summaryrefslogtreecommitdiffstatshomepage
path: root/3rdparty/asio/src/examples/cpp20/operations/composed_2.cpp
blob: ada029d38f0ddd5a5e1800612e9812c60319dfc1 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
//
// composed_2.cpp
// ~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2024 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#include <asio/deferred.hpp>
#include <asio/io_context.hpp>
#include <asio/ip/tcp.hpp>
#include <asio/use_future.hpp>
#include <asio/write.hpp>
#include <cstring>
#include <iostream>
#include <string>
#include <type_traits>
#include <utility>

using asio::ip::tcp;

// NOTE: This example requires the new asio::async_initiate function. For
// an example that works with the Networking TS style of completion tokens,
// please see an older version of asio.

//------------------------------------------------------------------------------

// This next simplest example of a composed asynchronous operation involves
// repackaging multiple operations but choosing to invoke just one of them. All
// of these underlying operations have the same completion signature. The
// asynchronous operation requirements are met by delegating responsibility to
// the underlying operations.

template <
  asio::completion_token_for<void(std::error_code, std::size_t)>
    CompletionToken>
auto async_write_message(tcp::socket& socket,
    const char* message, bool allow_partial_write,
    CompletionToken&& token)
  // The return type of the initiating function is deduced from the combination
  // of:
  //
  // - the CompletionToken type,
  // - the completion handler signature, and
  // - the asynchronous operation's initiation function object.
  //
  // When the completion token is a simple callback, the return type is void.
  // However, when the completion token is asio::yield_context (used for
  // stackful coroutines) the return type would be std::size_t, and when the
  // completion token is asio::use_future it would be std::future<std::size_t>.
  // When the completion token is asio::deferred, the return type differs for
  // each asynchronous operation.
  //
  // In C++20 we can omit the return type as it is automatically deduced from
  // the return type of asio::async_initiate.
{
  // In addition to determining the mechanism by which an asynchronous
  // operation delivers its result, a completion token also determines the time
  // when the operation commences. For example, when the completion token is a
  // simple callback the operation commences before the initiating function
  // returns. However, if the completion token's delivery mechanism uses a
  // future, we might instead want to defer initiation of the operation until
  // the returned future object is waited upon.
  //
  // To enable this, when implementing an asynchronous operation we must
  // package the initiation step as a function object. The initiation function
  // object's call operator is passed the concrete completion handler produced
  // by the completion token. This completion handler matches the asynchronous
  // operation's completion handler signature, which in this example is:
  //
  //   void(std::error_code error, std::size_t)
  //
  // The initiation function object also receives any additional arguments
  // required to start the operation. (Note: We could have instead passed these
  // arguments in the lambda capture set. However, we should prefer to
  // propagate them as function call arguments as this allows the completion
  // token to optimise how they are passed. For example, a lazy future which
  // defers initiation would need to make a decay-copy of the arguments, but
  // when using a simple callback the arguments can be trivially forwarded
  // straight through.)
  auto initiation = [](
      asio::completion_handler_for<void(std::error_code, std::size_t)>
        auto&& completion_handler,
      tcp::socket& socket,
      const char* message,
      bool allow_partial_write)
  {
    if (allow_partial_write)
    {
      // When delegating to an underlying operation we must take care to
      // perfectly forward the completion handler. This ensures that our
      // operation works correctly with move-only function objects as
      // callbacks.
      return socket.async_write_some(
          asio::buffer(message, std::strlen(message)),
          std::forward<decltype(completion_handler)>(completion_handler));
    }
    else
    {
      // As above, we must perfectly forward the completion handler when calling
      // the alternate underlying operation.
      return asio::async_write(socket,
          asio::buffer(message, std::strlen(message)),
          std::forward<decltype(completion_handler)>(completion_handler));
    }
  };

  // The asio::async_initiate function takes:
  //
  // - our initiation function object,
  // - the completion token,
  // - the completion handler signature, and
  // - any additional arguments we need to initiate the operation.
  //
  // It then asks the completion token to create a completion handler (i.e. a
  // callback) with the specified signature, and invoke the initiation function
  // object with this completion handler as well as the additional arguments.
  // The return value of async_initiate is the result of our operation's
  // initiating function.
  //
  // Note that we wrap non-const reference arguments in std::reference_wrapper
  // to prevent incorrect decay-copies of these objects.
  return asio::async_initiate<
    CompletionToken, void(std::error_code, std::size_t)>(
      initiation, token, std::ref(socket), message, allow_partial_write);
}

//------------------------------------------------------------------------------

void test_callback()
{
  asio::io_context io_context;

  tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
  tcp::socket socket = acceptor.accept();

  // Test our asynchronous operation using a lambda as a callback.
  async_write_message(socket, "Testing callback\r\n", false,
      [](const std::error_code& error, std::size_t n)
      {
        if (!error)
        {
          std::cout << n << " bytes transferred\n";
        }
        else
        {
          std::cout << "Error: " << error.message() << "\n";
        }
      });

  io_context.run();
}

//------------------------------------------------------------------------------

void test_deferred()
{
  asio::io_context io_context;

  tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
  tcp::socket socket = acceptor.accept();

  // Test our asynchronous operation using the deferred completion token. This
  // token causes the operation's initiating function to package up the
  // operation with its arguments to return a function object, which may then be
  // used to launch the asynchronous operation.
  asio::async_operation auto op = async_write_message(
      socket, "Testing deferred\r\n", false, asio::deferred);

  // Launch the operation using a lambda as a callback.
  std::move(op)(
      [](const std::error_code& error, std::size_t n)
      {
        if (!error)
        {
          std::cout << n << " bytes transferred\n";
        }
        else
        {
          std::cout << "Error: " << error.message() << "\n";
        }
      });

  io_context.run();
}

//------------------------------------------------------------------------------

void test_future()
{
  asio::io_context io_context;

  tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
  tcp::socket socket = acceptor.accept();

  // Test our asynchronous operation using the use_future completion token.
  // This token causes the operation's initiating function to return a future,
  // which may be used to synchronously wait for the result of the operation.
  std::future<std::size_t> f = async_write_message(
      socket, "Testing future\r\n", false, asio::use_future);

  io_context.run();

  try
  {
    // Get the result of the operation.
    std::size_t n = f.get();
    std::cout << n << " bytes transferred\n";
  }
  catch (const std::exception& e)
  {
    std::cout << "Error: " << e.what() << "\n";
  }
}

//------------------------------------------------------------------------------

int main()
{
  test_callback();
  test_deferred();
  test_future();
}