Does setprecision in c++ round? If so why am I seeing this?

Question

The following snippet outputs 0.29847 when I would have expected 0.29848:

double f = 0.298475;
cout << setprecision(5) << f << endl;

For other examples, however, I observe rounding:

double f = 0.123459;
cout << setprecision(5) << f << endl;

outputs: 0.12346

and

double f = 0.123454;
cout << setprecision(5) << f << endl;

outputs: 0.12345

Answer 1

The number 0.298475 isn't representable exactly in a double (since it's not a fraction whose denominator is a power of two, being 11939/40000), and the actual number that is stored is actually closer to 0.29847 than to 0.29848.

Answer 2

As far I can see from the documentation setprecision() doesn't perform any rounding, but just sets the maximum number of digits displayed after the decimal point. Possible representations of double and float numbers may differ from the constant initializers you use.