Consider the following syntax used in a C$++$ header file.

What is the purpose of the lines #ifndef MYCLASS$\_$H$,$ #define MYCLASS$\_$H$,$ and #endif in this header file?

#ifndef MYCLASS$\_$H

#define MYCLASS$\_$H

class MyClass $\{$

public:

$$ MyClass$()$;

$$ void showMessage$()$;

$\}$;

#endif