QFile file("file.dat");
  file.open(QIODevice::WriteOnly);
  QDataStream out(&file);   // we will serialize the data into the file
  out << QString("the answer is");   // serialize a string
  out << (qint32)42;        // serialize an integer

  QFile file("file.dat");
  file.open(QIODevice::ReadOnly);
  QDataStream in(&file);    // read the data serialized from the file
  QString str;
  qint32 a;
  in >> str >> a;           // extract "the answer is" and 42

stream.setVersion(QDataStream::Qt_4_0);

  QFile file("file.xxx");
  file.open(QIODevice::WriteOnly);
  QDataStream out(&file);
 
  // Write a header with a "magic number" and a version
  out << (quint32)0xA0B0C0D0;
  out << (qint32)123;
 
  out.setVersion(QDataStream::Qt_4_0);
 
  // Write the data
  out << lots_of_interesting_data;

  QFile file("file.xxx");
  file.open(QIODevice::ReadOnly);
  QDataStream in(&file);
 
  // Read and check the header
  quint32 magic;
  in >> magic;
  if (magic != 0xA0B0C0D0)
      return XXX_BAD_FILE_FORMAT;
 
  // Read the version
  qint32 version;
  in >> version;
  if (version < 100)
      return XXX_BAD_FILE_TOO_OLD;
  if (version > 123)
      return XXX_BAD_FILE_TOO_NEW;
 
  if (version <= 110)
      in.setVersion(QDataStream::Qt_3_2);
  else
      in.setVersion(QDataStream::Qt_4_0);
 
  // Read the data
  in >> lots_of_interesting_data;
  if (version >= 120)
      in >> data_new_in_XXX_version_1_2;
  in >> other_interesting_data;

  in.startTransaction();
  QString str;
  qint32 a;
  in >> str >> a; // try to read packet atomically
 
  if (!in.commitTransaction())
      return;     // wait for more data

#include <QCoreApplication>
#include <QDebug>
#include <QDataStream>
#include <QFile>
#include <QVector>
#include <QMap>
 
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
 
    //write
    QFile file("test.dat");
    if (!file.open(QIODevice::ReadWrite))
    {
        qDebug() << "open file failed";
        return 0;
    }
    QDataStream ds(&file);
    const char *wstr = "hello-world";
    quint32 wi = 1234;
    double wd = 1.1;
    float wf = 2.2f;
    QVector<int> wvector;
    wvector.push_back(1);
    wvector.push_back(2);
    wvector.push_back(3);
    QMap<int,int> wmap;
    wmap.insert(4, 4);
    wmap.insert(5, 5);
    wmap.insert(6, 6);
    ds << wstr;
    ds << wi;
    ds << wd;
    ds << wf;
    ds << wvector;
    ds << wmap;
    ds.writeBytes("file end ", qstrlen("file end "));
    ds.writeRawData("really end", qstrlen("really end"));
 
    //read
    file.seek(0);
    char *rstr;
    quint32 ri;
    double rd;
    float rf;
    QVector<int> rvector;
    QMap<int, int> rmap;
    char *rbytes;
    uint len;
    char *rraw = new char[100]{0};
    int rlen;
    ds >> rstr;
    ds >> ri;
    ds >> rd;
    ds >> rf;
    ds >> rvector;
    ds >> rmap;
    ds.readBytes(rbytes, len);
    ds.readRawData(rraw, rlen);
    qDebug() << rstr;
    qDebug() << ri;
    qDebug() << rd;
    qDebug() << rf;
    qDebug() << rvector;
    qDebug() << rmap;
    qDebug() << rbytes;
    qDebug() << rraw;
 
    return a.exec();
}