Refactor WebSocket, HTTP examples:

fix #297, fix #488

* Errors are checked and reported
* More comments explaining what is going on
* The connection is gracefully closed

WebSocket:

* Messages are drained before closing

example/http-client/http_client.cpp

@@ -11,35 +11,75 @@
 #include <beast/http.hpp>
 #include <boost/asio.hpp>
 #include <boost/lexical_cast.hpp>
+#include <cstdlib>
 #include <iostream>
 #include <string>
 
 int main()
 {
-    // Normal boost::asio setup
-    std::string const host = "www.example.com";
+    // A helper for reporting errors
+    auto const fail =
+        [](std::string what, beast::error_code ec)
+        {
+            std::cerr << what << ": " << ec.message() << std::endl;
+            std::cerr.flush();
+            return EXIT_FAILURE;
+        };
+
+    boost::system::error_code ec;
+
+    // Set up an asio socket
     boost::asio::io_service ios;
     boost::asio::ip::tcp::resolver r{ios};
     boost::asio::ip::tcp::socket sock{ios};
-    boost::asio::connect(sock,
-        r.resolve(boost::asio::ip::tcp::resolver::query{host, "http"}));
 
-    // Send HTTP request using beast
+    // Look up the domain name
+    std::string const host = "www.example.com";
+    auto const lookup = r.resolve(boost::asio::ip::tcp::resolver::query{host, "http"}, ec);
+    if(ec)
+        return fail("resolve", ec);
+
+    // Make the connection on the IP address we get from a lookup
+    boost::asio::connect(sock, lookup, ec);
+    if(ec)
+        return fail("connect", ec);
+
+    // Set up an HTTP GET request message
     beast::http::request<beast::http::string_body> req;
     req.method(beast::http::verb::get);
     req.target("/");
     req.version = 11;
-    req.insert(beast::http::field::host, host + ":" +
+    req.set(beast::http::field::host, host + ":" +
         boost::lexical_cast<std::string>(sock.remote_endpoint().port()));
-    req.insert(beast::http::field::user_agent, "Beast");
+    req.set(beast::http::field::user_agent, "Beast");
     req.prepare();
-    beast::http::write(sock, req);
 
-    // Receive and print HTTP response using beast
+    // Write the HTTP request to the remote host
+    beast::http::write(sock, req, ec);
+    if(ec)
+        return fail("write", ec);
+
+    // This buffer is used for reading and must be persisted
     beast::flat_buffer b;
+
+    // Declare a container to hold the response
     beast::http::response<beast::http::dynamic_body> res;
-    beast::http::read(sock, b, res);
+
+    // Read the response
+    beast::http::read(sock, b, res, ec);
+    if(ec)
+        return fail("read", ec);
+
+    // Write the message to standard out
     std::cout << res << std::endl;
+
+    // Gracefully close the socket
+    sock.shutdown(boost::asio::ip::tcp::socket::shutdown_both, ec);
+    if(ec)
+        return fail("shutdown", ec);
+
+    // If we get here then the connection is closed gracefully
+    return EXIT_SUCCESS;
 }
 
 //]