double d = 8.19999999999999999999;
cout << d  << "n";
cout << fixed << d  << "n";
cout << setprecision(15) << d  << "n";
cout << fixed << setprecision(15) << d  << "n";

8.2
8.200000
8.199999999999999
8.199999999999999

8.2
 8.200000
 8.200000000000000
 8.200000000000000