True, We can prevent the same file from getting included again by using a preprocessor directive called
#ifndef (short for "if not defined") to determine whether we've already defined a preprocessor symbol called
XSTRING_H. If we have already defined this symbol, the compiler will ignore the rest of the file until it sees a
#endif (which in this case is at the end of the file).
#ifndef XSTRING_H
#define XSTRING_H defines the same preprocessor symbol,
Finally, the last line of the file, #endif
