The lambda selectors sigc::_1, sigc::_2, ..., sigc::_9 are used to select the first, second, ..., nineth argument from a list.
std::cout << sigc::_1(10,20,30); // returns 10 std::cout << sigc::_2(10,20,30); // returns 20 ...
std::cout << (sigc::_1 + 5)(3); // returns (3 + 5) std::cout << (sigc::_1 * sigc::_2)(7,10); // returns (7 * 10)