std::cout << std::fixed << std::setprecision(std::numeric_limits<double>::digits10 + 1) << value << std::endl;

#include <iostream>
#include <iomanip>
#include <limits>

void print(double value)
{
    const auto digits = std::numeric_limits<double>::digits10;
    std::cout << std::setfill(' ') << std::setw(digits + 4);
    std::cout << std::fixed << std::setprecision(digits) << value << std::endl;
}

int main()
{
    double value = 9.12;
    print(value);
}

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

class  IM {
    double d;
    size_t ww;
    char c;
public:
    IM(double val, size_t whole_size, char imbue = ' ')
        : d(val), ww(whole_size), c(imbue) {}
    int get_whole() const
    {
        return d;
    }
    int get_fraction() const
    {
        stringstream s;
        s << d - get_whole();
        int k;
        s.ignore(2); // пропускаем '0' и '.'
        s >> k;
        return k;
    }
    friend ostream& operator <<(ostream& os, const IM& m)
    {
        os << setw(m.ww) << setiosflags(ios_base::left) <<setfill(m.c)
           << m.get_whole() << '.' << m.get_fraction();
        return os;
    }
};

double d = 2.45;
IM dd(d, 10, '5');
cout << dd << endl << dd.get_whole() << endl << dd.get_fraction();
/* вывод:
   2555555555.45
   2
   45
*/