braces), which is executed when the function call is made.
</p><p>The parameter types, as well as the return type of a function cannot be <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete</a> <a href="class.html" title="cpp/language/class">class types</a>, <span class="t-rev-inl t-since-cxx11"><span>except for deleted functions</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>. The completeness check is made in the context of the function <i>body</i>, which allows <a href="member_functions.html" title="cpp/language/member functions">member functions</a> to return the class in which they are defined (or its enclosing class), even if it's incomplete at the point of definition (it is complete in the function body).
</p><p>The parameters declared in the <span class="t-spar">declarator</span> of a function definition are <a href="scope.html" title="cpp/language/scope">in scope</a> within the body. If a parameter is not used in the function body, it does not need to be named (it's sufficient to use an abstract declarator)
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> print<span class="br0">(</span><span class="kw4">int</span> a, <span class="kw4">int</span><span class="br0">)</span> <span class="co1">// second parameter is not used</span>
<span class="br0">{</span>
<a href="../io/c/printf.html"><span class="kw1568">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"a = %d<span class="es1">\n</span>"</span>,a<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Even though top-level <a href="cv.html" title="cpp/language/cv">cv-qualifiers</a> on the parameters are discarded in function declarations, they modify the type of the parameter as visible in the body of a function:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> n<span class="br0">)</span> <span class="co1">// declares function of type void(int)</span>
<span class="br0">{</span>
<span class="co1">// but in the body, the type of n is const int</span>
<span class="br0">}</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<h3> <span class="mw-headline" id="Deleted_functions">Deleted functions</span></h3>
<p>If, instead of a function body, the special syntax <code><b>= delete ;</b></code> is used, the function is defined as <i>deleted</i>. Any use of a deleted function is ill-formed (the program will not compile). This includes calls, both explicit (with a function call operator) and implicit (a call to deleted overloaded operator, special member function, allocation function etc), constructing a pointer or pointer-to-member to a deleted function, and even the use of a deleted function in an unevaluated expression. However, implicit <a href="definition.html#ODR-use" title="cpp/language/definition">ODR-use</a> of a non-pure virtual member function that happens to be deleted is allowed.
</p><p>If the function is overloaded, <a href="overload_resolution.html" title="cpp/language/overload resolution">overload resolution</a> takes place first, and the program is only ill-formed if the deleted function was selected.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> sometype
<span class="br0">{</span>
<span class="kw4">void</span><span class="sy2">*</span> <a href="../memory/new/operator_new.html"><span class="kw589">operator new</span></a><span class="br0">(</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
<span class="kw4">void</span><span class="sy2">*</span> <a href="../memory/new/operator_new.html"><span class="kw589">operator new</span></a><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
sometype<span class="sy2">*</span> p <span class="sy1">=</span> new sometype<span class="sy4">;</span> <span class="co1">// error, attempts to call deleted sometype::operator new</span></pre></div></div>
<p>The deleted definition of a function must be the first declaration in a translation unit: a previously-declared function cannot be redeclared as deleted:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> sometype <span class="br0">{</span> sometype<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
sometype<span class="sy4">::</span><span class="me2">sometype</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span> <span class="co1">// error: must be deleted on the first declaration</span></pre></div></div>
<h3> <span class="mw-headline" id="func">__func__</span></h3>
<p>Within the function body, the function-local predefined variable <span class="t-lc">__func__</span> is defined as if by
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">char</span> __func__<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"function-name"</span><span class="sy4">;</span></pre></div></div>
<p>This variable has block scope and static storage duration:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
S<span class="br0">(</span><span class="br0">)</span><span class="sy4">:</span> s<span class="br0">(</span>__func__<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// OK: initializer-list is part of function body</span>
<span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> s<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> s <span class="sy1">=</span> __func__<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// error: parameter-list is part of declarator</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/function&action=edit&section=7" title="Edit section: Example 1: non-member functions">edit</a>]</span> <span class="mw-headline" id="Example_1:_non-member_functions">Example 1: non-member functions</span></h3>
<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: move/cleanup </td></tr></table>
<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="co2">#include <string></span>
 
<span class="co1">// declaration in namespace(file) scope</span>
<span class="co1">// (the definition is provided later)</span>
<span class="kw4">int</span> f1<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// simple function with a default argument, returning nothing</span>
<span class="kw4">void</span> f0<span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span> arg <span class="sy1">=</span> <span class="st0">"world"</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Hello, "</span> <span class="sy1"><<</span> arg <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// function returning a pointer to f0</span>
<span class="kw4">auto</span> fp11<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw4">void</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> f0<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// function returning a pointer to f0, pre-C++11 style</span>
<span class="kw4">void</span> <span class="br0">(</span><span class="sy2">*</span>fp03<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="sy3">&</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> f0<span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
f0<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
fp11<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="st0">"test"</span><span class="br0">)</span><span class="sy4">;</span>
fp03<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="st0">"again"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> f2<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declaration in function scope</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> f2<span class="br0">(</span><span class="st0">"bad12"</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// simple non-member function returning int</span>
<span class="kw4">int</span> f1<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">return</span> <span class="nu0">42</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="co1">// function with an exception specification and a function try block</span>
<span class="kw4">int</span> f2<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> str<span class="br0">)</span> <span class="kw1">noexcept</span> <span class="kw1">try</span>
<span class="br0">{</span>
<span class="kw1">return</span> <a href="../string/basic_string/stol.html"><span class="kw1081">std::<span class="me2">stoi</span></span></a><span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw1">catch</span><span class="br0">(</span><span class="kw4">const</span> <a href="../error/exception.html"><span class="kw644">std::<span class="me2">exception</span></span></a><span class="sy3">&</span> e<span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/cerr.html"><span class="kw1483">std::<span class="me2">cerr</span></span></a> <span class="sy1"><<</span> <span class="st0">"stoi() failed!<span class="es1">\n</span>"</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
Hello, test
Hello, again
stoi() failed!
0</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/function&action=edit&section=8" title="Edit section: Defect reports">edit</a>]</span> <span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1394">CWG 1394</a>
</td>
<td> c++11
</td>
<td> deleted function could not return an incomplete type
</td>
<td> incomplete return type allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#577">CWG 577</a>
</td>
<td> c++11
</td>
<td> dependent type <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> could be used to declare a no-parameter function
</td>
<td> only non-dependent void is allowed
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#393">CWG 393</a>
</td>
<td> c++14
</td>
<td> types that include pointers/references to array of unknown bound can't be parameters
</td>
<td> such types are allowed
</td></tr></table>
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