Table of contents

Appendices

5 Data Model

XPath operates on an XML document as a tree. This section describes how XPath models an XML document as a tree. This model is conceptual only and does not mandate any particular implementation. The relationship of this model to the XML Information Set XINFO is described in XML Information Set Mapping.

XML documents operated on by XPath must conform to the XML Namespaces Recommendation XMLNAMES.

The tree contains nodes. There are seven types of node:

• root nodes

• element nodes

• text nodes

• attribute nodes

• namespace nodes

• processing instruction nodes

• comment nodes

For every type of node, there is a way of determining a string-value for a node of that type. For some types of node, the string-value is part of the node; for other types of node, the string-value is computed from the string-value of descendant nodes.

NOTE: 

For element nodes and root nodes, the string-value of a node is not the same as the string returned by the DOM nodeValue method (see DOM).

Some types of node also have an expanded-name, which is a pair consisting of a local part and a namespace URI. The local part is a string. The namespace URI is either null or a string. The namespace URI specified in the XML document can be a URI reference as defined in RFC2396; this means it can have a fragment identifier and can be relative. A relative URI should be resolved into an absolute URI during namespace processing: the namespace URIs of [expanded-name] s of nodes in the data model should be absolute. Two [expanded-name] s are equal if they have the same local part, and either both have a null namespace URI or both have non-null namespace URIs that are equal.

There is an ordering, document order, defined on all the nodes in the document corresponding to the order in which the first character of the XML representation of each node occurs in the XML representation of the document after expansion of general entities. Thus, the root node will be the first node. Element nodes occur before their children. Thus, document order orders element nodes in order of the occurrence of their start-tag in the XML (after expansion of entities). The attribute nodes and namespace nodes of an element occur before the children of the element. The namespace nodes are defined to occur before the attribute nodes. The relative order of namespace nodes is implementation-dependent. The relative order of attribute nodes is implementation-dependent. Reverse document order is the reverse of [document order] .

Root nodes and element nodes have an ordered list of child nodes. Nodes never share children: if one node is not the same node as another node, then none of the children of the one node will be the same node as any of the children of another node. Every node other than the root node has exactly one parent, which is either an element node or the root node. A root node or an element node is the parent of each of its child nodes. The descendants of a node are the children of the node and the descendants of the children of the node.