visual c++ - C++ Header #define compiler error -

i'm learning c++ , i'm @ low level still. - i'm looking @ using header files , have 2 part question.

part 1

as understand them far, header file definitions similar hard coded strings within vb.net? example can following #define stock_quantity 300 , reference definition when using within functions? assume works same way vb.net strings need change value in 1 place should need make changes definition , program references number 300 on few hundred different lines?

part 2

now, said i'm still learning i'm still doing ye old multiplication tasks. i'm within using functions main .cpp file i'm not moving on header files. code snippet far.

add.h

#ifndef add_h #define add_h  int add(int x, int y);  #endif  

main.cpp

#include "stdafx.h" #include <iostream> #include "add.h"  int main() {     using namespace std;     cout << add(3, 4) << endl;     return 0; } 

when trying run receive 2 build errors , not compile.

apologies these silly questions, appreciate insight, tips or other things should consider. thanks.

edit

based on answer i've changed add.h too

#ifndef add_h #define add_h  int add(int x, int y) {     return x + y; }  #endif 

as read answer realised hadn't told function :( - gosh.

you have not added function body function add

int add(int x, int y) {     // add stuff here } 

this can done in either header file, or seperate cpp file add. trying call function has no code. known function prototype.