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<h1 class="topictitle1">atomic Template Class</h1>
<div>
<div class="section"><h2 class="sectiontitle">Summary</h2>
<p>Template class for atomic operations.
</p>
</div>
<div class="section"><h2 class="sectiontitle">Syntax</h2>
<pre>template<typename T> atomic;</pre>
</div>
<div class="section"><h2 class="sectiontitle">Header</h2>
<pre>#include "tbb/atomic.h"</pre>
</div>
<div class="section"><h2 class="sectiontitle">Description</h2>
<p>An
<samp class="codeph">atomic<T></samp> supports atomic read, write,
fetch-and-add, fetch-and-store, and compare-and-swap. Type T may be an integral
type, enumeration type, or a pointer type. When T is a pointer type, arithmetic
operations are interpreted as pointer arithmetic. For example, if
<samp class="codeph"><em>x</em></samp> has type
<samp class="codeph">atomic<float*></samp> and a float occupies four bytes,
then
<samp class="codeph"><em>++x</em></samp> advances
<samp class="codeph"><em>x</em></samp> by four bytes. Arithmetic on
<samp class="codeph">atomic<T></samp> is not allowed if
<samp class="codeph">T</samp> is an enumeration type,
<samp class="codeph">void*</samp>, or
<samp class="codeph">bool</samp>.
</p>
<p>Some of the methods have template method variants
that permit more selective memory fencing. On IA-32 and Intel® 64 architecture
processors, they have the same effect as the non-templated variants. On
processors with IA-64 architecture, they may improve performance by allowing
the memory subsystem more latitude on the orders of reads and write. Using them
may improve performance. The table below shows the fencing for the non-template
form.
</p>
<div class="tablenoborder"><a name="tbl29"><!-- --></a><table cellpadding="4" summary="" id="tbl29" width="100%" frame="hsides" border="1" rules="all"><caption><span class="tablecap">Operation Order Implied by Non-Template
Methods</span></caption>
<thead align="left">
<tr>
<th class="cellrowborder" valign="top" width="NaN%" id="d95291e103">
<p>Kind
</p>
</th>
<th class="cellrowborder" valign="top" width="NaN%" id="d95291e109">
<p>Description
</p>
</th>
<th class="row-nocellborder" valign="top" width="NaN%" id="d95291e115">
<p>Default For
</p>
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d95291e103 ">
<p>acquire
</p>
</td>
<td class="cellrowborder" valign="top" width="NaN%" headers="d95291e109 ">
<p>Operations after the atomic operation never
move over it.
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d95291e115 ">
<p>read
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d95291e103 ">
<p>release
</p>
</td>
<td class="cellrowborder" valign="top" width="NaN%" headers="d95291e109 ">
<p>Operations before the atomic operation
never move over it.
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d95291e115 ">
<p>write
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="NaN%" headers="d95291e103 ">
<p>sequentially consistent
</p>
</td>
<td class="cellrowborder" valign="top" width="NaN%" headers="d95291e109 ">
<p>Operations on either side never move over
it and furthermore, the sequentially consistent atomic operations have a global
order.
</p>
</td>
<td class="row-nocellborder" valign="top" width="NaN%" headers="d95291e115 "><samp class="codeph"> fetch_and_store,
fetch_and_add, compare_and_swap
</samp>
</td>
</tr>
</tbody>
</table>
</div>
<div class="Note"><h3 class="NoteTipHead">
Caution</h3>
<p>The copy constructor for class
<samp class="codeph">atomic<T></samp> is not atomic. To atomically copy
an<samp class="codeph"> atomic<T></samp>, default-construct the copy first and
assign to it. Below is an example that shows the difference.
</p>
</div>
<pre>
atomic<T> y(x); // Not atomic
atomic<T> z;
z=x; // Atomic assignment</pre>
<p>The copy constructor is not atomic because it is
compiler generated. In C++03 introducing any non-trivial constructors might
remove an important property of
<samp class="codeph">atomic<T></samp>: namespace scope instances are
zero-initialized before namespace scope dynamic initializers run. This property
can be essential for code executing early during program startup.
</p>
<p>In C++03, to create an
<samp class="codeph">atomic<T></samp> with a specific value,
default-construct it first, and afterwards assign a value to it. In C++11 there
is
<samp class="codeph">constexpr</samp> single argument constructor.
</p>
</div>
<div class="section"><h2 class="sectiontitle">Members</h2>
<pre>namespace tbb {
enum memory_semantics {
acquire,
release
};
struct atomic<T> {
typedef T value_type;
//C++11 specific:
atomic() = default;
constexpr atomic(T arg)
//end of C++11 specific
template<memory_semantics M>
value_type compare_and_swap( value_type new_value,
value_type comparand );
value_type compare_and_swap( value_type new_value,
value_type comparand );
template<memory_semantics M>
value_type fetch_and_store( value_type new_value );
value_type fetch_and_store( value_type new_value );
operator value_type() const;
value_type operator=( value_type new_value );
atomic<T>& operator=( const atomic<T>& value );
// The following members exist only if T is an integral
// or pointer type.
template<memory_semantics M>
value_type fetch_and_add( value_type addend );
value_type fetch_and_add( value_type addend );
template<memory_semantics M>
value_type fetch_and_increment();
value_type fetch_and_increment();
template<memory_semantics M>
value_type fetch_and_decrement();
value_type fetch_and_decrement();
value_type operator+=(value_type);
value_type operator-=(value_type);
value_type operator++();
value_type operator++(int);
value_type operator--();
value_type operator--(int);
};
} </pre>
<p>So that an atomic<T*> can be used like a
pointer to T, the specialization atomic<T*> also defines:
</p>
<pre> T* operator->() const;</pre>
<div class="tablenoborder"><table cellpadding="4" summary="" frame="border" border="1" cellspacing="0" rules="all"><span class="tabledesc">The following table provides additional information on the
members of this template class.
</span><thead align="left">
<tr>
<th class="cellrowborder" valign="top" width="33.89830508474576%" id="d95291e248">Member
</th>
<th class="cellrowborder" valign="top" width="66.10169491525423%" id="d95291e251">Description
</th>
</tr>
</thead>
<tbody>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword">memory_semantics Enum
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>Defines values used to select the
template variants that permit more selective control over visibility of
operations (see the table above).
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword">atomic() = default
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>C++11 Specific;
</p>
<p>Default constructor generated by
compiler. This constructor behaves same as if there were no user defined
constrcutors declared at all.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword">constexpr atomic(value_type arg)
</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>C++11 Specific;
</p>
<p>Initialize
<samp class="codeph">*this</samp> with value of arg. If the argument is
a translation time constant, then initialization is performed during
translation time, overwise initialization is performed at run time.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword">value_type fetch_and_add(
value_type addend )</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>Let
<samp class="codeph"><em>x</em></samp> be the value of
<samp class="codeph">*this</samp>. Atomically updates
<samp class="codeph"><em>x = x +</em></samp> addend.
</p>
<p><strong>Returns</strong>: Original value of
<samp class="codeph"><em>x</em></samp>.
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword"> value_type
fetch_and_increment()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>Let x be the value of
<samp class="codeph">*this</samp>. Atomically updates
<samp class="codeph"><em>x = x + 1</em>.
</samp>
</p>
<p><strong>Returns</strong>: Original value
of<samp class="codeph"><em> x</em>.</samp>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword">value_type
fetch_and_decrement()</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>Let
<samp class="codeph"><em>x</em></samp> be the value of *this. Atomically
updates
<samp class="codeph"><em>x = x</em> - 1.
</samp>
</p>
<p><strong>Returns</strong>: Original value
of<samp class="codeph"><em> x.</em></samp>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword">value_type
compare_and_swap</span>
<p>value_type compare_and_swap( value_type
new_value, value_type comparand )
</p>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>Let
<em>x</em> be the value of
<samp class="codeph">*this</samp>. Atomically compares
<samp class="codeph"><em>x</em></samp> with comparand, and if they are
equal, sets<samp class="codeph"><em> x</em></samp>=new_value.
</p>
<p><strong>Returns</strong>: Original value of
<samp class="codeph">x.</samp>
</p>
</td>
</tr>
<tr>
<td class="cellrowborder" valign="top" width="33.89830508474576%" headers="d95291e248 "><span class="keyword">value_type fetch_and_store(
value_type new_value )</span>
</td>
<td class="cellrowborder" valign="top" width="66.10169491525423%" headers="d95291e251 ">
<p>Let
<samp class="codeph">x</samp> be the value of
<samp class="codeph">*this</samp>. Atomically exchanges old value of
<samp class="codeph"><em>x</em></samp> with new_value.
</p>
<p><strong>Returns</strong>: Original value of
<samp class="codeph"><em>x</em></samp>.
</p>
</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../../reference/synchronization.htm">Synchronization</a></div>
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