How to access method from another source file?

[WingYn]

Hi...

I have been struggling concerning using methods from another source file...

For example I have a file called a.cpp, and I have a function in it
int sum(int a, int b)
{
............
}

I tried to access this method from another b.cpp.

But I get errors o.O

Can I get a small example of how to do this correctly?

Thanks...

[modoran]

Make a header file, say h1.h and put this in it:
int sum(int, int);

and then just #include "h1.h" in every .cpp file you want.

[Galik]

The way to do this is to create a 'header' file for your source, called a.h

// a.h
#ifndef _A_H // must be unique name in the project
#define _A_H

int sum(int a, int b); // prototype declaration of the function in a.cpp

#endif 

You then include that header file in any file you want to access the function in:

// b.cpp
#include "a.h" // get the prototype declaration for the functions in a.cpp

int main()
{
    sum(2, 8); // use the function
}

[WingYn]

Thanks for the answers. But I tried that, I think it's because I haven't initialized my object?

I have my code down below:

#pragma once
class people
{
public:
	people(void);
	~people(void);
	int sum(int a, int b);
};

#include "StdAfx.h"
#include "people.h"


people::people(void)
{
}



people::~people(void)
{
}

int sum(int a, int b)
{
	int result;
	int result = a + b;

	return result;
}

#include "stdafx.h"
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "people.h"

int _tmain(int argc, _TCHAR* argv[])
{
  srand ( time(NULL) );
  printf ("First number: %d\n", rand() % 2 + 1);
  
  sum(3,4);	

  system("PAUSE");	
  return 0;
}

I get red lines under my "sum"

I try to use my function sum() in my main method. But it says "Error:identifier "sum" is undefined. I tried to put "int" infront, then my comma gets an error "Error: expected a')'"

Thanks in advance...

[Moschops]

At that line in your main function, the function "sum" does not exist.

This is because it has not been declared anywhere, or defined. You have defined a function called sum in what I assume is your people.cpp file, but your main code cannot see that. Your main code can see the header file, which also does not declare or define a function called sum (although you do declare a class function within your people class called sum, but
sum(3,4);
is not trying to use that class function).

Solution; add declaration of the sum function, like this,

int sum(int a, int b);

to the top of your main code file (sorry, I don't know what you've called it, that one with _tmain in) or to your people.h file

It is true that you haven't initialised your people object, but you're also not even trying to create one or use one.

[Moschops]

Try this:

// main.cpp
#include <iostream>
#include "sum.h"

int main ()
{
  int x;
  x = sum(3,4);
  std::cout << x;
  return 0;
}

// sum.h

//declare sum function
int sum(int a, int b);

// sum.cpp

//define sum function
int sum(int a, int b)
{
  return(a+b);
}

I've not compiled it to test, but it should give you an idea of how this works. Be sure you compile both sum.cpp and main.cpp in your project.

Edit: I have now compiled and tested, and on my system at least this is happy. Hopefully this makes it clear how to go about doing this. I think you were confusing yourself by having that unused "people" class in there

[WingYn]

Thank alot for the help, with the help of you guys' code examples, I found out it was because I couldn't do this:

//sum.h

class sum
{
public:
	sum(void);
	~sum(void);
	int sum(int a, int b);
};

I had to make it this way.

int sum(int a, int b);

class sum
{

public:
	sum(void);
	~sum(void);

};

Although I dont really understand why :P

But thanks alot

[Moschops]

I don't think you quite understand what a class is for. There is no need at all for you to be defining a class if you're not going to use a function from within it.

[Zhuge]

That's right. This is not Java; you do not have to have a class containing every function you ever use, and you don't even use the class in this case anyway.