Add asio standalone example (#133)

* Add asio standalone example

* Mark VERSION explicitly

Without this version information, CPM's versioning mechanism may break
in some cases.

Also add comments for compile definition on Windows.

* Use aync-tcp-echo-server as the example

* Add boost software license to the demo source

Co-authored-by: Lars Melchior <TheLartians@users.noreply.github.com>

Co-authored-by: Lars Melchior <TheLartians@users.noreply.github.com>

examples/asio-standalone/main.cpp

@@ -0,0 +1,118 @@
+//
+// async_tcp_echo_server.cpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2020 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <cstdlib>
+#include <iostream>
+#include <memory>
+#include <utility>
+
+#include "asio.hpp"
+
+// An asynchronous tcp echo server.
+// See https://think-async.com/Asio/asio-1.16.1/src/examples/cpp11/echo/async_tcp_echo_server.cpp
+
+using asio::ip::tcp;
+
+class session
+  : public std::enable_shared_from_this<session>
+{
+public:
+  session(tcp::socket socket)
+    : socket_(std::move(socket))
+  {
+  }
+
+  void start()
+  {
+    do_read();
+  }
+
+private:
+  void do_read()
+  {
+    auto self(shared_from_this());
+    socket_.async_read_some(asio::buffer(data_, max_length),
+        [this, self](std::error_code ec, std::size_t length)
+        {
+          if (!ec)
+          {
+            do_write(length);
+          }
+        });
+  }
+
+  void do_write(std::size_t length)
+  {
+    auto self(shared_from_this());
+    asio::async_write(socket_, asio::buffer(data_, length),
+        [this, self](std::error_code ec, std::size_t /*length*/)
+        {
+          if (!ec)
+          {
+            do_read();
+          }
+        });
+  }
+
+  tcp::socket socket_;
+  enum { max_length = 1024 };
+  char data_[max_length];
+};
+
+class server
+{
+public:
+  server(asio::io_context& io_context, short port)
+    : acceptor_(io_context, tcp::endpoint(tcp::v4(), port))
+  {
+    do_accept();
+  }
+
+private:
+  void do_accept()
+  {
+    acceptor_.async_accept(
+        [this](std::error_code ec, tcp::socket socket)
+        {
+          if (!ec)
+          {
+            std::make_shared<session>(std::move(socket))->start();
+          }
+
+          do_accept();
+        });
+  }
+
+  tcp::acceptor acceptor_;
+};
+
+int main(int argc, char* argv[])
+{
+  try
+  {
+    if (argc != 2)
+    {
+      std::cerr << "Usage: async_tcp_echo_server <port>\n";
+      return 1;
+    }
+
+    asio::io_context io_context;
+
+    server s(io_context, std::atoi(argv[1]));
+
+    io_context.run();
+  }
+  catch (std::exception& e)
+  {
+    std::cerr << "Exception: " << e.what() << "\n";
+  }
+
+  return 0;
+}