16 #ifndef QGSOPTIONALEXPRESSION_H    17 #define QGSOPTIONALEXPRESSION_H    22 #include "qgis_core.h"    69     void writeXml( QDomElement &element );
    76     void readXml( 
const QDomElement &element );
    87 #endif // QGSOPTIONALEXPRESSION_H Class for parsing and evaluation of expressions (formerly called "search strings"). 
 
An expression with an additional enabled flag. 
 
QgsOptional is a container for other classes and adds an additional enabled/disabled flag...