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<h1 id="firstHeading" class="firstHeading">noexcept operator <span class="t-mark-rev t-since-cxx11">(since C++11)</span></h1>
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<p>The <code>noexcept</code> operator performs a compile-time check that returns true if an expression is declared to not throw any exceptions.
</p><p>It can be used within a function template's <a href="noexcept_spec.html" title="cpp/language/noexcept spec"> noexcept specifier</a> to declare that the function will throw exceptions for some types but not others.
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<li class="toclevel-1 tocsection-1"><a href="noexcept.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="noexcept.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="noexcept.html#Keywords"><span class="tocnumber">3</span> <span class="toctext">Keywords</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="noexcept.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="noexcept.html#See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li>
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/noexcept&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
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<td class="t-sdsc-nopad"> <code><b>noexcept(</b></code> <span class="t-spar">expression</span> <code><b>)</b></code>
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<p>Returns a <a href="value_category.html" title="cpp/language/value category">prvalue</a> of type <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>.
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/noexcept&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The <code>noexcept</code> operator does not evaluate <span class="t-spar">expression</span>.
</p>
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<p>The result is <code><b>false</b></code> if the <span class="t-spar">expression</span> contains at least one of the following potentially evaluated constructs:
</p>
<dl><dd><ul><li> call to any type of function that does not have non-throwing exception specification, unless it is a <a href="constexpr.html" title="cpp/language/constexpr"> constant expression</a>.
</li><li> <a href="throw.html" title="cpp/language/throw"><code>throw</code> expression</a>.
</li><li> <a href="dynamic_cast.html" title="cpp/language/dynamic cast"><code>dynamic_cast</code> expression</a> when the target type is a reference type, and conversion needs a run time check
</li><li> <a href="typeid.html" title="cpp/language/typeid"><code>typeid</code> expression</a> when argument type is polymorphic class type
</li></ul>
</dd></dl>
<p>In all other cases the result is <code><b>true</b></code>.
</p>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td></tr>
<tr class="t-rev t-since-cxx17"><td>
<p>The result is <code><b>true</b></code> if the set of <a href="except_spec.html" title="cpp/language/except spec"><i>potential exceptions</i></a> of the <span class="t-spar">expression</span> is empty, and <code><b>false</b></code> otherwise.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/noexcept&action=edit&section=3" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../keyword/noexcept.html" title="cpp/keyword/noexcept"><tt>noexcept</tt></a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/noexcept&action=edit&section=4" 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="co2">#include <utility></span>
<span class="co2">#include <vector></span>
 
<span class="kw4">void</span> may_throw<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">void</span> no_throw<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
<span class="kw4">auto</span> lmay_throw <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">auto</span> lno_throw <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> T<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
~T<span class="br0">(</span><span class="br0">)</span><span class="br0">{</span><span class="br0">}</span> <span class="co1">// dtor prevents move ctor</span>
<span class="co1">// copy ctor is noexcept</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> U<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
~U<span class="br0">(</span><span class="br0">)</span><span class="br0">{</span><span class="br0">}</span> <span class="co1">// dtor prevents move ctor</span>
<span class="co1">// copy ctor is noexcept(false)</span>
<a href="../container/vector.html"><span class="kw1094">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> V<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
<a href="../container/vector.html"><span class="kw1094">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
T t<span class="sy4">;</span>
U u<span class="sy4">;</span>
V v<span class="sy4">;</span>
 
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="../io/manip/boolalpha.html"><span class="kw1487">std::<span class="me2">boolalpha</span></span></a>
<span class="sy1"><<</span> <span class="st0">"Is may_throw() noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>may_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is no_throw() noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>no_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is lmay_throw() noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>lmay_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is lno_throw() noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>lno_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is ~T() noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span><a href="../utility/declval.html"><span class="kw950">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.~T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="co1">// note: the following tests also require that ~T() is noexcept because</span>
<span class="co1">// the expression within noexcept constructs and destroys a temporary</span>
<span class="sy1"><<</span> <span class="st0">"Is T(rvalue T) noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>T<span class="br0">(</span><a href="../utility/declval.html"><span class="kw950">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is T(lvalue T) noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>T<span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is U(rvalue U) noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>U<span class="br0">(</span><a href="../utility/declval.html"><span class="kw950">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is U(lvalue U) noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>U<span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is V(rvalue V) noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>V<span class="br0">(</span><a href="../utility/declval.html"><span class="kw950">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>V<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
<span class="sy1"><<</span> <span class="st0">"Is V(lvalue V) noexcept? "</span> <span class="sy1"><<</span> <span class="kw1">noexcept</span><span class="br0">(</span>V<span class="br0">(</span>v<span class="br0">)</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></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">Is may_throw() noexcept? false
Is no_throw() noexcept? true
Is lmay_throw() noexcept? false
Is lno_throw() noexcept? true
Is ~T() noexcept? true
Is T(rvalue T) noexcept? true
Is T(lvalue T) noexcept? true
Is U(rvalue U) noexcept? false
Is U(lvalue U) noexcept? false
Is V(rvalue V) noexcept? true
Is V(lvalue V) noexcept? false</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/noexcept&action=edit&section=5" 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> <a href="noexcept_spec.html" title="cpp/language/noexcept spec"> noexcept specifier </a>
</td>
<td> requires a function to not throw any exceptions <span class="t-mark-rev t-since-cxx11">(C++11)</span> <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/dsc_noexcept_spec&action=edit">[edit]</a></span>
</td></tr>
<tr class="t-dsc">
<td> <a href="except_spec.html" title="cpp/language/except spec"> exception specification </a>
</td>
<td> specifies what exceptions are thrown by a function <span class="t-mark">(deprecated)</span> <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/dsc_except_spec&action=edit">[edit]</a></span>
</td></tr>
</table>
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