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<p>Sets the initial value of an object to zero
</p>
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<li class="toclevel-1 tocsection-1"><a href="zero_initialization.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="zero_initialization.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="zero_initialization.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="zero_initialization.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="zero_initialization.html#Defect_reports"><span class="tocnumber">5</span> <span class="toctext">Defect reports</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="zero_initialization.html#See_also"><span class="tocnumber">6</span> <span class="toctext">See also</span></a></li>
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</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/zero_initialization&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>static</b></code> <span class="t-spar">T</span> <span class="t-spar">object</span> <code><b>;</b></code>
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<td> (1)
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<td> <span class="t-spar">int</span> <code><b>()</b></code> <code><b>;</b></code>
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<td> (2)
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<td> <code><b>char</b></code> <span class="t-spar">array</span> <code><b>[</b></code> <span class="t-spar">n</span> <code><b>]</b></code> <code><b>= "";</b></code>
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<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/zero_initialization&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Zero initialization is performed in the following situations:
</p>
<div class="t-li1"><span class="t-li">1)</span> For every named variable with static or thread-local <a href="storage_duration.html" title="cpp/language/storage duration">storage duration</a> <span class="t-rev-inl t-since-cxx14"><span>that is not subject to <a href="constant_initialization.html" title="cpp/language/constant initialization">constant initialization</a></span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>, before any other initialization.</div>
<div class="t-li1"><span class="t-li">2)</span> As part of <a href="value_initialization.html" title="cpp/language/value initialization">value-initialization</a> sequence for non-class types and for members of value-initialized class types that have no constructors.</div>
<div class="t-li1"><span class="t-li">3)</span> When a character array is <a href="aggregate_initialization.html#Character_arrays" title="cpp/language/aggregate initialization">initialized with a string literal</a> that is too short, the remainder of the array is zero-initialized.</div>
<p>The effects of zero initialization are:
</p>
<ul><li> If <code>T</code> is a scalar type, the object's initial value is the integral constant zero <a href="explicit_cast.html" title="cpp/language/explicit cast">explicitly converted</a> to <code>T</code>.
</li></ul>
<ul><li> If <code>T</code> is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.
</li></ul>
<ul><li> If <code>T</code> is a union type, the first non-static named data member is zero-initialized and all padding is initialized to zero bits.
</li></ul>
<ul><li> If <code>T</code> is array type, each element is zero-initialized
</li></ul>
<ul><li> If <code>T</code> is reference type, nothing is done.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/zero_initialization&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>As described in <a href="initialization.html#Non-local_variables" title="cpp/language/initialization">non-local initialization</a>, static and thread-local variables <span class="t-rev-inl t-since-cxx14"><span>that aren't constant-initialized</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span> are zero-initialized before any other initialization takes place. If the definition of a non-class non-local variable has no initializer, then default initialization does nothing, leaving the result of the earlier zero-initialization unmodified.
</p><p>A zero-initialized pointer is the null pointer value of its type, even if the value of the null pointer is not integral zero.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/zero_initialization&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 <string></span>
 
<span class="kw4">double</span> f<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// zero-initialized to three 0.0's</span>
<span class="kw4">int</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// zero-initialized to null pointer value</span>
<a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span> <span class="co1">// zero-initialized to indeterminate value</span>
<span class="co1">// then default-initialized to ""</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">int</span> argc, <span class="kw4">char</span><span class="sy2">*</span> argv<span class="br0">[</span><span class="br0">]</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">static</span> <span class="kw4">int</span> n <span class="sy1">=</span> argc<span class="sy4">;</span> <span class="co1">// zero-initialized to 0</span>
<span class="co1">// then copy-initialized to argc</span>
delete p<span class="sy4">;</span> <span class="co1">// safe to delete a null pointer</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/zero_initialization&action=edit&section=5" 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#2026">CWG 2026</a>
</td>
<td> C++14
</td>
<td> zero-init was specified to always occur first, even before constant-init
</td>
<td> no zero-init if constant init applies
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/zero_initialization&action=edit&section=6" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="default_initialization.html" title="cpp/language/default initialization">default initialization</a>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization">direct initialization</a>
</li><li> <a href="value_initialization.html" title="cpp/language/value initialization">value initialization</a>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization">copy initialization</a>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization">list initialization</a>
</li></ul>
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