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?

• 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

How it is done?

Example

Employee Schema

<?xml version="1.0" ?>
<Schema name="Employee-Schema"
			xmlns="urn:schemas-microsoft-com:xml-data"
			xmlns:dt="urn:schemas-microsoft-com:datatypes">

	<ElementType name="Id" dt:type="int"/>
	<ElementType name="FirstName" dt:type="string"/>
	<ElementType name="LastName" dt:type="string"/>
	<ElementType name="Address" dt:type="string"/>

	<ElementType name="Employee" model="closed" content="eltOnly">
		<element type="Id"/>
		<element type="FirstName"/>
		<element type="LastName"/>
		<element type="Address"/>
	</ElementType>
</Schema>

Employee Object

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

That's it. Happy XMLization.

Downloads