why is the loop started a third time?

[amatorc++]

conff is an opened file which contains 4 lines only

	    short int nubses = 0; string line;
	    while (conff)
		{
		getline(conff, line); OPT[nubses] = line.substr(4, line.size());
		getline(conff, line); OPT_Vuln[nubses] = line.substr(9, line.size());
		nubses++;
		}

but the program goes into the loop a third time and crashes... why? it should be end of file