can't get the trained parameter alpha and rho by any function

I have checked that the CvSVM member desicion_func is protected but I want to get the alpha and rho. But I cannot find any function which allows me to do that. I can save the svm parameters to an xml file. and I have tried to create a new class which derive the CvSVM to get the desicion_func but it still didn't work. so the only way is to read the xml file ? I am using opencv2.4.9 + VS2013