How to properly use QWebSocket::sendBinaryMessage() method from a C++ client?

Question

There are C++ Qt client & server. Following code works fine and the connection happens between the client and the server:

QWebSocket webSocket;  // defined somewhere
...
QUrl url;
url.setScheme("ws");  // SSL encryption disabled
url.setHost(serverName);  // "127.0.0.1" (can be "www.abc.com" too)
url.setPort(portNumber);  // 2000
webSocket.open(url);  // connects with the server properly
PRINT(url.toString());  // output: "ws://127.0.0.1:2000"

While sending the binary data, the function returns 0 instead of the number of bytes:

// though the message.size() is 80 bytes; the method returns 0
webSocket.sendBinaryMessage(QByteArray(message.data(), message.size()));

Note that, the QWebSocketServer works as expected.
We also have a Javascript client. That connects & sends the binary message properly. The only addition in that client is below:

webSocketJS.binaryType = "arraybuffer";  // <--- Javascript code

But such provision is not found in QWebSocket or I may have missed it.

Question: How to correctly send the binary data over the web connection?

---

For those interested, the server [pseudo] code is like below:

auto pWebSocket = WebServer.nextPendingConnection();
QObject::connect(pWebSocket, &QWebSocket::binaryMessageReceived,
                 [&] (const QByteArray& message) { DataRead(message, rManager); });  // This slot is not called as of now

Answer 1

It seems that there is no mention of how the QWebSocket::connected() signal is treated.
Due to internet delay and initial handshakes, the WebSocketServer may take some time to establish a connection. Ideally the binary/text message should be sent only after the connected() is received.

Before making a connection using webSocket.open(url), you should be handling this signal:

... // same code
   QObject::connect(&webSocket, &QWebSocket::connected,
                    [&] () 
                    {
                      webSocket.sendBinaryMessage(QByteArray(message.data(), message.size()));
                      // ... set some internal state suggesting the established connection
                    }
   webSocket.open(url);

Above is just a pseudo code to show that the first sendBinaryMessage() should be sent after the connect() signal. Ideally in real world code, you may want to set some state, which informs the client that the connection is established.

Similarly as mentioned in the comments, we should be checking for errors and disconnections as well.