isnan isinf

在linux下有两个函数

isnan(x) 
isinf(x)

对应在windows下的函数：

 _isnan(x)
!_finite(x)

为了跨平台，要么用boost要么这样写：

int isnan(double x) { return x != x; }
int isinf(double x) { return !isnan(x) && isnan(x - x); }

int iserr(double x) {return isnan(x)||isinf(x);}

http://stackoverflow.com/questions/2249110/how-do-i-make-a-portable-isnan-isinf-function