You only declared field eventXml
. You need to define it somewhere. In some .cpp file you must put line like
xml ParsingEventsTest::eventXml;
EDIT
As for the reason why it is needed: declaration of static member in class can not define it. If it did, every translation unit which includes class definition would define it. But each program needs exactly one definition, so you have to choose place where member is defined outside the class definition (usually in some .cpp file. Best is the one with member functions definition). And the syntax looks like above.