How can I POST data to a url using QNetworkAccessManager

I used:

QByteArray postData;
postData.append("param1=string&");
postData.append("param2=string");

So & instead of newline after each parameter.

Updating alexisdm answer to Qt5:

// Setup the webservice url
QUrl serviceUrl = QUrl("http://your.url");
QByteArray postData;

QUrlQuery query;
query.addQueryItem("param1","string1");
query.addQueryItem("param2","string2");

postData = query.toString(QUrl::FullyEncoded).toUtf8();

// Call the webservice
QNetworkAccessManager *networkManager = new QNetworkAccessManager(this);
connect(networkManager, SIGNAL(finished(QNetworkReply*)),
        SLOT(onPostAnswer(QNetworkReply*)));

QNetworkRequest networkRequest(serviceUrl);
networkRequest.setHeader(QNetworkRequest::ContentTypeHeader,"application/x-www-form-urlencoded");

networkManager->post(networkRequest,postData);

Don't forget to include

QT += network

in .pro.

Since some parameters and values might need to be utf-8 and percent encoded (spaces, &, =, special chars...), you should rather use QUrl (for Qt 4) or QUrlQuery (for Qt 5) to build the posted string.

Example code for Qt 4:

QUrl postData;
postData.addQueryItem("param1", "string");
postData.addQueryItem("param2", "string");
...
QNetworkRequest request(serviceUrl);    
request.setHeader(QNetworkRequest::ContentTypeHeader, 
    "application/x-www-form-urlencoded");
networkManager->post(request, postData.encodedQuery());

and for Qt 5:

QUrlQuery postData;
postData.addQueryItem("param1", "string");
postData.addQueryItem("param2", "string");
...
QNetworkRequest request(serviceUrl);
request.setHeader(QNetworkRequest::ContentTypeHeader, 
    "application/x-www-form-urlencoded");
networkManager->post(request, postData.toString(QUrl::FullyEncoded).toUtf8());

Starting with Qt 4.8 you can also use QHttpMultiPart if you need to upload files.