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<p>Lists the exceptions that a function might directly or indirectly throw.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="except_spec.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="except_spec.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1"><a href="except_spec.html#Potential_exceptions"><span class="tocnumber">3</span> <span class="toctext">Potential exceptions</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="except_spec.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="except_spec.html#See_also"><span class="tocnumber">5</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/except_spec&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> <code><b>throw(</b></code><span class="t-spar">typeid, typeid, ...</span><code><b>)</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td> <span class="t-mark">(deprecated)</span>
</td></tr>
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<p>This specification may appear only on lambda-declarator or on a function declarator that is the <span class="t-rev-inl t-until-cxx17"><span>top-level</span> <span><span class="t-mark-rev t-until-cxx17">(until C++17)</span></span></span> declarator of a function, variable, or non-static data member, whose type is a function type, a pointer to function type, a reference to function type, a pointer to member function type. It may appear on the declarator of a parameter or on the declarator of a return type.
</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="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: function declaration</span>
<span class="kw4">void</span> <span class="br0">(</span><span class="sy2">*</span>fp<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span> <span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: pointer to function declaration</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="kw4">void</span> pfa<span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: pointer to function parameter declaration</span>
<span class="kw1">typedef</span> <span class="kw4">int</span> <span class="br0">(</span><span class="sy2">*</span>pf<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Error: typedef declaration</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/except_spec&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>If a function is declared with type <code>T</code> listed in its exception specification, the function may throw exceptions of that type or a type derived from it.
</p><p><a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">Incomplete types</a>, pointers or references to incomplete types other than cv <code>void*</code>, and rvalue reference types are not allowed in the exception specification. Array and function types, if used, are adjusted to corresponding pointer types. <span class="t-rev-inl t-since-cxx11"><span><a href="parameter_pack.html" title="cpp/language/parameter pack">parameter packs</a> are allowed</span> <span><span class="t-mark-rev t-since-cxx11">(since C++11)</span></span></span>.
</p><p>If the function throws an exception of the type not listed in its exception specification, the function <span class="t-lc"><a href="../error/unexpected.html" title="cpp/error/unexpected">std::unexpected</a></span> is called. The default function calls <span class="t-lc"><a href="../error/terminate.html" title="cpp/error/terminate">std::terminate</a></span>, but it may be replaced by a user-provided function (via <span class="t-lc"><a href="../error/set_unexpected.html" title="cpp/error/set unexpected">std::set_unexpected</a></span>) which may call <span class="t-lc"><a href="../error/terminate.html" title="cpp/error/terminate">std::terminate</a></span> or throw an exception. If the exception thrown from <span class="t-lc"><a href="../error/unexpected.html" title="cpp/error/unexpected">std::unexpected</a></span> is accepted by the exception specification, stack unwinding continues as usual. If it isn't, but <span class="t-lc"><a href="../error/bad_exception.html" title="cpp/error/bad exception">std::bad_exception</a></span> is allowed by the exception specification, <span class="t-lc"><a href="../error/bad_exception.html" title="cpp/error/bad exception">std::bad_exception</a></span> is thrown. Otherwise, <span class="t-lc"><a href="../error/terminate.html" title="cpp/error/terminate">std::terminate</a></span> is called.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<h3> <span class="mw-headline" id="Potential_exceptions">Potential exceptions</span></h3>
<p>Each function <code>f</code>, pointer to function <code>fp</code>, and pointer to member function <code>mfp</code> has a <i>set of potential exceptions</i>, which consists of types that might be thrown and possibly the "any" placeholder for the possibility of an exception of arbitrary type. This set is defined as follows:
</p>
<div class="t-li1"><span class="t-li">1)</span> If the declaration of <code>f</code>, <code>fp</code>, or <code>mfp</code> uses <code>throw()</code><span class="t-mark">(deprecated)</span> or <a href="noexcept.html" title="cpp/language/noexcept">noexcept</a>, the set is empty.</div>
<div class="t-li1"><span class="t-li">2)</span> Otherwise, if the declaration of <code>f</code>, <code>fp</code>, or <code>mfp</code> uses a dynamic exception specification<span class="t-mark">(deprecated)</span>, the set consists of the types listed in that specification</div>
<div class="t-li1"><span class="t-li">3)</span> Otherwise, the set consists of the placeholder "any"</div>
<p>Note: for implicitly-declared special member functions (constructors, assignment operators, and destructors) and for the inheriting constructors, the set of potential exceptions is a combination of the sets of the potential exceptions of everything they would call: constructors/assignment operators/destructors of non-variant non-static data members, direct bases, and, where appropriate, virtual bases (including default argument expressions, as always)
</p><p>Each expression <code>e</code> has a <i>set of potential exceptions</i>, defined as follows:
</p>
<div class="t-li1"><span class="t-li">1)</span> If <code>e</code> is a <a href="constexpr.html" title="cpp/language/constexpr">core constant expression</a>, the set is empty</div>
<div class="t-li1"><span class="t-li">2)</span> Otherwise, the set is the union of the sets of potential exceptions of all immediate subexpressions of <code>e</code> (including <a href="default_arguments.html" title="cpp/language/default arguments">default argument expressions</a>), combined with another set that depends on the form of <code>e</code>, as follows:</div>
<div class="t-li2"><span class="t-li">1)</span> If <code>e</code> is a function call expression, and
<dl><dd><dl><dd><ul><li> the function is named by an id-expression (either directly or as part of member access or pointer-to-member access expression), the set of potential exceptions of the named function is added to the list.
</li><li> if the function is named by an expression of type noexcept function or by an expression of type pointer to noexcept function, the set is empty
</li><li> otherwise, the set consists of the placeholder "any"</div>
</li></ul>
</dd></dl>
</dd></dl>
<div class="t-li2"><span class="t-li">2)</span> If <code>e</code> calls a function implicitly (it's an operator expression and the operator is overloaded, it is a <a href="new.html" title="cpp/language/new">new-expression</a> and the allocation function is overloaded, or it is a full expression and the destructor of a temporary is called)), then the set is the set of that function.</div>
<div class="t-li2"><span class="t-li">3)</span> If <code>e</code> is a <a href="throw.html" title="cpp/language/throw">throw-expression</a>, the set is the exception that would be initialized by its operand, or "any" for the re-throwing throw-expression (with no operand)</div>
<div class="t-li2"><span class="t-li">4)</span> If <code>e</code> is a <a href="dynamic_cast.html" title="cpp/language/dynamic cast">dynamic_cast</a> to a reference to a polymorphic type, the set consists of <span class="t-lc"><a href="../types/bad_cast.html" title="cpp/types/bad cast">std::bad_cast</a></span></div>
<div class="t-li2"><span class="t-li">5)</span> If <code>e</code> is a <a href="typeid.html" title="cpp/language/typeid">typeid</a> applied to a polymorphic glvalue, the set consists of <span class="t-lc"><a href="../types/bad_typeid.html" title="cpp/types/bad typeid">std::bad_typeid</a></span></div>
<div class="t-li2"><span class="t-li">6)</span> If <code>e</code> is a <a href="new.html" title="cpp/language/new">new-expression</a> with a non-constant size, the set consists of <span class="t-lc"><a href="../memory/new/bad_array_new_length.html" title="cpp/memory/new/bad array new length">std::bad_array_new_length</a></span></div>
<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="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// f()'s set is "int"</span>
<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// g()'s set is "any"</span>
<span class="kw1">struct</span> A <span class="br0">{</span> A<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// "new A"'s set is "any"</span>
<span class="kw1">struct</span> B <span class="br0">{</span> B<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// "B()"'s set is empty</span>
<span class="kw1">struct</span> D<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> D<span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span> <span class="br0">(</span><span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// new D's set is "double, any"</span></pre></div></div>
<p>All implicitly-declared member functions (and inheriting constructors) have exception specifications, selected as follows:
</p>
<ul><li> If the set of potential exceptions contains "any", the implicit exception specification is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span></span></span>.
</li><li> Otherwise, If the set of potential exceptions is not empty, the implicit exception specification lists every type from the set
</li><li> Otherwise, the implicit exception specification is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span><span class="kw2">true</span><span class="br0">)</span></span></span> and the function type is "noexcept function"
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
A<span class="br0">(</span><span class="kw4">int</span> <span class="sy1">=</span> <span class="br0">(</span>A<span class="br0">(</span><span class="nu0">5</span><span class="br0">)</span>, <span class="nu0">0</span><span class="br0">)</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
A<span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
A<span class="br0">(</span>A<span class="sy3">&&</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
~A<span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span>X<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="br0">{</span>
B<span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
B<span class="br0">(</span><span class="kw4">const</span> B<span class="sy3">&</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span> <span class="co1">// exception specification is "noexcept(true)"</span>
B<span class="br0">(</span>B<span class="sy3">&&</span>, <span class="kw4">int</span> <span class="sy1">=</span> <span class="br0">(</span><span class="kw1">throw</span> Y<span class="br0">(</span><span class="br0">)</span>, <span class="nu0">0</span><span class="br0">)</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span>Y<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
~B<span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span>Y<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
<span class="kw1">struct</span> D <span class="sy4">:</span> <span class="kw1">public</span> A, <span class="kw1">public</span> B <span class="br0">{</span>
<span class="kw4">int</span> <span class="sy2">*</span> p <span class="sy1">=</span> new <span class="br0">(</span><a href="../memory/new/nothrow.html"><span class="kw600">std::<span class="me2">nothrow</span></span></a><span class="br0">)</span> <span class="kw4">int</span><span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span>
<span class="co1">// D has the following implicitly-declared members:</span>
<span class="co1">// D::D() throw(X, std::bad_array_new_length);</span>
<span class="co1">// D::D(const D&) noexcept(true);</span>
<span class="co1">// D::D(D&&) throw(Y);</span>
<span class="co1">// D::~D() throw(X, Y);</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</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/except_spec&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 <exception></span>
<span class="co2">#include <cstdlib></span>
 
<span class="kw1">class</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> Y <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> Z <span class="sy4">:</span> <span class="kw1">public</span> X <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> W <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span>X, Y<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw1">if</span> <span class="br0">(</span>n<span class="br0">)</span> <span class="kw1">throw</span> X<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
<span class="kw1">if</span> <span class="br0">(</span>n<span class="br0">)</span> <span class="kw1">throw</span> Z<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// also OK</span>
<span class="kw1">throw</span> W<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// will call std::unexpected()</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<a href="../error/set_unexpected.html"><span class="kw671">std::<span class="me2">set_unexpected</span></span></a><span class="br0">(</span><span class="br0">[</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">"That was unexpected<span class="es1">\n</span>"</span><span class="sy4">;</span>
<a href="../utility/program/abort.html"><span class="kw844">std::<span class="me2">abort</span></span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
f<span class="br0">(</span><span class="br0">)</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">That was unexpected</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/except_spec&action=edit&section=5" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li><a href="noexcept_spec.html" title="cpp/language/noexcept spec"> <code>noexcept</code> specifier</a>
</li></ul>
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