c++中nan，inf 详解

参考 

c++中nan，inf详解_sunyoop的博客-优快云博客_c++ nan

c++ isnan和isinf_明夷TEC的博客-优快云博客_c++ isnan

C++ std::isnan等函数的使用_ccf19881030的专栏-优快云博客_c++ isnan

c isnormal_weixin_30807779的博客-优快云博客 

isfinite()测试某个浮点数是不是有限的数
isfinite(float x);
isfinite(double x);
isfinite（long double x);

isinf()测试某个浮点数是否是无限大
isinf(float x);
isinf(double x);
isinf（long double x);

isnan()测试某个浮点数是否是 非数字
isnan(float x);
isnan(double x);
isnan（long double x);

isnormal()测试某个浮点数是否被规格化: >0且不是inf
isnormal(float x);
isnormal(double x);
isnormal（long double x);

signbit()测试某个浮点数是否为负数
signbit(float x);
signbit(double x);
signbit（long double x);