Serialisation of user-defined data in Qt

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Article Metadata

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Tested with

Devices(s): All (S60)

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Compatibility

Platform(s): S60 3rd Edition, FP1
S60 3rd Edition, FP2
S60 5th Edition

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Article

Keywords: QDataStream

Created: User:Kbwiki (27 Jan 2010)

Last edited: hamishwillee (11 Oct 2012)

Overview

This article describes how to provide a serialisation of user-defined custom classes in Qt.

Description

To serialise our own custom Qt classes, we need to write appropriate stream operators:

 QDataStream &operator<<(QDataStream &, const MyClass &);
 QDataStream &operator>>(QDataStream &, MyClass &);

Solution

Let's assume that MyClass is a user-defined class.

 class MyClass
 {
  public:
   int m_int;
   QString m_str;
 };

Writing into streams - Externalisation:

 QDataStream &operator<<(QDataStream &s, const MyClass& myClass)
 {
  //Extracting from the stream
  int i = myClass.m_int;
  QString str = myClass.m_str;
  s<<i;
  s<<str;
  return s;
 }

Reading from the stream - Internalisation:

 QDataStream &operator>>(QDataStream& s, MyClass& myClass)
 {
  s>>myClass.m_int;
  s>>myClass.m_str;
  return s;
 }

Serialisation - Writing contents of the class into file:

 MyClass mClass;
 mClass.m_int = 100;
 mClass.m_str = "String sting";
 QString location = QDesktopServices::storageLocation(QDesktopServices::DataLocation);
 location += "/customDataStream.txt";
 QFile file;
 file.setFileName(location);
 bool err = file.open(QIODevice::WriteOnly);
 QDataStream out(&file);
 out<<mClass;
 file.close();

Serialisation - Reading into user-defined class from file:

 MyClass mClass;
 QString location = QDesktopServices::storageLocation(QDesktopServices::DataLocation);
 location += "/customDataStream.txt";
 QFile file(location);
 bool err = file.open(QIODevice::ReadOnly);
 QDataStream in(&file);
 in>>mClass;
 int i = mClass.m_int;
 QString str = mClass.m_str;
 file.close();