Probably your are observing the ('inizialization')
constructor[
^] of a
istringstream
object called
sin
and used to retrieve the corresponding number from its string representation (roughly speaking: 'conversion from string to number') e.g.
#include <iostream>
#include <sstream>
using namespace std;
int main()
{
string repr = "42.042";
istringstream sin (repr);
double d;
sin >> d;
cout << "d = " << d << endl;
}