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<p>Declares a class data member with explicit size, in bits. Adjacent bit field members may be packed to share and straddle the individual bytes.
</p><p>A bit field declaration is a <a href="data_members.html" title="cpp/language/data members">class data member declaration</a> which uses the following declarator:
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<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">identifier</span><span class="t-mark">(optional)</span> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>:</b></code> <span class="t-spar">size</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>The <i>type</i> of the bit field is introduced by the <span class="t-spar">decl-specifier-seq</span> of the <a href="declarations.html" title="cpp/language/declarations">declaration syntax</a>.
</p>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> optional sequence of any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">identifier</span>
</td>
<td> -
</td>
<td> the name of the bit field that is being declared. The name is optional: nameless bitfields introduce the specified number of bits of padding
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">size</span>
</td>
<td> -
</td>
<td> an integral constant expression with a value greater or equal to zero. When greater than zero, this is the number of bits that this bit field will occupy. The value zero is only allowed for nameless bitfields and has special meaning: it specifies that the next bit field in the class definition will begin at an allocation unit's boundary.
</td></tr></table>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="bit_field.html#Explanation"><span class="tocnumber">1</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="bit_field.html#Notes"><span class="tocnumber">2</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="bit_field.html#References"><span class="tocnumber">3</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="bit_field.html#See_also"><span class="tocnumber">4</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/bit_field&action=edit&section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The number of bits in a bit field sets the limit to the range of values it can hold:
</p>
<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="kw1">struct</span> S <span class="br0">{</span>
<span class="co1">// three-bit unsigned field,</span>
<span class="co1">// allowed values are 0...7</span>
<span class="kw4">unsigned</span> <span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">3</span><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>
S s <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">7</span><span class="br0">}</span><span class="sy4">;</span>
<span class="sy2">++</span>s.<span class="me1">b</span><span class="sy4">;</span> <span class="co1">// unsigned overflow (guaranteed wrap-around)</span>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> s.<span class="me1">b</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// output: 0</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<p>Multiple adjacent bit fields are usually packed together (although this behavior is implementation-defined):
</p>
<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="kw1">struct</span> S <span class="br0">{</span>
<span class="co1">// will usually occupy 2 bytes:</span>
<span class="co1">// 3 bits: value of b1</span>
<span class="co1">// 2 bits: unused</span>
<span class="co1">// 6 bits: value of b2</span>
<span class="co1">// 2 bits: value of b3</span>
<span class="co1">// 3 bits: unused</span>
<span class="kw4">unsigned</span> <span class="kw4">char</span> b1 <span class="sy4">:</span> <span class="nu0">3</span>, <span class="sy4">:</span> <span class="nu0">2</span>, b2 <span class="sy4">:</span> <span class="nu0">6</span>, b3 <span class="sy4">:</span> <span class="nu0">2</span><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>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> sizeof<span class="br0">(</span>S<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// usually prints 2</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<p>The special unnamed bit field of size zero can be forced to break up padding. It specifies that the next bit field begins at the beginning of its allocation unit:
</p>
<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="kw1">struct</span> S <span class="br0">{</span>
<span class="co1">// will usually occupy 2 bytes:</span>
<span class="co1">// 3 bits: value of b1</span>
<span class="co1">// 5 bits: unused</span>
<span class="co1">// 6 bits: value of b2</span>
<span class="co1">// 2 bits: value of b3</span>
<span class="kw4">unsigned</span> <span class="kw4">char</span> b1 <span class="sy4">:</span> <span class="nu0">3</span><span class="sy4">;</span>
<span class="kw4">unsigned</span> <span class="kw4">char</span> <span class="sy4">:</span><span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// start a new byte</span>
<span class="kw4">unsigned</span> <span class="kw4">char</span> b2 <span class="sy4">:</span> <span class="nu0">6</span><span class="sy4">;</span>
<span class="kw4">unsigned</span> <span class="kw4">char</span> b3 <span class="sy4">:</span> <span class="nu0">2</span><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>
<a href="../io/cout.html"><span class="kw1481">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> sizeof<span class="br0">(</span>S<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// usually prints 2</span>
<span class="br0">}</span></pre></div></div>
<p><br />
</p>
</div>
<p>If the specified size of the bit field is greater than the size of its type, the value is limited by the type: a <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/integer.html"><span class="kw120">std::<span class="me2">uint8_t</span></span></a> b <span class="sy4">:</span> <span class="nu0">1000</span><span class="sy4">;</span></span></span> would still hold values between 0 and 255. the extra bits become unused padding.
</p><p>Because bit fields do not necessarily begin at the beginning of a byte, address of a bit field cannot be taken. Pointers and non-const references to bit fields are not possible. When <a href="reference_initialization.html" title="cpp/language/reference initialization">initializing a const reference</a> from a bit field, a temporary is created (its type is the type of the bit field), copy initialized with the value of the bit field, and the reference is bound to that temporary.
</p><p>The type of a bit field can only be integral or enumeration type.
</p><p>A bit field cannot be a <a href="static.html" title="cpp/language/static">static data member</a>.
</p><p>There are no bit field <a href="value_category.html" title="cpp/language/value category">prvalues</a>: lvalue-to-rvalue conversion always produces an object of the underlying type of the bit field.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/bit_field&action=edit&section=2" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>There are no <a href="data_members.html#Member_initialization" title="cpp/language/data members">default member initializers</a> for bit fields: <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">1</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span></span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">1</span> <span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span></span></span> are ill-formed.
</p><p>The following properties of bit fields are <i>implementation-defined</i>
</p>
<ul><li> The value that results from assigning or initializing a bit field with a value out of range, or from incrementing a bit field past its range.
</li><li> Everything about the actual allocation details of bit fields within the class object
</li></ul>
<dl><dd><ul><li> For example, on some platforms, bit fields don't straddle bytes, on others they do
</li><li> Also, on some platforms, bit fields are packed left-to-right, on others right-to-left
</li></ul>
</dd></dl>
<table class="t-rev-begin">
<tr class="t-rev t-until-cxx14"><td>
<ul><li> Whether <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">short</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span></span></span>, and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">long</span></span></span> bit fields that aren't explicitly <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">signed</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span></span></span> are signed or unsigned.
</li></ul>
<dl><dd><ul><li> For example, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> b<span class="sy4">:</span><span class="nu0">3</span><span class="sy4">;</span></span></span> may have the range of values <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">0</span>..<span class="nu0">7</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="nu0">4</span>..<span class="nu0">3</span></span></span>.
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-until-cxx14">(until C++14)</span></td></tr>
</table>
<p>In the C programming language, the width of a bit field cannot exceed the width of the underlying type.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/bit_field&action=edit&section=3" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-c++11">
<ul><li> C++11 standard (ISO/IEC 14882:2011):
</li></ul>
<dl><dd><ul><li> 9.6 Bit-fields [class.bit]
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++98">
<ul><li> C++98 standard (ISO/IEC 14882:1998):
</li></ul>
<dl><dd><ul><li> 9.6 Bit-fields [class.bit]
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/bit_field&action=edit&section=4" 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/bit_field.html" title="c/language/bit field">C documentation</a></span> for <span class="t-dsc-see-tt"><span>Bit fields</span></span></div>
</td></tr>
</table>
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