format unix timestamp to string

unix_ts_to_str.cc

#include <iostream>
#include <sstream>
#include <iomanip>
#include <string>

// e.g. 1584773265.928826
std::stringstream oss;
oss << std::setfill('0') << std::setw(17) << std::fixed
    << std::setprecision(6) << timestamp << ".png";
std::string filename = oss.str();