double ldexp(double , int exp);

Returns x times 2 raised to n

#include <iostream> /* including standard library */
#include <cmath> /* including math library */
 
using namespace std;
int main ( void ) 
{
 
  //cout.setf(ios::fixed, ios::floatfield);
  cout.precision(8);
 
  cout << "4.2 raised to 2 is " <<  ldexp(4.2, 2) << endl;
  return 0;
}

 4.2 raised to 2 is 16.8