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/websocket-client/websocket_client.cpp

@@ -10,29 +10,89 @@
 #include <beast/core.hpp>
 #include <beast/websocket.hpp>
 #include <boost/asio.hpp>
+#include <cstdlib>
 #include <iostream>
 #include <string>
 
 int main()
 {
-    // Normal boost::asio setup
-    std::string const host = "echo.websocket.org";
+    // 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, "80"}));
 
-    // WebSocket connect and send message using beast
+    // Look up the domain name
+    std::string const host = "echo.websocket.org";
+    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);
+
+    // Wrap the now-connected socket in a websocket stream
     beast::websocket::stream<boost::asio::ip::tcp::socket&> ws{sock};
-    ws.handshake(host, "/");
-    ws.write(boost::asio::buffer(std::string("Hello, world!")));
 
-    // Receive WebSocket message, print and close using beast
+    // Perform the websocket handhskae
+    ws.handshake(host, "/", ec);
+    if(ec)
+        return fail("handshake", ec);
+
+    // Send a message
+    ws.write(boost::asio::buffer(std::string("Hello, world!")), ec);
+    if(ec)
+        return fail("write", ec);
+
+    // This buffer will hold the incoming message
     beast::multi_buffer b;
-    ws.read(b);
-    ws.close(beast::websocket::close_code::normal);
-    std::cout << beast::buffers(b.data()) << "\n";
+
+    // Read the message into our buffer
+    ws.read(b, ec);
+    if(ec)
+        return fail("read", ec);
+
+    // Send a "close" frame to the other end, this is a websocket thing
+    ws.close(beast::websocket::close_code::normal, ec);
+    if(ec)
+        return fail("close", ec);
+
+    // The buffers() function helps print a ConstBufferSequence
+    std::cout << beast::buffers(b.data()) << std::endl;
+
+    // WebSocket says that to close a connection you have
+    // to keep reading messages until you receive a close frame.
+    // Beast delivers the close frame as an error from read.
+    //
+    beast::drain_buffer drain; // Throws everything away efficiently
+    for(;;)
+    {
+        // Keep reading messages...
+        ws.read(drain, ec);
+
+        // ...until we get the special error code
+        if(ec == beast::websocket::error::closed)
+            break;
+
+        // Some other error occurred, report it and exit.
+        if(ec)
+            return fail("close", ec);
+    }
+
+    // If we get here the connection was cleanly closed
+    return EXIT_SUCCESS;
 }
 
 //]