main.cpp

/*
preprocessor is a separate program that runs before the compiler when you
compile your program. It’s purpose is to process directives. Directives are
specific instructions that start with a # symbol and end with a newline (NOT a
semicolon).

There are several different types of directives:
1)Includes
2)Macro defines
3)Conditional compilation
4)Header guards

The preprocessor manipulates text before the compiler gets to it.


For example, consider the following program:

add.h:
        #include "mymath.h"
        int add(int x, int y);

subtract.h:
        #include "mymath.h"
        int subtract(int x, int y);

main.cpp:
        #include "add.h"
        #include "subtract.h"

Consequently, all the contents of mymath.h will have been included twice in
main.cpp, which will cause the compiler to complain.

solution:

#ifndef SOME_UNIQUE_NAME_HERE
#define SOME_UNIQUE_NAME_HERE

// your declarations here

#endif

for example in our case:
add.h

        #ifndef ADD_H
        #define ADD_H

        // your declarations here

        #endif

subtract.h

        #ifndef SUBTRACT_H
        #define SUBTRACT_H

        // your declarations here

        #endif

and because in math.h we have

        #ifndef MATH_H
        #define MATH_H

        // your declarations here

        #endif

so after we include add.h, via add.h automatically we define MATH_H, so when we
include subtract.h, MATH_H already defined so #ifndef MATH_H would return false
and the content of math.h won't included again
*/

#include "add.h"
#include "subtract.h"

int main() {}