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<p>Sets the initial values of the static constants
</p>
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<li class="toclevel-1 tocsection-1"><a href="constant_initialization.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="constant_initialization.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="constant_initialization.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
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<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/constant_initialization&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>static</b></code> <span class="t-spar">T</span> <code><b>&</b></code> <span class="t-spar">ref</span> <code><b>= <span class="t-spar">constexpr</span>;</b></code>
</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> <code><b>static</b></code> <span class="t-spar">T</span> <span class="t-spar">object</span> <code><b>= <span class="t-spar">constexpr</span>;</b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<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/constant_initialization&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Constant initialization is performed <span class="t-rev-inl t-until-cxx14"><span>after</span> <span><span class="t-mark-rev t-until-cxx14">(until C++14)</span></span></span><span class="t-rev-inl t-since-cxx14"><span>instead of</span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span> zero initialization of the static and thread-local objects and before all other initialization. Only the following variables are constant initialized:
</p>
<div class="t-li1"><span class="t-li">1)</span> Static or thread-local references, if it is bound to static glvalue, to a temporary object (or its subobject), or to a function. and if every expression (including implicit conversions) in <a href="reference_initialization.html" title="cpp/language/reference initialization">the initializer</a> of the reference is a <a href="constant_expression.html" title="cpp/language/constant expression">constant expression</a>.</div>
<div class="t-li1"><span class="t-li">2)</span> Static or thread-local object of class type that is initialized by a constructor call, if the constructor is <a href="constexpr.html" title="cpp/language/constexpr">constexpr</a> and all constructor arguments (including implicit conversions) are <a href="constant_expression.html" title="cpp/language/constant expression">constant expressions</a>, and if the initializers in the constructor's initializer list and the brace-or-equal initializers of the class members only contain constant expressions.</div>
<div class="t-li1"><span class="t-li">3)</span> Static or thread-local object (not necessarily of class type), that is not initialized by a constructor call, if the object is <a href="value_initialization.html" title="cpp/language/value initialization">value-initialized</a> or if every expression in its initializer is a constant expression.</div>
<p>The effects of constant initialization are the same as the effects of the corresponding initialization, except that it's guaranteed that it is complete before any other initialization of a static or thread-local object begins, and it may be performed at compile time.
</p><p>The object that was initialized by constant initialization can be used in constant expressions, e.g. in an array declaration.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/constant_initialization&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>The compiler is permitted to initialize other static and thread-local objects using constant initialization, if it can guarantee that the value would be the same as if the standard order of initialization was followed. Such objects can be used in constant expressions, but this use is not portable.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/constant_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 <iostream></span>
<span class="co2">#include <array></span>
 
<span class="kw1">struct</span> S <span class="br0">{</span>
<span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> c<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">const</span> <span class="kw4">int</span> d <span class="sy1">=</span> <span class="nu0">10</span> <span class="sy2">*</span> S<span class="sy4">::</span><span class="me2">c</span><span class="sy4">;</span> <span class="co1">// not a constant expression: S::c has no preceding</span>
<span class="co1">// initializer, this initialization happens after const</span>
<span class="kw4">const</span> <span class="kw4">int</span> S<span class="sy4">::</span><span class="me2">c</span> <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span> <span class="co1">// constant initialization, guaranteed to happen first</span>
<span class="kw4">int</span> main<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">"d = "</span> <span class="sy1"><<</span> d <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<a href="../container/array.html"><span class="kw1092">std::<span class="me2">array</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, S<span class="sy4">::</span><span class="me2">c</span><span class="sy1">></span> a1<span class="sy4">;</span> <span class="co1">// OK: S::c is a constant expression</span>
<span class="co1">// std::array<int, d> a2; // error: d is not a constant expression</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">d = 50</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/constant_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/constant_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="constexpr.html" title="cpp/language/constexpr">constexpr specifier</a>
</li><li> <a href="zero_initialization.html" title="cpp/language/zero initialization">zero initialization</a>
</li></ul>
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