const char* filename1 = "myfile";
ifstream ifile(filename1);
string filename2 = filename1 + ".out"
ofstream ofile(filename2);  // new output file
ofstream ofile(filename2, ios_base::app);  // append

//      Read something from the file:

double a; int b; char c[200];
ifile >> a >> b >> c;  // skips white space in between

//      Can test on success of reading:

if (!(ifile >> a >> b >> c)) ok = 0;