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<p><i>Initialization</i> of a variable provides its initial value at the time of construction.
</p><p>The initial value may be provided in the initializer section of a <a href="declarations.html" title="cpp/language/declarations">declarator</a> or a <a href="new.html" title="cpp/language/new">new expression</a>. It also takes place during function calls: function parameters and the function return values are also initialized.
</p><p>For each declarator, the initializer may be one of the following:
</p>
<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>(</b></code> <span class="t-spar">expression-list</span> <code><b>)</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
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<td colspan="10" class="t-sdsc-sep">
</td></tr>
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<td> <code><b>=</b></code> <span class="t-spar">expression</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
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<td> <code><b>{ </b></code> <span class="t-spar">initializer-list</span> <code><b>} </b></code>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>
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<td colspan="10" class="t-sdsc-sep">
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<div class="t-li1"><span class="t-li">1)</span> comma-separated list of arbitrary expressions and braced-init-lists in parentheses</div>
<div class="t-li1"><span class="t-li">2)</span> the equals sign followed by an expression</div>
<div class="t-li1"><span class="t-li">3)</span> braced-init-list: possibly empty, comma-separated list of expressions and other braced-init-lists</div>
<p>Depending on context, the initializer may invoke one of the following:
</p>
<ul><li> <a href="value_initialization.html" title="cpp/language/value initialization">Value initialization</a>, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></span></span>
</li><li> <a href="direct_initialization.html" title="cpp/language/direct initialization">Direct initialization</a>, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s<span class="br0">(</span><span class="st0">"hello"</span><span class="br0">)</span><span class="sy4">;</span></span></span>
</li><li> <a href="copy_initialization.html" title="cpp/language/copy initialization">Copy initialization</a>, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">"hello"</span><span class="sy4">;</span></span></span>
</li><li> <a href="list_initialization.html" title="cpp/language/list initialization">List initialization</a>, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../string/basic_string.html"><span class="kw1072">std::<span class="me2">string</span></span></a> s<span class="br0">{</span><span class="st0">'a'</span>, <span class="st0">'b'</span>, <span class="st0">'c'</span><span class="br0">}</span><span class="sy4">;</span></span></span>
</li><li> <a href="aggregate_initialization.html" title="cpp/language/aggregate initialization">Aggregate initialization</a>, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span> a<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'a'</span>, <span class="st0">'b'</span><span class="br0">}</span><span class="sy4">;</span></span></span>
</li><li> <a href="reference_initialization.html" title="cpp/language/reference initialization">Reference initialization</a>, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span><span class="sy3">&</span> c <span class="sy1">=</span> a<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="sy4">;</span></span></span>
</li></ul>
<p>If no initializer is provided, the rules of <a href="default_initialization.html" title="cpp/language/default initialization">default initialization</a> apply.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="initialization.html#Non-local_variables"><span class="tocnumber">1</span> <span class="toctext">Non-local variables</span></a>
<ul>
<li class="toclevel-2 tocsection-2"><a href="initialization.html#Static_initialization"><span class="tocnumber">1.1</span> <span class="toctext">Static initialization</span></a></li>
<li class="toclevel-2 tocsection-3"><a href="initialization.html#Dynamic_initialization"><span class="tocnumber">1.2</span> <span class="toctext">Dynamic initialization</span></a></li>
<li class="toclevel-2 tocsection-4"><a href="initialization.html#Early_dynamic_initialization"><span class="tocnumber">1.3</span> <span class="toctext">Early dynamic initialization</span></a></li>
<li class="toclevel-2 tocsection-5"><a href="initialization.html#Deferred_dynamic_initialization"><span class="tocnumber">1.4</span> <span class="toctext">Deferred dynamic initialization</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-6"><a href="initialization.html#Static_local_variables"><span class="tocnumber">2</span> <span class="toctext">Static local variables</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="initialization.html#Class_members"><span class="tocnumber">3</span> <span class="toctext">Class members</span></a></li>
<li class="toclevel-1 tocsection-8"><a href="initialization.html#Notes"><span class="tocnumber">4</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-9"><a href="initialization.html#Defect_reports"><span class="tocnumber">5</span> <span class="toctext">Defect reports</span></a></li>
<li class="toclevel-1 tocsection-10"><a href="initialization.html#See_also"><span class="tocnumber">6</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/initialization&action=edit&section=1" title="Edit section: Non-local variables">edit</a>]</span> <span class="mw-headline" id="Non-local_variables">Non-local variables</span></h3>
<p>All non-local variables with static <a href="storage_duration.html" title="cpp/language/storage duration">storage duration</a> are initialized as part of program startup, before the execution of the <a href="main_function.html" title="cpp/language/main function">main function</a> begins (unless deferred, see below). All variables with thread-local storage duration are initialized as part of thread launch, sequenced-before the execution of the thread function begins. For both of these classes of variables, initialization occurs in two distinct stages:
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=2" title="Edit section: Static initialization">edit</a>]</span> <span class="mw-headline" id="Static_initialization">Static initialization</span></h4>
<div class="t-li1"><span class="t-li">1)</span> If permitted, <a href="constant_initialization.html" title="cpp/language/constant initialization">Constant initialization</a> takes place first (see <a href="constant_initialization.html" title="cpp/language/constant initialization">Constant initialization</a> for the list of those situations).
In practice, constant initialization is usually performed at compile time, and pre-calculated object representations are stored as part of the program image. If the compiler doesn't do that, it still has to guarantee that this initialization happens before any dynamic initialization.</div>
<div class="t-li1"><span class="t-li">2)</span> For all other non-local static and thread-local variables, <a href="zero_initialization.html" title="cpp/language/zero initialization">Zero initialization</a> takes place.
In practice, variables that are going to be zero-initialized are placed in the <code>.bss</code> segment of the program image, which occupies no space on disk, and is zeroed out by the OS when loading the program.</div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=3" title="Edit section: Dynamic initialization">edit</a>]</span> <span class="mw-headline" id="Dynamic_initialization">Dynamic initialization</span></h4>
<p>After all static initialization is completed, dynamic initialization of non-local variables occurs in the following situations:
</p>
<div class="t-li1"><span class="t-li">1)</span> <i>Unordered dynamic initialization</i>, which applies only to (static/thread-local) class template data members that aren't <a href="template_specialization.html" title="cpp/language/template specialization">explicitly specialized</a>. Initialization of such static variables is indeterminately sequenced with respect to all other dynamic initialization. Initialization of such thread-local variables is unsequenced with respect to all other dynamic initialization.</div>
<div class="t-li1"><span class="t-li">2)</span> <i>Ordered dynamic initialization</i>, which applies to all other non-local variables: within a single translation unit, these variables are initialized in exact order their definitions appear in the source code. Initialization of static variables in different translation units is indeterminately sequenced. Initialization of thread-local variables in different translation units is unsequenced.</div>
<p>If the initialization of a non-local variable with static or thread storage duration exits via an exception, <span class="t-lc"><a href="../error/terminate.html" title="cpp/error/terminate">std::terminate</a></span> is called.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=4" title="Edit section: Early dynamic initialization">edit</a>]</span> <span class="mw-headline" id="Early_dynamic_initialization">Early dynamic initialization</span></h4>
<p>The compilers are allowed to initialize dynamically-initialized variables as part of static initialization (essentially, at compile time), if the following conditions are both true:
</p>
<div class="t-li1"><span class="t-li">1)</span> the dynamic version of the initialization does not change the value of any other object of namespace scope prior to its initialization</div>
<div class="t-li1"><span class="t-li">2)</span> the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically.</div>
<p>Because of the rule above, if initialization of some object <code>o1</code> refers to an namespace-scope object <code>o2</code>, which potentially requires dynamic initialization, but is defined later in the same translation unit, it is unspecified whether the value of <code>o2</code> used will be the value of the fully initialized <code>o2</code> (because the compiler promoted initialization of <code>o2</code> to compile time) or will be the value of <code>o2</code> merely zero-initialized.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">inline</span> <span class="kw4">double</span> fd<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu16">1.0</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="kw4">extern</span> <span class="kw4">double</span> d1<span class="sy4">;</span>
<span class="kw4">double</span> d2 <span class="sy1">=</span> d1<span class="sy4">;</span> <span class="co1">// unspecified:</span>
<span class="co1">// may be statically initialized to 0.0 or</span>
<span class="co1">// dynamically initialized to 0.0 if d1 is dynamically initialized, or</span>
<span class="co1">// dynamically initialized to 1.0 if d1 is dynamically initialized</span>
<span class="kw4">double</span> d1 <span class="sy1">=</span> fd<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// may be initialized statically or dynamically to 1.0</span></pre></div></div>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=5" title="Edit section: Deferred dynamic initialization">edit</a>]</span> <span class="mw-headline" id="Deferred_dynamic_initialization">Deferred dynamic initialization</span></h4>
<p>It is implementation-defined whether dynamic initialization happens-before the first statement of the main function (for statics) or the initial function of the thread (for thread-locals), or deferred to happen after.
</p><p>If the initialization is deferred to happen after the first statement of main/thread function, it happens before the first <a href="definition.html#ODR-use" title="cpp/language/definition">odr-use</a> of any variable with static/thread storage duration defined in the same translation unit as the variable to be initialized. If no variable or function is odr-used from a given translation unit, the non-local variables defined in that translation unit may never be initialized (this models the behavior of an on-demand dynamic library). However, as long as anything from a TU is odr-used, all non-local variables whose initialization or destruction has side effects will be initialized even if they are not used in the program.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co1">// - File 1 -</span>
<span class="co2">#include "a.h"</span>
<span class="co2">#include "b.h"</span>
B b<span class="sy4">;</span>
A<span class="sy4">::</span><span class="me2">A</span><span class="br0">(</span><span class="br0">)</span><span class="br0">{</span> b.<span class="me1">Use</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
 
<span class="co1">// - File 2 -</span>
<span class="co2">#include "a.h"</span>
A a<span class="sy4">;</span>
 
<span class="co1">// - File 3 -</span>
<span class="co2">#include "a.h"</span>
<span class="co2">#include "b.h"</span>
<span class="kw4">extern</span> A a<span class="sy4">;</span>
<span class="kw4">extern</span> B b<span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
a.<span class="me1">Use</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
b.<span class="me1">Use</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// if a is initialized before main is entered, b may still be uninitialized</span>
<span class="co1">// at the point where A::A() uses it (because dynamic init is indeterminately sequenced</span>
<span class="co1">// across translation units)</span>
 
<span class="co1">// If a is initialized at some point after the first statement of main (which odr-uses</span>
<span class="co1">// a function defined in File 1, forcing its dynamic initialization to run),</span>
<span class="co1">// then b will be initialized prior to its use in A::A</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=6" title="Edit section: Static local variables">edit</a>]</span> <span class="mw-headline" id="Static_local_variables">Static local variables</span></h3>
<p>For initialization of locals (that is, block scope) static and thread-local variables, see <a href="storage_duration.html#Static_local_variables" title="cpp/language/storage duration">static local variables</a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=7" title="Edit section: Class members">edit</a>]</span> <span class="mw-headline" id="Class_members">Class members</span></h3>
<p>Non-static data members can be initialized with <a href="constructor.html" title="cpp/language/initializer list">member initializer list</a> or with a <a href="data_members.html#Member_initialization" title="cpp/language/data members">default member initializer</a>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=8" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>The order of destruction of non-local variables is described in <span class="t-lc"><a href="../utility/program/exit.html" title="cpp/utility/program/exit">std::exit</a></span>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/initialization&action=edit&section=9" 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/initialization&action=edit&section=10" 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 colspan="2"> <div class="t-dsc-see"><span><a href="../../c/language/initialization.html" title="c/language/initialization">C documentation</a></span> for <span class="t-dsc-see-tt"><span>initialization</span></span></div>
</td></tr>
</table>
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