double a = 6.352356663353535;
double b = a.precision(5);

double a = 6.352356663353535;
std::cout.precision(5);
std::cout << a << std::endl;

#include <cmath>
#include <cstdio>
using namespace std;
int main()
{
   double a = 6.352356663353535;
   double  intpart;
   double fractpart = modf (a, &intpart);
   fractpart  = roundf(fractpart * 100000.0)/100000.0; // Round to 5 decimal places
   double b = intpart + fractpart;
   printf("%.5lf", b);
}

6.35236