Even if SSL is enabled when configuring Qt for compilation in Linux, it is not guaranteed that SSL support is enabled in Qt, if for example libssl-dev package on Ubuntu/Debian is missing while compiling Qt. Now it is impossible to discover this issue when using QNetworkAccessManager. For example:

QNetworkAccessManager *networkManager = new QNetworkAccessManager(this);
QNetworkRequest networkRequest;
networkRequest.setUrl( QUrl("https://api.twitter.com/oauth/authorize") );
networkRequest.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlencoded");

...
...
connect(networkManager, SIGNAL(finished(QNetworkReply *)),
this, SLOT(onRequestReplyReceived(QNetworkReply*) ));
QNetworkReply *reply = networkManager->post(networkRequest, QByteArray("Some body"));

signal "finished()" is never emitted and reply->error() gives NoError. Also Wireshark shows that the packets are never submitted through the ethernet controller, so QNetworkAccessManager cannot tell me that everything is fine.