XML Data Binding for C++

Strigi has reached the point that the configuration files for it should be more advanced than a text file with one directory per line. Because I have good experience with using XML Schema for mapping from XML to java and back using JAXB, I’d been looking for a good toolkit that does the same in C++. The requirements for such a tool are:

• Simple

• Small

• Few dependencies

• Use XML Schema

A tool I’ve found is called xsd. Behind this trivially simple name is a suite of free software tools that generate different types of C++ code. The latest version of this tool is 2.2.0 and can be gotten here. There’s an extensive manual starting at a simple Hello World application.

To test the feasibility of this code for Strigi I designed a small XML Schema file and wrote a simple program to accompany it. For Strigi, I’d like to have a configuration file that describes what directories and other sources are indexed and how. Below I’ve written the different file I used for testing xsd. The workflow is pretty easy:

1. write XML Schema

2. compile c++ from schema

3. use data classes in c++ code

If the requirements for the configuration file change, the XML Schema is changed and the c++ code regenerated. Any changes required in the c++ code that uses the data classes will be caught by the compiler.
Nothing is for free: using xsd introduces dependencies in your code. When using xsd, you will need to link your executable with xerces-c. Since this is a widespread library, this is not a big problem.
An example configuration file might look like this:

<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<s:daemonConfiguration xmlns:s="http://www.vandenoever.info/strigi">

  <repository repositoryLocation="/home/tux/.strigi/mainindex" repositoryType="CLucene">
    <fileSystemSource baseURI="file:/home/tux"/>
    <httpSource baseURI="http://www.kde.org/"/>
  </repository>

</s:daemonConfiguration>

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
 targetNamespace="http://www.vandenoever.info/strigi"
 xmlns:tns="http://www.vandenoever.info/strigi">

 <element name="daemonConfiguration" type="tns:daemonConfigurationType"/>

 <complexType name="daemonConfigurationType">
   <sequence>
   <!-- a repository contains one index -->
   <element name="repository" type="tns:repositoryType"
       minOccurs="0" maxOccurs="unbounded">
   </element>
  </sequence>
 </complexType>

 <complexType name="repositoryType">
  <sequence>
   <element name="fileSystemSource" type="tns:fileSystemSourceType"
       minOccurs="0" maxOccurs="unbounded"/>
   <element name="httpSource" type="tns:httpSourceType"
       minOccurs="0" maxOccurs="unbounded"/>
  </sequence>
  <attribute name="repositoryLocation" type="anyURI" use='required'/>
  <attribute name="repositoryType" type="tns:repositoryTypeType"
    use='required'/>
 </complexType>

 <simpleType name="repositoryTypeType">
  <restriction base='string'>
   <enumeration value='CLucene'/>
   <enumeration value='HyperEstraier'/>
   <enumeration value='Xapian'/>
   <enumeration value='Sqlite'/>
  </restriction>
 </simpleType>

 <complexType name="fileSourceType">
  <attribute name="baseURI" type="anyURI" use='required'/>
  <!-- time between updates for this directory, <= 0 means never -->
  <attribute name="autoUpdateFrequence" type="int"/>
 </complexType>

 <complexType name="fileSystemSourceType">
  <complexContent>
   <extension base="tns:fileSourceType">
    <sequence>
     <element name="fileEventListener" minOccurs="0" maxOccurs="1">
      <complexType>
      </complexType>
     </element>
    </sequence>
   </extension>
  </complexContent>
 </complexType>

 <complexType name="httpSourceType">
  <complexContent>
   <extension base="tns:fileSourceType">
   </extension>
  </complexContent>
 </complexType>

</schema>

#include "strigidaemon.hxx"
#include <iostream>
using namespace std;

int
main (int argc, char* argv[]) {
    auto_ptr<strigi::daemonConfigurationType> config;

    if (argc > 1) {
        // load an object
        try {
            config = strigi::daemonConfiguration(argv[1],
                xml_schema::flags::dont_validate);
        } catch (struct xsd::cxx::tree::exception<char>& e) {
            cerr << e << endl;
        }
    }
    if (!config.get()) {
        // create an object
        config = auto_ptr<strigi::daemonConfigurationType>(
            new strigi::daemonConfigurationType());
    }

    // add elements to the object
    string repositoryLocation = "/home/tux/.strigi/mainindex";
    strigi::repositoryTypeType repositoryType("CLucene");
    strigi::repositoryType repo(repositoryLocation, repositoryType);

    strigi::fileSystemSourceType fs("file:/home/tux");
    repo.fileSystemSource().push_back(fs);

    strigi::httpSourceType hs("http://www.kde.org/");
    repo.httpSource().push_back(hs);

    config->repository().push_back(repo);

    // provide a mapping for the namespace we use
    xml_schema::namespace_infomap map;
    map["s"].name = "http://www.vandenoever.info/strigi";

    // output the object to the standard output iostream
    daemonConfiguration(cout, *config, map);

    return 0;
}

LDFLAGS=-lxerces-c
CXXFLAGS=-Wall -O2
main: main.cpp strigidaemon.cxx

strigidaemon.cxx: strigidaemon.xsd
        xsd cxx-tree --generate-serialization strigidaemon.xsd

clean:
        rm strigidaemon.cxx strigidaemon.hxx main