1 //
2 // composed_2.cpp
3 // ~~~~~~~~~~~~~~
4 //
5 // Copyright (c) 2003-2021 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/io_context.hpp>
12 #include <boost/asio/ip/tcp.hpp>
13 #include <boost/asio/use_future.hpp>
14 #include <boost/asio/write.hpp>
15 #include <cstring>
16 #include <iostream>
17 #include <string>
18 #include <type_traits>
19 #include <utility>
20
21 using boost::asio::ip::tcp;
22
23 //------------------------------------------------------------------------------
24
25 // This next simplest example of a composed asynchronous operation involves
26 // repackaging multiple operations but choosing to invoke just one of them. All
27 // of these underlying operations have the same completion signature. The
28 // asynchronous operation requirements are met by delegating responsibility to
29 // the underlying operations.
30
31 template <typename CompletionToken>
async_write_message(tcp::socket & socket,const char * message,bool allow_partial_write,CompletionToken && token)32 auto async_write_message(tcp::socket& socket,
33 const char* message, bool allow_partial_write,
34 CompletionToken&& token)
35 // The return type of the initiating function is deduced from the combination
36 // of CompletionToken type and the completion handler's signature. When the
37 // completion token is a simple callback, the return type is void. However,
38 // when the completion token is boost::asio::yield_context (used for stackful
39 // coroutines) the return type would be std::size_t, and when the completion
40 // token is boost::asio::use_future it would be std::future<std::size_t>.
41 -> typename boost::asio::async_result<
42 typename std::decay<CompletionToken>::type,
43 void(boost::system::error_code, std::size_t)>::return_type
44 {
45 // As the return type of the initiating function is deduced solely from the
46 // CompletionToken and completion signature, we know that two different
47 // asynchronous operations having the same completion signature will produce
48 // the same return type, when passed the same CompletionToken. This allows us
49 // to trivially delegate to alternate implementations.
50 if (allow_partial_write)
51 {
52 // When delegating to an underlying operation we must take care to
53 // perfectly forward the completion token. This ensures that our operation
54 // works correctly with move-only function objects as callbacks, as well as
55 // other completion token types.
56 return socket.async_write_some(
57 boost::asio::buffer(message, std::strlen(message)),
58 std::forward<CompletionToken>(token));
59 }
60 else
61 {
62 // As above, we must perfectly forward the completion token when calling
63 // the alternate underlying operation.
64 return boost::asio::async_write(socket,
65 boost::asio::buffer(message, std::strlen(message)),
66 std::forward<CompletionToken>(token));
67 }
68 }
69
70 //------------------------------------------------------------------------------
71
test_callback()72 void test_callback()
73 {
74 boost::asio::io_context io_context;
75
76 tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
77 tcp::socket socket = acceptor.accept();
78
79 // Test our asynchronous operation using a lambda as a callback.
80 async_write_message(socket, "Testing callback\r\n", false,
81 [](const boost::system::error_code& error, std::size_t n)
82 {
83 if (!error)
84 {
85 std::cout << n << " bytes transferred\n";
86 }
87 else
88 {
89 std::cout << "Error: " << error.message() << "\n";
90 }
91 });
92
93 io_context.run();
94 }
95
96 //------------------------------------------------------------------------------
97
test_future()98 void test_future()
99 {
100 boost::asio::io_context io_context;
101
102 tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
103 tcp::socket socket = acceptor.accept();
104
105 // Test our asynchronous operation using the use_future completion token.
106 // This token causes the operation's initiating function to return a future,
107 // which may be used to synchronously wait for the result of the operation.
108 std::future<std::size_t> f = async_write_message(
109 socket, "Testing future\r\n", false, boost::asio::use_future);
110
111 io_context.run();
112
113 try
114 {
115 // Get the result of the operation.
116 std::size_t n = f.get();
117 std::cout << n << " bytes transferred\n";
118 }
119 catch (const std::exception& e)
120 {
121 std::cout << "Error: " << e.what() << "\n";
122 }
123 }
124
125 //------------------------------------------------------------------------------
126
main()127 int main()
128 {
129 test_callback();
130 test_future();
131 }
132