1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Proposed Standard Executors</title> 5<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../boost_asio.html" title="Boost.Asio"> 9<link rel="prev" href="net_ts.html" title="Networking TS compatibility"> 10<link rel="next" href="history.html" title="Revision History"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 15<td align="center"><a href="../../../index.html">Home</a></td> 16<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="net_ts.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_asio.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../boost_asio.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="history.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 27<a name="boost_asio.std_executors"></a><a class="link" href="std_executors.html" title="Proposed Standard Executors">Proposed Standard Executors</a> 28</h2></div></div></div> 29<p> 30 Boost.Asio provides a complete implementation of the proposed standard executors, 31 as described in <a href="http://wg21.link/P0443r13" target="_top">P0443r13</a>, <a href="http://wg21.link/P1348r0" target="_top">P1348r0</a>, and <a href="http://wg21.link/P1393r0" target="_top">P1393r0</a>. 32 </p> 33<p> 34 Just as with executors under the Networking TS model, a standard executor represents 35 a policy as to how, when, and where a piece of code should be executed. Most 36 existing code should continue to work with little or no change. 37 </p> 38<h4> 39<a name="boost_asio.std_executors.h0"></a> 40 <span class="phrase"><a name="boost_asio.std_executors.standard_executor_implementations_in_boost_asio"></a></span><a class="link" href="std_executors.html#boost_asio.std_executors.standard_executor_implementations_in_boost_asio">Standard 41 Executor Implementations in Boost.Asio</a> 42 </h4> 43<p> 44 The <a class="link" href="reference/io_context/executor_type.html" title="io_context::executor_type"><code class="computeroutput">io_context::executor_type</code></a>, 45 <a class="link" href="reference/thread_pool/executor_type.html" title="thread_pool::executor_type"><code class="computeroutput">thread_pool::executor_type</code></a>, 46 <a class="link" href="reference/system_executor.html" title="system_executor"><code class="computeroutput">system_executor</code></a>, 47 and <a class="link" href="reference/strand.html" title="strand"><code class="computeroutput">strand</code></a> 48 executors meet the requirements for the proposed standard executors. For compatibility, 49 these classes also meet the requirements for the Networking TS model of executors. 50 </p> 51<h4> 52<a name="boost_asio.std_executors.h1"></a> 53 <span class="phrase"><a name="boost_asio.std_executors.standard_executor_use_in_boost_asio"></a></span><a class="link" href="std_executors.html#boost_asio.std_executors.standard_executor_use_in_boost_asio">Standard 54 Executor Use in Boost.Asio</a> 55 </h4> 56<p> 57 All I/O objects such as <a class="link" href="reference/ip__tcp/socket.html" title="ip::tcp::socket"><code class="computeroutput">ip::tcp::socket</code></a>, 58 asynchronous operations, and utilities including <a class="link" href="reference/dispatch.html" title="dispatch"><code class="computeroutput">dispatch</code></a>, 59 <a class="link" href="reference/post.html" title="post"><code class="computeroutput">post</code></a>, <a class="link" href="reference/defer.html" title="defer"><code class="computeroutput">defer</code></a>, 60 <a class="link" href="reference/get_associated_executor.html" title="get_associated_executor"><code class="computeroutput">get_associated_executor</code></a>, 61 <a class="link" href="reference/bind_executor.html" title="bind_executor"><code class="computeroutput">bind_executor</code></a>, 62 <a class="link" href="reference/make_work_guard.html" title="make_work_guard"><code class="computeroutput">make_work_guard</code></a>, 63 <a class="link" href="reference/spawn.html" title="spawn"><code class="computeroutput">spawn</code></a>, <a class="link" href="reference/co_spawn.html" title="co_spawn"><code class="computeroutput">co_spawn</code></a>, <a class="link" href="reference/async_compose.html" title="async_compose"><code class="computeroutput">async_compose</code></a>, 64 <a class="link" href="reference/use_future.html" title="use_future"><code class="computeroutput">use_future</code></a>, 65 etc., can interoperate with both proposed standard executors, and with Networking 66 TS executors. Boost.Asio's implementation determines at compile time which 67 model a particular executor meets; the proposed standard executor model is 68 used in preference if both are detected. 69 </p> 70<p> 71 Support for the existing Networking TS model of executors can be disabled by 72 defining <code class="computeroutput">BOOST_ASIO_NO_TS_EXECUTORS</code>. 73 </p> 74<h4> 75<a name="boost_asio.std_executors.h2"></a> 76 <span class="phrase"><a name="boost_asio.std_executors.polymorphic_i_o_executor"></a></span><a class="link" href="std_executors.html#boost_asio.std_executors.polymorphic_i_o_executor">Polymorphic 77 I/O Executor</a> 78 </h4> 79<p> 80 The <a class="link" href="reference/any_io_executor.html" title="any_io_executor"><code class="computeroutput">any_io_executor</code></a> 81 type alias is the default runtime-polymorphic executor for all I/O objects. 82 This type alias points to the <a class="link" href="reference/execution__any_executor.html" title="execution::any_executor"><code class="computeroutput">execution::any_executor<></code></a> 83 template with a set of supportable properties specified for use with I/O. 84 </p> 85<p> 86 This new name may break existing code that directly uses the old polymorphic 87 wrapper, <a class="link" href="reference/executor.html" title="executor"><code class="computeroutput">executor</code></a>. 88 If required for backward compatibility, <code class="computeroutput">BOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT</code> 89 can be defined, which changes the <code class="computeroutput">any_io_executor</code> type alias to 90 instead point to the <code class="computeroutput">executor</code> polymorphic wrapper. 91 </p> 92<h4> 93<a name="boost_asio.std_executors.h3"></a> 94 <span class="phrase"><a name="boost_asio.std_executors.implementing_a_minimal_i_o_executor"></a></span><a class="link" href="std_executors.html#boost_asio.std_executors.implementing_a_minimal_i_o_executor">Implementing 95 a Minimal I/O Executor</a> 96 </h4> 97<p> 98 Standard executor properties make what were previously hard requirements on 99 an executor (such as work counting, or the ability to distinguish between 100 <code class="computeroutput">post</code>, <code class="computeroutput">dispatch</code>, and <code class="computeroutput">defer</code>) into optional 101 facilities. With this relaxation, the minimal requirements for an I/O executor 102 are: 103 </p> 104<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 105<li class="listitem"> 106 Conformance to the <a class="link" href="reference/Executor1.html#boost_asio.reference.Executor1.standard_executors"><code class="computeroutput">executor</code></a> 107 concept. 108 </li> 109<li class="listitem"> 110 The ability to query the <a class="link" href="reference/execution__context.html" title="execution::context"><code class="computeroutput">execution::context</code></a> 111 property, with the result being <a class="link" href="reference/execution_context.html" title="execution_context"><code class="computeroutput">execution_context&</code></a> 112 or a reference to a class that is derived from <code class="computeroutput">execution_context</code>. 113 </li> 114<li class="listitem"> 115 The <code class="computeroutput">execute</code> operation having, at minimum, the <a class="link" href="reference/execution__blocking_t/never.html" title="execution::blocking_t::never"><code class="computeroutput">execution::blocking.never</code></a> 116 semantic. 117 </li> 118</ul></div> 119<p> 120 The following example shows a minimal I/O executor. Given a queue submission 121 operation implemented elsewhere: 122 </p> 123<p> 124</p> 125<pre class="programlisting">queue_t queue_create(); 126template <typename F> void queue_submit(queue_t q, F f); 127</pre> 128<p> 129 </p> 130<p> 131 the executor may be defined as follows: 132 </p> 133<p> 134</p> 135<pre class="programlisting">struct minimal_io_executor 136{ 137 boost::asio::execution_context* context_; 138 queue_t queue_; 139 140 bool operator==(const minimal_io_executor& other) const noexcept 141 { 142 return context_ == other.context_ && queue_ == other.queue_; 143 } 144 145 bool operator!=(const minimal_io_executor& other) const noexcept 146 { 147 return !(*this == other); 148 } 149 150 boost::asio::execution_context& query( 151 boost::asio::execution::context_t) const noexcept 152 { 153 return *context_; 154 } 155 156 static constexpr boost::asio::execution::blocking_t::never_t query( 157 boost::asio::execution::blocking_t) noexcept 158 { 159 // This executor always has blocking.never semantics. 160 return boost::asio::execution::blocking.never; 161 } 162 163 template <class F> 164 void execute(F f) const 165 { 166 queue_submit(queue_, std::move(f)); 167 } 168}; 169</pre> 170<p> 171 </p> 172<p> 173 This executor may be created as follows: 174 </p> 175<p> 176</p> 177<pre class="programlisting">boost::asio::execution_context context; 178queue_t queue = queue_create(); 179minimal_io_executor executor{&context, queue}; 180</pre> 181<p> 182 </p> 183<p> 184 and then used with I/O objects: 185 </p> 186<p> 187</p> 188<pre class="programlisting">boost::asio::ip::tcp::acceptor acceptor(executor); 189</pre> 190<p> 191 </p> 192<p> 193 or assigned into the <a class="link" href="reference/any_io_executor.html" title="any_io_executor"><code class="computeroutput">any_io_executor</code></a> 194 polymorphic wrapper: 195 </p> 196<p> 197</p> 198<pre class="programlisting">boost::asio::any_io_executor poly_executor = executor; 199</pre> 200<p> 201 </p> 202<h4> 203<a name="boost_asio.std_executors.h4"></a> 204 <span class="phrase"><a name="boost_asio.std_executors.traits_for_deducing_conformance_to_the_executor_concept"></a></span><a class="link" href="std_executors.html#boost_asio.std_executors.traits_for_deducing_conformance_to_the_executor_concept">Traits 205 for Deducing Conformance to the Executor Concept</a> 206 </h4> 207<p> 208 Older C++ standards and compilers require some assistance to determine whether 209 an executor implementation conforms to the <code class="computeroutput">executor</code> concept and 210 type requirements. This is achieved through specialisation of traits. The following 211 code shows a specialisation of these traits for the <code class="computeroutput">minimal_io_executor</code> 212 example from above: 213 </p> 214<p> 215</p> 216<pre class="programlisting">namespace boost { namespace asio { 217namespace traits { 218 219#if !defined(BOOST_ASIO_HAS_DEDUCED_EXECUTE_MEMBER_TRAIT) 220 221template <typename F> 222struct execute_member<minimal_io_executor, F> 223{ 224 static constexpr bool is_valid = true; 225 static constexpr bool is_noexcept = true; 226 typedef void result_type; 227}; 228 229#endif // !defined(BOOST_ASIO_HAS_DEDUCED_EXECUTE_MEMBER_TRAIT) 230#if !defined(BOOST_ASIO_HAS_DEDUCED_EQUALITY_COMPARABLE_TRAIT) 231 232template <> 233struct equality_comparable<minimal_io_executor> 234{ 235 static constexpr bool is_valid = true; 236 static constexpr bool is_noexcept = true; 237}; 238 239#endif // !defined(BOOST_ASIO_HAS_DEDUCED_EQUALITY_COMPARABLE_TRAIT) 240#if !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_MEMBER_TRAIT) 241 242template <> 243struct query_member<minimal_io_executor, 244 boost::asio::execution::context_t> 245{ 246 static constexpr bool is_valid = true; 247 static constexpr bool is_noexcept = true; 248 typedef boost::asio::execution_context& result_type; 249}; 250 251#endif // !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_MEMBER_TRAIT) 252#if !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT) 253 254template <typename Property> 255struct query_static_constexpr_member<minimal_io_executor, Property, 256 typename enable_if< 257 std::is_convertible<Property, boost::asio::execution::blocking_t>::value 258 >::type> 259{ 260 static constexpr bool is_valid = true; 261 static constexpr bool is_noexcept = true; 262 typedef boost::asio::execution::blocking_t::never_t result_type; 263 static constexpr result_type value() noexcept { return result_type(); } 264}; 265 266#endif // !defined(BOOST_ASIO_HAS_DEDUCED_QUERY_STATIC_CONSTEXPR_MEMBER_TRAIT) 267 268} // namespace traits 269} } // namespace boost::asio 270</pre> 271<p> 272 </p> 273<p> 274 Boost.Asio uses an extensive set of traits to implement all of the proposed 275 standard executor functionality on older C++ standards. These traits may be 276 found under the <code class="literal">boost/asio/traits</code> include directory. 277 </p> 278</div> 279<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 280<td align="left"></td> 281<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 282 Kohlhoff<p> 283 Distributed under the Boost Software License, Version 1.0. (See accompanying 284 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>) 285 </p> 286</div></td> 287</tr></table> 288<hr> 289<div class="spirit-nav"> 290<a accesskey="p" href="net_ts.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../boost_asio.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../boost_asio.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="history.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 291</div> 292</body> 293</html> 294
