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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.execution_context"></a><a class="link" href="execution_context.html" title="execution_context">execution_context</a>
</h3></div></div></div>
<p>
A context for function object execution.
</p>
<pre class="programlisting">class execution_context :
noncopyable
</pre>
<h5>
<a name="boost_asio.reference.execution_context.h0"></a>
<span class="phrase"><a name="boost_asio.reference.execution_context.types"></a></span><a class="link" href="execution_context.html#boost_asio.reference.execution_context.types">Types</a>
</h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a class="link" href="execution_context__id.html" title="execution_context::id"><span class="bold"><strong>id</strong></span></a>
</p>
</td>
<td>
<p>
Class used to uniquely identify a service.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context__service.html" title="execution_context::service"><span class="bold"><strong>service</strong></span></a>
</p>
</td>
<td>
<p>
Base class for all io_context services.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context/fork_event.html" title="execution_context::fork_event"><span class="bold"><strong>fork_event</strong></span></a>
</p>
</td>
<td>
<p>
Fork-related event notifications.
</p>
</td>
</tr>
</tbody>
</table></div>
<h5>
<a name="boost_asio.reference.execution_context.h1"></a>
<span class="phrase"><a name="boost_asio.reference.execution_context.member_functions"></a></span><a class="link" href="execution_context.html#boost_asio.reference.execution_context.member_functions">Member
Functions</a>
</h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody><tr>
<td>
<p>
<a class="link" href="execution_context/notify_fork.html" title="execution_context::notify_fork"><span class="bold"><strong>notify_fork</strong></span></a>
</p>
</td>
<td>
<p>
Notify the execution_context of a fork-related event.
</p>
</td>
</tr></tbody>
</table></div>
<h5>
<a name="boost_asio.reference.execution_context.h2"></a>
<span class="phrase"><a name="boost_asio.reference.execution_context.protected_member_functions"></a></span><a class="link" href="execution_context.html#boost_asio.reference.execution_context.protected_member_functions">Protected
Member Functions</a>
</h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a class="link" href="execution_context/destroy.html" title="execution_context::destroy"><span class="bold"><strong>destroy</strong></span></a>
</p>
</td>
<td>
<p>
Destroys all services in the context.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context/execution_context.html" title="execution_context::execution_context"><span class="bold"><strong>execution_context</strong></span></a>
</p>
</td>
<td>
<p>
Constructor.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context/shutdown.html" title="execution_context::shutdown"><span class="bold"><strong>shutdown</strong></span></a>
</p>
</td>
<td>
<p>
Shuts down all services in the context.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context/_execution_context.html" title="execution_context::~execution_context"><span class="bold"><strong>~execution_context</strong></span></a>
</p>
</td>
<td>
<p>
Destructor.
</p>
</td>
</tr>
</tbody>
</table></div>
<h5>
<a name="boost_asio.reference.execution_context.h3"></a>
<span class="phrase"><a name="boost_asio.reference.execution_context.friends"></a></span><a class="link" href="execution_context.html#boost_asio.reference.execution_context.friends">Friends</a>
</h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
Description
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a class="link" href="execution_context/add_service.html" title="execution_context::add_service"><span class="bold"><strong>add_service</strong></span></a>
</p>
</td>
<td>
<p>
(Deprecated: Use make_service().) Add a service object to the execution_context.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context/has_service.html" title="execution_context::has_service"><span class="bold"><strong>has_service</strong></span></a>
</p>
</td>
<td>
<p>
Determine if an execution_context contains a specified service
type.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context/make_service.html" title="execution_context::make_service"><span class="bold"><strong>make_service</strong></span></a>
</p>
</td>
<td>
<p>
Creates a service object and adds it to the execution_context.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="execution_context/use_service.html" title="execution_context::use_service"><span class="bold"><strong>use_service</strong></span></a>
</p>
</td>
<td>
<p>
Obtain the service object corresponding to the given type.
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
An execution context represents a place where function objects will be executed.
An <code class="computeroutput">io_context</code> is an example of an execution context.
</p>
<h5>
<a name="boost_asio.reference.execution_context.h4"></a>
<span class="phrase"><a name="boost_asio.reference.execution_context.the_execution_context_class_and_services"></a></span><a class="link" href="execution_context.html#boost_asio.reference.execution_context.the_execution_context_class_and_services">The
execution_context class and services</a>
</h5>
<p>
Class <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
implements an extensible, type-safe, polymorphic set of services, indexed
by service type.
</p>
<p>
Services exist to manage the resources that are shared across an execution
context. For example, timers may be implemented in terms of a single timer
queue, and this queue would be stored in a service.
</p>
<p>
Access to the services of an <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
is via three function templates, <code class="computeroutput">use_service()</code>, <code class="computeroutput">add_service()</code>
and <code class="computeroutput">has_service()</code>.
</p>
<p>
In a call to <code class="computeroutput">use_service<Service>()</code>, the type argument
chooses a service, making available all members of the named type. If <code class="computeroutput">Service</code>
is not present in an <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>,
an object of type <code class="computeroutput">Service</code> is created and added to the <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>.
A C++ program can check if an <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
implements a particular service with the function template <code class="computeroutput">has_service<Service>()</code>.
</p>
<p>
Service objects may be explicitly added to an <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
using the function template <code class="computeroutput">add_service<Service>()</code>. If
the <code class="computeroutput">Service</code> is already present, the <a class="link" href="service_already_exists.html" title="service_already_exists"><code class="computeroutput">service_already_exists</code></a>
exception is thrown. If the owner of the service is not the same object as
the <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
parameter, the <a class="link" href="invalid_service_owner.html" title="invalid_service_owner"><code class="computeroutput">invalid_service_owner</code></a>
exception is thrown.
</p>
<p>
Once a service reference is obtained from an <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
object by calling <code class="computeroutput">use_service()</code>, that reference remains usable
as long as the owning <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
object exists.
</p>
<p>
All service implementations have <a class="link" href="execution_context__service.html" title="execution_context::service"><code class="computeroutput">execution_context::service</code></a>
as a public base class. Custom services may be implemented by deriving from
this class and then added to an <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
using the facilities described above.
</p>
<h5>
<a name="boost_asio.reference.execution_context.h5"></a>
<span class="phrase"><a name="boost_asio.reference.execution_context.the_execution_context_as_a_base_class"></a></span><a class="link" href="execution_context.html#boost_asio.reference.execution_context.the_execution_context_as_a_base_class">The
execution_context as a base class</a>
</h5>
<p>
Class <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
may be used only as a base class for concrete execution context types. The
<code class="computeroutput">io_context</code> is an example of such a derived type.
</p>
<p>
On destruction, a class that is derived from <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput">execution_context</code></a>
must perform <code class="computeroutput">execution_context::shutdown()</code> followed by <code class="computeroutput">execution_context::destroy()</code>.
</p>
<p>
This destruction sequence permits programs to simplify their resource management
by using <code class="computeroutput">shared_ptr<></code>. Where an object's lifetime is tied
to the lifetime of a connection (or some other sequence of asynchronous operations),
a <code class="computeroutput">shared_ptr</code> to the object would be bound into the handlers
for all asynchronous operations associated with it. This works as follows:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
When a single connection ends, all associated asynchronous operations
complete. The corresponding handler objects are destroyed, and all <code class="computeroutput">shared_ptr</code>
references to the objects are destroyed.
</li>
<li class="listitem">
To shut down the whole program, the <a class="link" href="io_context.html" title="io_context"><code class="computeroutput">io_context</code></a>
function stop() is called to terminate any run() calls as soon as possible.
The <a class="link" href="io_context.html" title="io_context"><code class="computeroutput">io_context</code></a>
destructor calls <code class="computeroutput">shutdown()</code> and <code class="computeroutput">destroy()</code> to
destroy all pending handlers, causing all <code class="computeroutput">shared_ptr</code> references
to all connection objects to be destroyed.
</li>
</ul></div>
<h5>
<a name="boost_asio.reference.execution_context.h6"></a>
<span class="phrase"><a name="boost_asio.reference.execution_context.requirements"></a></span><a class="link" href="execution_context.html#boost_asio.reference.execution_context.requirements">Requirements</a>
</h5>
<p>
<span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/execution_context.hpp</code>
</p>
<p>
<span class="emphasis"><em>Convenience header: </em></span><code class="literal">boost/asio.hpp</code>
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2003-2017 Christopher M. Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
</div></td>
</tr></table>
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