XMLize Your Class

Environment: Visual C++ 6, ATL, MS XML

You may be already familiar with serializing C++ objects using MFC's CArchive class. Serialization is the process of storing the object properties and contained objects persistently, so that the object can be reconstructed to its previous state by loading the persistently stored data. If you use CArchive to serialize, the object will be serialized in to a binary form.

In this article I will show you how to serialize/deserialize a class into/from XML string.

Why Serialize a object in to a XML?

With the widespread usage and acceptance of XML as the format to exchange data over disparate systems, serializing a object in to a XML has many advantages:
  • XML documents can be easily exchanged over the web
  • Can work on disparate platforms
  • XML tree can be parsed easily by using readily available XML parsers in the market such as Microsoft XML Parser, Java XML Parser(JAXP) etc
Now, Let me show you how to do the XMLizing.

How it is done?

The generic class CXMLArchive has most of the functionality to XMLize(i.e. serialize) a class and deXMLize(i.e. deserialize) a class. Just derive your class from CXMLArchive and override the pure virtual methods to provide class specific implementation. Let's take an example and see how it works.

Example

Let's take a simple employee object and see how it can be serialized in to the schema format shown below.

Employee Schema

Uses Microsoft XDR Schema Standard

Employee Object

In employee object, do the following things to XMLize it:

In Header file

  • Include XMLArchive.h file
  • Derive employee class from CXMLArchive
  • Call XML_DECLARE_NODE_VAR_MAP(n), where n is the size of nodevarmap structure
  • Declare the following virtual functions from CXMLArchive class
    • InitXMLNodeVariableMap()
    • FillAttribute()
    • Save()

In CPP file

  • Implement the virtual methods

  • InitXMLNodeVariableMap()
    The purpose of this method is to maintain a dictionary of which XML node is related to which object variable, so that the XML node value can be correctly transferred to the appropriate object variable. Pass XML Node , data type and variable as parameters to XML_NODE_VAR_MAP macro. Note that variant data types(VT_xx) is used to specify data type.
    void CEmployee::InitXMLNodeVariableMap()
    {
    START_XML_NODE_VAR_MAP()
     //Specify xml node name, data type and the variable to hold the node value
     XML_NODE_VAR_MAP(NODE_ID, VT_I4, &m_lId)
     XML_NODE_VAR_MAP(NODE_FIRSTNAME, VT_BSTR, &m_szFirstName)
     XML_NODE_VAR_MAP(NODE_LASTNAME, VT_BSTR, &m_szLastName)
     XML_NODE_VAR_MAP(NODE_ADDRESS, VT_BSTR, &m_szAddress)
     END_XML_NODE_VAR_MAP()
    }
    
    FillAttribute()
    This method helps to initialize variables of the employee object from the XML tree. Note that all the meat is in base class and we just need call the base class helper method FillAttributeHelper() here.
    BOOL CEmployee::FillAttribute(MSXML::IXMLDOMNode* pXMLNode)
    {
     ATLASSERT(pXMLNode);
    
     return FillAttributeHelper(pXMLNode,
                                XML_NODEVARMAP_STRUCT, 
                                XML_NODE_VAR_MAP_COUNT);
    }
    
    Save()
    The purpose of Save() method is build XML tree to represent employee object. Build the XML tree using helper macros like START_XML, START_XML_ELEMENT etc.
    long CEmployee::Save(MSXML::IXMLDOMNode* pCurrentNode)
    {
     //Empty DOM tree
     EmptyDOM();
    
     //Block of code below will build xml tree
     //
     XML_SET_CURRENTNODE(pCurrentNode)
     START_XML()
      START_XML_ELEMENT(NODE_EMPLOYEE)
      XML_ELEMENT_LONG(NODE_ID, m_lId)
      XML_ELEMENT_STR(NODE_FIRSTNAME, m_szFirstName)
      XML_ELEMENT_STR(NODE_LASTNAME, m_szLastName)
      XML_ELEMENT_STR(NODE_ADDRESS, m_szAddress)
      END_XML_ELEMENT()
     END_XML()
     
     return S_OK;
    }
    

  • XMLize() method
  • XMLize() and deXMLize() are exposed methods in employee object to serialize and deserialize. In XMLize() method, call Save() method to build XML Tree. Once the XML tree is build, it is easy to get the XML string by calling IXMLDOMDocument::get_xml() method.

  • deXMLize() method
  • In deXMLize() method, Just call the base class method LoadXML(), which takes care of exploring each XML node and transferring the values from XML node to appropriate object variable.

Contained Objects

So far so good. Will this framework work if employee object contains any child objects?. The answer is a resounding yes. This framework supports contained objects XMLization too. You just have to implement ReadContainedObject() in your derived class(employee in this case). In overridden ReadContainedObject() method, just check if the node name is of the contained object and if yes call the contained object LoadXMLNode() method and set bContainedObjectFound to TRUE.

That's it. Happy XMLization.

Downloads

Download source - 35 Kb
Download demo project - 39 Kb


Comments

  • Updated Code??

    Posted by Legacy on 03/30/2001 12:00am

    Originally posted by: Alan

    The same article at CodeProject seems to have updated example and more questions/answers. The article text is same though.

    Check out http://www.codeproject.com/useritems/xmlize.asp

    Reply
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