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                <h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ concepts:</span> Allocator</h1>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="CopyInsertable.html" title="cpp/concept/CopyInsertable"><span class="t-lines"><span>CopyInsertable</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="MoveInsertable.html" title="cpp/concept/MoveInsertable"><span class="t-lines"><span>MoveInsertable</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="EmplaceConstructible.html" title="cpp/concept/EmplaceConstructible"><span class="t-lines"><span>EmplaceConstructible</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Eraseable.html" title="cpp/concept/Erasable"><span class="t-lines"><span>Erasable</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Iterator</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Iterator.html" title="cpp/concept/Iterator"><span class="t-lines"><span>Iterator</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="OutputIterator.html" title="cpp/concept/OutputIterator"><span class="t-lines"><span>OutputIterator</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="InputIterator.html" title="cpp/concept/InputIterator"><span class="t-lines"><span>InputIterator</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="ForwardIterator.html" title="cpp/concept/ForwardIterator"><span class="t-lines"><span>ForwardIterator</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="BidirectionalIterator.html" title="cpp/concept/BidirectionalIterator"><span class="t-lines"><span>BidirectionalIterator</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="RandomAccessIterator.html" title="cpp/concept/RandomAccessIterator"><span class="t-lines"><span>RandomAccessIterator</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/ContiguousIterator&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/ContiguousIterator (page does not exist)"><span class="t-lines"><span>ContiguousIterator</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Stream I/O</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="FormattedInputFunction.html" title="cpp/concept/FormattedInputFunction"><span class="t-lines"><span>FormattedInputFunction</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="UnformattedInputFunction.html" title="cpp/concept/UnformattedInputFunction"><span class="t-lines"><span>UnformattedInputFunction</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="FormattedOutputFunction.html" title="cpp/concept/FormattedOutputFunction"><span class="t-lines"><span>FormattedOutputFunction</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="UnformattedOutputFunction.html" title="cpp/concept/UnformattedOutputFunction"><span class="t-lines"><span>UnformattedOutputFunction</span></span></a></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Random Numbers</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="SeedSequence.html" title="cpp/concept/SeedSequence"><span class="t-lines"><span>SeedSequence</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/RandomNumberEngine&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/RandomNumberEngine (page does not exist)"><span class="t-lines"><span>RandomNumberEngine</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="RandomNumberDistribution.html" title="cpp/concept/RandomNumberDistribution"><span class="t-lines"><span>RandomNumberDistribution</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="UniformRandomNumberGenerator.html" title="cpp/concept/UniformRandomNumberGenerator"><span class="t-lines"><span>UniformRandomNumberGenerator</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/RandomNumberEngineAdaptor&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/RandomNumberEngineAdaptor (page does not exist)"><span class="t-lines"><span>RandomNumberEngineAdaptor</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Concurrency</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="BasicLockable.html" title="cpp/concept/BasicLockable"><span class="t-lines"><span>BasicLockable</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Lockable.html" title="cpp/concept/Lockable"><span class="t-lines"><span>Lockable</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="TimedLockable.html" title="cpp/concept/TimedLockable"><span class="t-lines"><span>TimedLockable</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Mutex.html" title="cpp/concept/Mutex"><span class="t-lines"><span>Mutex</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="TimedMutex.html" title="cpp/concept/TimedMutex"><span class="t-lines"><span>TimedMutex</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="SharedMutex.html" title="cpp/concept/SharedMutex"><span class="t-lines"><span>SharedMutex</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="SharedTimedMutex.html" title="cpp/concept/SharedTimedMutex"><span class="t-lines"><span>SharedTimedMutex</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Other</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/CharTraits&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/CharTraits (page does not exist)"><span class="t-lines"><span>CharTraits</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/pos_type&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/pos type (page does not exist)"><span class="t-lines"><span>pos_type</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/off_type&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/off type (page does not exist)"><span class="t-lines"><span>off_type</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="Clock.html" title="cpp/concept/Clock"><span class="t-lines"><span>Clock</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="BitmaskType.html" title="cpp/concept/BitmaskType"><span class="t-lines"><span>BitmaskType</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/UnaryTypeTrait&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/UnaryTypeTrait (page does not exist)"><span class="t-lines"><span>UnaryTypeTrait</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/BinaryTypeTrait&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/BinaryTypeTrait (page does not exist)"><span class="t-lines"><span>BinaryTypeTrait</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/TransformationTrait&amp;action=edit&amp;redlink=1" class="new" title="cpp/concept/TransformationTrait (page does not exist)"><span class="t-lines"><span>TransformationTrait</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="TrivialClock.html" title="cpp/concept/TrivialClock"><span class="t-lines"><span>TrivialClock</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="LiteralType.html" title="cpp/concept/LiteralType"><span class="t-lines"><span>LiteralType</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
</table></div></td></tr>
</table></div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/concept/navbar_content&amp;action=edit">&#91;edit&#93;</a></span></div></div></div></div><div class="t-navbar-sep">&#160;</div></div>
<p>Encapsulates a memory allocation and deallocation strategy.
</p><p>Every standard library component that may need to allocate or release storage, from <code>std::string</code>, <code>std::vector</code>, and every container except <code>std::array</code>, to <code>std::shared_ptr</code> and <code>std::function</code>, does so through an <strong class="selflink"><code>Allocator</code></strong>: an object of a class type that satisfies the following requirements.
</p><p>Some requirements are optional: the template <span class="t-lc"><a href="../memory/allocator_traits.html" title="cpp/memory/allocator traits">std::allocator_traits</a></span> supplies the default implementations for all optional requirements, and all standard library containers and other allocator-aware classes access the allocator through <code>std::allocator_traits</code>, not directly.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/concept/Allocator&amp;action=edit&amp;section=1" title="Edit section: Requirements">edit</a>]</span> <span class="mw-headline" id="Requirements">Requirements</span></h3>
<p>Given
</p>
<ul><li> <code>T</code>, a cv-unqualified object type
</li><li> <code>A</code>, an Allocator type for type <code>T</code>
</li><li> <code>a</code>, an object of type <code>A</code>
</li><li> <code>B</code>, the corresponding Allocator type for some cv-unqualified object type <code>U</code> (as obtained by rebinding <code>A</code>)
</li><li> <code>ptr</code>, a value of type <code>allocator_traits&lt;A&gt;::pointer</code>, obtained by calling <span class="t-c"><span class="mw-geshi cpp source-cpp">allocator_traits<span class="sy1">&lt;</span>A<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">allocate</span><span class="br0">&#40;</span><span class="br0">&#41;</span></span></span>
</li><li> <code>cptr</code>, a value of type <code>allocator_traits&lt;A&gt;::const_pointer</code>, obtained by conversion from <code>ptr</code>
</li><li> <code>vptr</code>, a value of type <code>allocator_traits&lt;A&gt;::void_pointer</code>, obtained by conversion from <code>ptr</code>
</li><li> <code>cvptr</code>, a value of type <code>allocator_traits&lt;A&gt;::const_void_pointer</code>, obtained by conversion from <code>cptr</code> or from <code>vptr</code>
</li><li> <code>xptr</code>, a dereferencable pointer to some cv-unqualified object type <code>X</code>
</li></ul>
<table class="wikitable">

<tr>
<th>Expression</th>
<th>Requirements</th>
<th>Return type
</th></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">pointer</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> Satisfies <a href="NullablePointer.html" title="cpp/concept/NullablePointer"><code>NullablePointer</code></a> and <a href="RandomAccessIterator.html" title="cpp/concept/RandomAccessIterator"><code>RandomAccessIterator</code></a>
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">const_pointer</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <code>A::pointer</code> is convertible to <code>A::const_pointer</code>. Satisfies <a href="NullablePointer.html" title="cpp/concept/NullablePointer"><code>NullablePointer</code></a> and <a href="RandomAccessIterator.html" title="cpp/concept/RandomAccessIterator"><code>RandomAccessIterator</code></a>
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">void_pointer</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <code>A::pointer</code> is convertible to <code>A::void_pointer</code>
<p><code>B::void_pointer</code> and <code>A::void_pointer</code> are the same type. Satisfies <a href="NullablePointer.html" title="cpp/concept/NullablePointer"><code>NullablePointer</code></a>
</p>
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">const_void_pointer</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <code>A::pointer</code>, <code>A::const_pointer</code>, and <code>A::void_pointer</code> are convertible to <code>A::const_void_pointer</code>
<p><code>B::const_void_pointer</code> and <code>A::const_void_pointer</code> are the same type. Satisfies <a href="NullablePointer.html" title="cpp/concept/NullablePointer"><code>NullablePointer</code></a>
</p>
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">value_type</span></span></span>
</td>
<td>
</td>
<td> the type <code>T</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">size_type</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <code>A::size_type</code> can represent the size of the largest object <code>A</code> can allocate
</td>
<td> unsigned integer type
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">difference_type</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <code>A::difference_type</code> can represent the difference of any two pointers to the objects allocated by <code>A</code>
</td>
<td> signed integer type
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="kw1">template</span> rebind<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">other</span></span></span> <span class="t-mark">(optional[1])</span>
</td>
<td> for any <code>U</code>, <code>B::template rebind&lt;T&gt;::other</code> is <code>A</code>
</td>
<td> the type <code>B</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>ptr</span></span>
</td>
<td>
</td>
<td> <code>T&amp;</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>cptr</span></span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>cptr</span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>ptr</span></span> identify the same object
</td>
<td> <code>const T&amp;</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">ptr<span class="sy2">-</span><span class="sy1">&gt;</span>m</span></span>
</td>
<td> same as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">&#40;</span><span class="sy2">*</span>ptr<span class="br0">&#41;</span>.<span class="me1">m</span></span></span>, if <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">&#40;</span><span class="sy2">*</span>ptr<span class="br0">&#41;</span>.<span class="me1">m</span></span></span> is well-defined
</td>
<td> the type of <code>T::m</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">cptr<span class="sy2">-</span><span class="sy1">&gt;</span>m</span></span>
</td>
<td> same as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">&#40;</span><span class="sy2">*</span>cptr<span class="br0">&#41;</span>.<span class="me1">m</span></span></span>, if <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">&#40;</span><span class="sy2">*</span>cptr<span class="br0">&#41;</span>.<span class="me1">m</span></span></span> is well-defined
</td>
<td> the type of <code>T::m</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1">&lt;</span>A<span class="sy4">::</span><span class="me2">pointer</span><span class="sy1">&gt;</span><span class="br0">&#40;</span>vptr<span class="br0">&#41;</span></span></span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1">&lt;</span>A<span class="sy4">::</span><span class="me2">pointer</span><span class="sy1">&gt;</span><span class="br0">&#40;</span>vptr<span class="br0">&#41;</span> <span class="sy1">==</span> ptr</span></span>
</td>
<td> <code>A::pointer</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1">&lt;</span>A<span class="sy4">::</span><span class="me2">const_pointer</span><span class="sy1">&gt;</span><span class="br0">&#40;</span>cvptr<span class="br0">&#41;</span></span></span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1">&lt;</span>A<span class="sy4">::</span><span class="me2">const_pointer</span><span class="sy1">&gt;</span><span class="br0">&#40;</span>vptr<span class="br0">&#41;</span> <span class="sy1">==</span> cptr</span></span>
</td>
<td> <code>A::const_pointer</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">allocate</span><span class="br0">&#40;</span>n<span class="br0">&#41;</span></span></span>
</td>
<td> allocates storage suitable for <code>n</code> objects of type <code>T</code>, but does not construct them. May throw exceptions.
</td>
<td> <code>A::pointer</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">allocate</span><span class="br0">&#40;</span>n, cvptr<span class="br0">&#41;</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> same as <code>a.allocate(n)</code>, but may use <code>cvptr</code> (a pointer obtained from <code>a.allocate()</code> or <code>nullptr_t</code>) in unspecified manner to aid locality
</td>
<td> <code>A::pointer</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">deallocate</span><span class="br0">&#40;</span>ptr, n<span class="br0">&#41;</span></span></span>
</td>
<td> deallocates storage pointed to <code>ptr</code>, which must be a value returned by a previous call to <span class="t-c"><span class="mw-geshi cpp source-cpp">allocate</span></span> that has not been invalidated by an intervening call to <code>deallocate</code>. <code>n</code> must match the value previously passed to <code>allocate</code>. Does not throw exceptions.
</td>
<td> (not used)
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">max_size</span><span class="br0">&#40;</span><span class="br0">&#41;</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> the largest value that can be passed to <code>A::allocate()</code>
</td>
<td> <code>A::size_type</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a1 <span class="sy1">==</span> a2</span></span>
</td>
<td> returns <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> only if the storage allocated by the allocator <code>a1</code> can be deallocated through <code>a2</code>. Establishes reflexive, symmetric, and transitive relationship. Does not throw exceptions.
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a1 <span class="sy3">!</span><span class="sy1">=</span> a2</span></span>
</td>
<td> same as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="br0">&#40;</span>a1<span class="sy1">==</span>a2<span class="br0">&#41;</span></span></span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A a1<span class="br0">&#40;</span>a<span class="br0">&#41;</span></span></span>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">A a1 <span class="sy1">=</span> a</span></span>
</p>
</td>
<td> Copy-constructs <code>a1</code> such that <span class="t-c"><span class="mw-geshi cpp source-cpp">a1 <span class="sy1">==</span> a</span></span>. Does not throw exceptions. (Note: every <code>Allocator</code> also satisfies <a href="CopyConstructible.html" title="cpp/concept/CopyConstructible"><code>CopyConstructible</code></a>)
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A a<span class="br0">&#40;</span>b<span class="br0">&#41;</span></span></span>
</td>
<td> Constructs <code>a</code> such that <span class="t-c"><span class="mw-geshi cpp source-cpp">B<span class="br0">&#40;</span>a<span class="br0">&#41;</span><span class="sy1">==</span>b</span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="br0">&#40;</span>b<span class="br0">&#41;</span><span class="sy1">==</span>a</span></span>. Does not throw exceptions. (Note: this implies that all allocators related by rebind maintain each other's resources, such as memory pools)
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A a1<span class="br0">&#40;</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">&#40;</span>a<span class="br0">&#41;</span><span class="br0">&#41;</span></span></span>
<p><span class="t-c"><span class="mw-geshi cpp source-cpp">A a1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">&#40;</span>a<span class="br0">&#41;</span></span></span>
</p>
</td>
<td> Constructs <code>a1</code> such that it equals the prior value of <code>a</code>. Does not throw exceptions.
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A a<span class="br0">&#40;</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">&#40;</span>b<span class="br0">&#41;</span><span class="br0">&#41;</span></span></span>
</td>
<td> Constructs <code>a</code> such that it equals the prior value of <code>A(b)</code>. Does not throw exceptions.
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">construct</span><span class="br0">&#40;</span>xptr, args<span class="br0">&#41;</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> Constructs an object of type <code>X</code> in previously-allocated storage at the address pointed to by <code>xptr</code>, using args as the constructor arguments
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">destroy</span><span class="br0">&#40;</span>xptr<span class="br0">&#41;</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> Destructs an object of type <code>X</code> pointed to by <code>xptr</code>, but does not deallocate any storage.
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">select_on_container_copy_construction</span><span class="br0">&#40;</span><span class="br0">&#41;</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> Provides an instance of <code>A</code> to be used by the container that is copy-constructed from the one that uses <code>a</code> currently. Usually returns either a copy of <code>a</code> or a default-constructed <code>A()</code>.
</td>
<td> <code>A</code>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">propagate_on_container_copy_assignment</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> if the allocator of type <code>A</code> needs to be copied when the container that uses it is copy-assigned. Note that if the allocators of the source and the target containers do not compare equal, copy assignment has to deallocate the target's memory using the old allocator and then allocate it using the new allocator before copying the elements (and the allocator).
</td>
<td> <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::true_type</a></span> or <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::false_type</a></span> or derived from such
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">propagate_on_container_move_assignment</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> if the allocator of type <code>A</code> needs to be copied when the container that uses it is move-assigned. If this member is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> and the allocators of the source and the target containers do not compare equal, move assignment cannot take ownership of the source memory and must move-assign or move-construct the elements individually, resizing its own memory as needed.
</td>
<td> <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::true_type</a></span> or <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::false_type</a></span> or derived from such
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">propagate_on_container_swap</span></span></span> <span class="t-mark">(optional)</span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> if the allocators of type <code>A</code> need to be swapped when two containers that use them are swapped. If this member is <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> and the allocators of the two containers do not compare equal, the behavior of container swap is undefined.
</td>
<td> <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::true_type</a></span> or <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::false_type</a></span> or derived from such
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">A<span class="sy4">::</span><span class="me2">is_always_equal</span></span></span> <span class="t-mark">(since c++17)</span> <span class="t-mark">(optional)</span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> if any two allocators of type <code>A</code> always compare equal. If not provided, <span class="t-lc"><a href="../memory/allocator_traits.html" title="cpp/memory/allocator traits">std::allocator_traits</a></span> defaults this to equal <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/is_empty.html"><span class="kw470">std::<span class="me2">is_empty</span></span></a><span class="sy1">&lt;</span>A<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span></span></span>
</td>
<td> <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::true_type</a></span> or <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::false_type</a></span> or derived from such
</td></tr></table>
<p>Notes:
[1] rebind is only optional (provided by <span class="t-lc"><a href="../memory/allocator_traits.html" title="cpp/memory/allocator traits">std::allocator_traits</a></span>) if this allocator is a template of the form <span class="t-c"><span class="mw-geshi cpp source-cpp">SomeAllocator<span class="sy1">&lt;</span>T, Args<span class="sy1">&gt;</span></span></span>, where <code>Args</code> is zero or more additional template parameters.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<p>Additionally, in order for the type <code>A</code> to satisfy <code>Allocator</code>
</p>
<ul><li> If <code>A::propagate_on_container_copy_assignment::value</code> is <code>true</code>, then <code>A</code> must satisfy <a href="CopyAssignable.html" title="cpp/concept/CopyAssignable"><code>CopyAssignable</code></a> and the copy operation must not throw exceptions
</li><li> If <code>A::propagate_on_container_move_assignment::value</code> is <code>true</code>, then <code>A</code> must satisfy <a href="MoveAssignable.html" title="cpp/concept/MoveAssignable"><code>MoveAssignable</code></a> and the move operation must not throw exceptions.
</li><li> If <code>A::propagate_on_container_swap::value</code> is <code>true</code>, lvalues of <code>A</code> must be <a href="Swappable.html" title="cpp/concept/Swappable"><code>Swappable</code></a> and the swap operation must not throw exceptions
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p>Given
</p>
<ul><li> <code>x1</code> and <code>x2</code>, objects of (possibly different) types <code>X::void_pointer</code>, <code>X::const_void_pointer</code>, <code>X::pointer</code>, or <code>X::const_pointer</code>
</li></ul>
<p>Then, x1 and x2 are <i>equivalently-valued</i> pointer values, if and only if both <code>x1</code> and <code>x2</code> can be explicitly converted to the two corresponding objects <code>px1</code> and <code>px2</code> of type <code>X::const_pointer</code>, using a sequence of <code>static_casts</code> using only these four types, and the expression <span class="t-c"><span class="mw-geshi cpp source-cpp">px1 <span class="sy1">==</span> px2</span></span> evaluates to true
</p><p>Given
</p>
<ul><li> <code>w1</code> and <code>w2</code>, objects of type <code>X::void_pointer</code>.
</li></ul>
<p>Then for the expression <span class="t-c"><span class="mw-geshi cpp source-cpp">w1 <span class="sy1">==</span> w2</span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp">w1 <span class="sy3">!</span><span class="sy1">=</span> w2</span></span> either or both objects may be replaced by an <i>equivalently-valued</i> object of type <code>X::const_void_pointer</code> with no change in semantics.
</p><p>Given
</p>
<ul><li> <code>p1</code> and <code>p2</code>, objects of type <code>X::pointer</code>
</li></ul>
<p>Then, for the expressions <span class="t-c"><span class="mw-geshi cpp source-cpp">p1 <span class="sy1">==</span> p2</span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp">p1 <span class="sy3">!</span><span class="sy1">=</span> p2</span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp">p1 <span class="sy1">&lt;</span> p2</span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">p1 <span class="sy1">&lt;=</span> p2</span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp">p1 <span class="sy1">&gt;=</span> p2</span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp">p1 <span class="sy1">&gt;</span> p2</span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp">p1 <span class="sy2">-</span> p2</span></span>} either or both objects may be replaced by an <i>equivalently-valued</i> object of type <code>X::const_pointer</code> with no change in semantics.
</p><p>The above requirements make it possible to compare <a href="Container.html" title="cpp/concept/Container"><code>Container</code></a>'s iterators and const_iterators.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<p><br />
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<h3> <span class="mw-headline" id="Allocator_completeness_requirements">Allocator completeness requirements</span></h3>
<p>An allocator type <code>X</code> for type <code>T</code> additionally satisfies the <i>allocator completeness requirements</i> if both of the following are true regardless of whether <code>T</code> is a complete type:
</p>
<ul><li> <code>X</code> is a complete type
</li><li> Except for <code>value_type</code>, all the member types of <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../memory/allocator_traits.html"><span class="kw602">std::<span class="me2">allocator_traits</span></span></a><span class="sy1">&lt;</span>X<span class="sy1">&gt;</span></span></span> are complete types.
</li></ul>
</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/concept/Allocator&amp;action=edit&amp;section=3" title="Edit section: Examples">edit</a>]</span> <span class="mw-headline" id="Examples">Examples</span></h3>
<div class="t-example"><p>a minimum C++11 allocator 
</p><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 &lt;cstddef&gt;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Tp<span class="sy1">&gt;</span>
<span class="kw1">struct</span> SimpleAllocator <span class="br0">&#123;</span>
  <span class="kw1">typedef</span> Tp value_type<span class="sy4">;</span>
  SimpleAllocator<span class="br0">&#40;</span><span class="coMULTI">/*ctor args*/</span><span class="br0">&#41;</span><span class="sy4">;</span>
  <span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> SimpleAllocator<span class="br0">&#40;</span><span class="kw4">const</span> SimpleAllocator<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> other<span class="br0">&#41;</span><span class="sy4">;</span>
  Tp<span class="sy2">*</span> allocate<span class="br0">&#40;</span><a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">&#41;</span><span class="sy4">;</span>
  <span class="kw4">void</span> deallocate<span class="br0">&#40;</span>Tp<span class="sy2">*</span> p, <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">&#41;</span><span class="sy4">;</span>
<span class="br0">&#125;</span><span class="sy4">;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span>
<span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">&#40;</span><span class="kw4">const</span> SimpleAllocator<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, <span class="kw4">const</span> SimpleAllocator<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="br0">&#41;</span><span class="sy4">;</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">&gt;</span>
<span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">&#40;</span><span class="kw4">const</span> SimpleAllocator<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span>, <span class="kw4">const</span> SimpleAllocator<span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="br0">&#41;</span><span class="sy4">;</span></pre></div></div>
<p><br /> 
</p>
</div>

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