What is the reason for including the #ifndef, #define, and #endif in the header file Animal.h$?$

#ifndef ANIMAL$\_$H

#define ANIMAL$\_$H

class Animal

$\{$

public:

Animal$()$;

Animal$($double new$\_$area$\_$hunt$)$;

double get$\_$area$\_$hunt$()$ const;

private:

double area$\_$hunt;

$\}$;

#endif