指定尾数的precision

#include <iostream>
#include <sstream>
#include <iomanip>
using namespace std;

int main()
{
    while( true )
    {
        double d;
        cin >> d;
        std::ostringstream os;
        os << std::fixed << std::setprecision( 3 );
        os << d;
        cout << os.str() << endl;
    }
}

1
1.000
2.2
2.200
3.33
3.330
4.444
4.444
5.5555
5.556