noob problem with header file

[frogboy77]

Hi, new to this so go easy.
I'm trying to store a function i wrote in a file and call it with a header file rather than copy and paste as ive been doing but i keep getting error messages from the compiler.
Can anyone point out what i'm doing wrong?

the header file is:

#ifndef GUARD_is_prime_h
#define GUARD_is_prime_h

bool is_prime(unsigned long long int);

#endif

this is saved as is_prime.h

is_prime.cpp is;

bool is_prime(unsigned long long int a)
{
     if(a==1){return false;}
     if(a<4){return true;}
     if(a%2==0){return false;}
     if(a<9){return true;}
     if(a%3==0){return false;}
     unsigned long long int i;
     for(i=5;(i*i)<=a;i+=6)
     {
                           if(a%i==0){return false;}
                           if(a%(i+2)==0){return false;}
     }
     return true;
}

i then tried to write a simple program that asks if a number is prime having used
#include "is_prime.h" and i get the following error

In function main
[Linker error]undefined reference to is_prime(unsigned long long)
Id returned 1 exit status

any help greatly appreciated.:)

[frogboy77]

this is the test program i wrote

#include <iostream>
#include "is_prime.h"

using namespace std;

int main()
{
    if(is_prime(50)){cout<<"yes"<<endl;}
    else{cout<<"no"<<endl;}
    
    
    system("pause");
    return 0;
}

[NathanOliver]

you need to include is_prime.h in your is_prime.cpp file.

[frogboy77]

ok tried that and still getting same error message.

[Nick Evan]

Can you post the 3 files exactly as you're using them?

[frogboy77]

don't understand thought i had.

[Fbody]

The issue is most-likely that "is_prime.cpp" is not getting compiled as part of your compilation process.

What OS and IDE are you using?

Is the file "is_prime.cpp" included as part of your "project" in your IDE? If not, it doesn't get compiled and the linker can't find the object code that it needs to finish the job.

[frogboy77]

using dev on windows

the files are just saved in the directory

isn't a project (unsure what that means, have only been writing one off programs up to this point) just three seperate files.

[Fbody]

No, projects are actually an organizational system within the IDE. You add files to a "project" then the IDE's project system automatically manages creation of the make file used during compilation for you. If you have more than 1 *.cpp file for the program you are working on, all of those files should be part of the "project".

Take a look at the "screenie" that I've attached. This particular one is for MS-VC++ 2008, but the concept is the same. Notice how I have added all 3 files to the project. If I remove "is_prime.cpp" from the project, it will cause the same linker error you are experiencing.

[frogboy77]

thanks. put it all in one project and it seems to work fine.