In your python options file, you may want to pass values to a certain used C++ class (e.g. in DaVinci in the TupleToolMCTruth you may want to switch on the association by Chi2 or not).
The following example treats the case, if you want to pass a string to your C++ class (to save output, for instance).
In the .h-file of your class, add:
private: std::string m_outputName;
Of course, it does not necessarily need to be private.
In the constructor, add:
declareProperty( "OutputName", m_outputName = "defaultOutputName");
The first argument defines the name in the python options file, the second argument sets the variable to a default value, if there is no input from the python options file.
In the initialize() add (for example):
fstream out = new fstream(m_outputName, ios::out);
Note: Only add the declareProperty in the constructor, but don't do anything with the variable, it won't work (at least considering my experience…)
To your instance of your class, add:
myInstance.OutputName = "myOutputName.txt"