【error】'isnan' was not declared in this scope

error问题

'isnan' was not declared in this scope

isnan在cmath中被取消宏定义；

// These are possible macros imported from C99-land.
#undef fpclassify
#undef isfinite
#undef isinf
#undef isnan

使用的时候可以在isnan前加上std命名空间即可；