deal with multiple includes

I dont get how to avoid multiple includes.
The task is simple:
We have 2 (for example) source files and one header file. We want to use this header for both sourse files.
I have found some explanation of this issue but its hard to understand.
All I know is :
1) Using #pragma once is a bad practice for some reason.
2) I must to use #ifdef (ifndef actually), #define, and #endif.
But how?! I dont get it...
Any help is appreciate.

well all you do is this in your header

Code:

---

ifndef _HEADER_H
#define _HEADER_H
//prototypes
#endif//_HEADER_H

---

Where HEADER is the name of you .h file. The way i do things is
you make your header file with your prototypes then you define them in your .cpp file eg:

Code:

---

#ifndef _TEST_H
#define _TEST_H
void printNumber();
#endif//_TEST_H

---

Code:

---

#include <iostream>
#include "test.h"
using std::cout;
void printNUmber()
{
  cout<<"20"<<endl;
}

---

THE SAME example I have in my book! I dont understand what I do wrong.
_HEADER_H - Is it actually header name or just something? If it is, its not working for me...
I type:

Code:

---

#ifndef _MAIN_H
#define _MAIN_H
void callFunc();
#endif

---

... and the same error appear on my screen - "one or more multiply defined symbols found"

OH SHAME ON ME! Do you know what I do wrong? I just made two files with main() function and use both in the same program @_@

You can't use main twice in the same program. There really is no need to either here let me whip something up for you to look at

Code:

---

//This is the header file which will allow you to call a function
//defined in another cpp file
#ifndef _TEST_H
#define _TEST_H
int add(int a, int b);
int subtract(int a, int b);
#endif//_TEST_H

---

Code:

---

//This is the cpp file which defines add and subtract
#include "test.h"
int add(int a, int b)
{
  return (a+b)
}
int subtract(int a, int b)
{
  return (a-b)
}

---

Code:

---

//this is your main.cpp file
#include <iostream>
#include "test.h"
using std::cout;
using std::cin;

int main()
{
  cout<<add(1,9);
  cout<<subtract(9,1);
  cin.get();
  return 0;
}

---

prog-bman, THANK you! I wonder why its not cover in my book.