#include <string>
#include <iostream>

int main()
{
  std::string str = "123.45 ist gleich ";
  std::size_t endpos;
  double d = std::stod(str, &endpos);
  std::cout << str << d << " before \"" << str.substr(endpos) << "\" : ";

  // einfach:
  std::cout << std::stod(str) << '\n';
}