1 //
2 // composed_6.cpp
3 // ~~~~~~~~~~~~~~
4 //
5 // Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
6 //
7 // Distributed under the Boost Software License, Version 1.0. (See accompanying
8 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
9 //
10
11 #include <boost/asio/executor_work_guard.hpp>
12 #include <boost/asio/io_context.hpp>
13 #include <boost/asio/ip/tcp.hpp>
14 #include <boost/asio/steady_timer.hpp>
15 #include <boost/asio/use_future.hpp>
16 #include <boost/asio/write.hpp>
17 #include <functional>
18 #include <iostream>
19 #include <memory>
20 #include <sstream>
21 #include <string>
22 #include <type_traits>
23 #include <utility>
24
25 using boost::asio::ip::tcp;
26
27 // NOTE: This example requires the new boost::asio::async_initiate function. For
28 // an example that works with the Networking TS style of completion tokens,
29 // please see an older version of asio.
30
31 //------------------------------------------------------------------------------
32
33 // This composed operation shows composition of multiple underlying operations.
34 // It automatically serialises a message, using its I/O streams insertion
35 // operator, before sending it N times on the socket. To do this, it must
36 // allocate a buffer for the encoded message and ensure this buffer's validity
37 // until all underlying async_write operation complete. A one second delay is
38 // inserted prior to each write operation, using a steady_timer.
39
40 // In addition to determining the mechanism by which an asynchronous operation
41 // delivers its result, a completion token also determines the time when the
42 // operation commences. For example, when the completion token is a simple
43 // callback the operation commences before the initiating function returns.
44 // However, if the completion token's delivery mechanism uses a future, we
45 // might instead want to defer initiation of the operation until the returned
46 // future object is waited upon.
47 //
48 // To enable this, when implementing an asynchronous operation we must package
49 // the initiation step as a function object.
50 struct async_write_message_initiation
51 {
52 // The initiation function object's call operator is passed the concrete
53 // completion handler produced by the completion token. This completion
54 // handler matches the asynchronous operation's completion handler signature,
55 // which in this example is:
56 //
57 // void(boost::system::error_code error)
58 //
59 // The initiation function object also receives any additional arguments
60 // required to start the operation. (Note: We could have instead passed these
61 // arguments as members in the initiaton function object. However, we should
62 // prefer to propagate them as function call arguments as this allows the
63 // completion token to optimise how they are passed. For example, a lazy
64 // future which defers initiation would need to make a decay-copy of the
65 // arguments, but when using a simple callback the arguments can be trivially
66 // forwarded straight through.)
67 template <typename CompletionHandler>
operator ()async_write_message_initiation68 void operator()(CompletionHandler&& completion_handler, tcp::socket& socket,
69 std::unique_ptr<std::string> encoded_message, std::size_t repeat_count,
70 std::unique_ptr<boost::asio::steady_timer> delay_timer) const
71 {
72 // In this example, the composed operation's intermediate completion
73 // handler is implemented as a hand-crafted function object.
74 struct intermediate_completion_handler
75 {
76 // The intermediate completion handler holds a reference to the socket as
77 // it is used for multiple async_write operations, as well as for
78 // obtaining the I/O executor (see get_executor below).
79 tcp::socket& socket_;
80
81 // The allocated buffer for the encoded message. The std::unique_ptr
82 // smart pointer is move-only, and as a consequence our intermediate
83 // completion handler is also move-only.
84 std::unique_ptr<std::string> encoded_message_;
85
86 // The repeat count remaining.
87 std::size_t repeat_count_;
88
89 // A steady timer used for introducing a delay.
90 std::unique_ptr<boost::asio::steady_timer> delay_timer_;
91
92 // To manage the cycle between the multiple underlying asychronous
93 // operations, our intermediate completion handler is implemented as a
94 // state machine.
95 enum { starting, waiting, writing } state_;
96
97 // As our composed operation performs multiple underlying I/O operations,
98 // we should maintain a work object against the I/O executor. This tells
99 // the I/O executor that there is still more work to come in the future.
100 boost::asio::executor_work_guard<tcp::socket::executor_type> io_work_;
101
102 // The user-supplied completion handler, called once only on completion
103 // of the entire composed operation.
104 typename std::decay<CompletionHandler>::type handler_;
105
106 // By having a default value for the second argument, this function call
107 // operator matches the completion signature of both the async_write and
108 // steady_timer::async_wait operations.
109 void operator()(const boost::system::error_code& error, std::size_t = 0)
110 {
111 if (!error)
112 {
113 switch (state_)
114 {
115 case starting:
116 case writing:
117 if (repeat_count_ > 0)
118 {
119 --repeat_count_;
120 state_ = waiting;
121 delay_timer_->expires_after(std::chrono::seconds(1));
122 delay_timer_->async_wait(std::move(*this));
123 return; // Composed operation not yet complete.
124 }
125 break; // Composed operation complete, continue below.
126 case waiting:
127 state_ = writing;
128 boost::asio::async_write(socket_,
129 boost::asio::buffer(*encoded_message_), std::move(*this));
130 return; // Composed operation not yet complete.
131 }
132 }
133
134 // This point is reached only on completion of the entire composed
135 // operation.
136
137 // We no longer have any future work coming for the I/O executor.
138 io_work_.reset();
139
140 // Deallocate the encoded message before calling the user-supplied
141 // completion handler.
142 encoded_message_.reset();
143
144 // Call the user-supplied handler with the result of the operation.
145 handler_(error);
146 }
147
148 // It is essential to the correctness of our composed operation that we
149 // preserve the executor of the user-supplied completion handler. With a
150 // hand-crafted function object we can do this by defining a nested type
151 // executor_type and member function get_executor. These obtain the
152 // completion handler's associated executor, and default to the I/O
153 // executor - in this case the executor of the socket - if the completion
154 // handler does not have its own.
155 using executor_type = boost::asio::associated_executor_t<
156 typename std::decay<CompletionHandler>::type,
157 tcp::socket::executor_type>;
158
159 executor_type get_executor() const noexcept
160 {
161 return boost::asio::get_associated_executor(
162 handler_, socket_.get_executor());
163 }
164
165 // Although not necessary for correctness, we may also preserve the
166 // allocator of the user-supplied completion handler. This is achieved by
167 // defining a nested type allocator_type and member function
168 // get_allocator. These obtain the completion handler's associated
169 // allocator, and default to std::allocator<void> if the completion
170 // handler does not have its own.
171 using allocator_type = boost::asio::associated_allocator_t<
172 typename std::decay<CompletionHandler>::type,
173 std::allocator<void>>;
174
175 allocator_type get_allocator() const noexcept
176 {
177 return boost::asio::get_associated_allocator(
178 handler_, std::allocator<void>{});
179 }
180 };
181
182 // Initiate the underlying async_write operation using our intermediate
183 // completion handler.
184 auto encoded_message_buffer = boost::asio::buffer(*encoded_message);
185 boost::asio::async_write(socket, encoded_message_buffer,
186 intermediate_completion_handler{
187 socket, std::move(encoded_message),
188 repeat_count, std::move(delay_timer),
189 intermediate_completion_handler::starting,
190 boost::asio::make_work_guard(socket.get_executor()),
191 std::forward<CompletionHandler>(completion_handler)});
192 }
193 };
194
195 template <typename T, typename CompletionToken>
async_write_messages(tcp::socket & socket,const T & message,std::size_t repeat_count,CompletionToken && token)196 auto async_write_messages(tcp::socket& socket,
197 const T& message, std::size_t repeat_count,
198 CompletionToken&& token)
199 // The return type of the initiating function is deduced from the combination
200 // of CompletionToken type and the completion handler's signature. When the
201 // completion token is a simple callback, the return type is always void.
202 // In this example, when the completion token is boost::asio::yield_context
203 // (used for stackful coroutines) the return type would be also be void, as
204 // there is no non-error argument to the completion handler. When the
205 // completion token is boost::asio::use_future it would be std::future<void>.
206 -> typename boost::asio::async_result<
207 typename std::decay<CompletionToken>::type,
208 void(boost::system::error_code)>::return_type
209 {
210 // Encode the message and copy it into an allocated buffer. The buffer will
211 // be maintained for the lifetime of the composed asynchronous operation.
212 std::ostringstream os;
213 os << message;
214 std::unique_ptr<std::string> encoded_message(new std::string(os.str()));
215
216 // Create a steady_timer to be used for the delay between messages.
217 std::unique_ptr<boost::asio::steady_timer> delay_timer(
218 new boost::asio::steady_timer(socket.get_executor()));
219
220 // The boost::asio::async_initiate function takes:
221 //
222 // - our initiation function object,
223 // - the completion token,
224 // - the completion handler signature, and
225 // - any additional arguments we need to initiate the operation.
226 //
227 // It then asks the completion token to create a completion handler (i.e. a
228 // callback) with the specified signature, and invoke the initiation function
229 // object with this completion handler as well as the additional arguments.
230 // The return value of async_initiate is the result of our operation's
231 // initiating function.
232 //
233 // Note that we wrap non-const reference arguments in std::reference_wrapper
234 // to prevent incorrect decay-copies of these objects.
235 return boost::asio::async_initiate<
236 CompletionToken, void(boost::system::error_code)>(
237 async_write_message_initiation(), token, std::ref(socket),
238 std::move(encoded_message), repeat_count, std::move(delay_timer));
239 }
240
241 //------------------------------------------------------------------------------
242
test_callback()243 void test_callback()
244 {
245 boost::asio::io_context io_context;
246
247 tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
248 tcp::socket socket = acceptor.accept();
249
250 // Test our asynchronous operation using a lambda as a callback.
251 async_write_messages(socket, "Testing callback\r\n", 5,
252 [](const boost::system::error_code& error)
253 {
254 if (!error)
255 {
256 std::cout << "Messages sent\n";
257 }
258 else
259 {
260 std::cout << "Error: " << error.message() << "\n";
261 }
262 });
263
264 io_context.run();
265 }
266
267 //------------------------------------------------------------------------------
268
test_future()269 void test_future()
270 {
271 boost::asio::io_context io_context;
272
273 tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
274 tcp::socket socket = acceptor.accept();
275
276 // Test our asynchronous operation using the use_future completion token.
277 // This token causes the operation's initiating function to return a future,
278 // which may be used to synchronously wait for the result of the operation.
279 std::future<void> f = async_write_messages(
280 socket, "Testing future\r\n", 5, boost::asio::use_future);
281
282 io_context.run();
283
284 try
285 {
286 // Get the result of the operation.
287 f.get();
288 std::cout << "Messages sent\n";
289 }
290 catch (const std::exception& e)
291 {
292 std::cout << "Error: " << e.what() << "\n";
293 }
294 }
295
296 //------------------------------------------------------------------------------
297
main()298 int main()
299 {
300 test_callback();
301 test_future();
302 }
303