http://zh.cppreference.com/w/cpp/chrono/c/time #include <ctime> isnan找不到 http://en.cppreference.com/w/cpp/numeric/math/isnan
std::isnan C++ Numerics library Common mathematical functions Defined in header<cmath> (1) boolisnan(floatnum); boolisnan(doublenum); boolisnan(longdoublenum); (since C++11) (until C++23) constexprboolisnan(/*floating-point-type*/num); ...