Linking problems

Hello again.

I have 3 files:
main.cpp:

Code:

#include <iostream>
#include "def.h"
using namespace std;

int main()
{
    cout << server_name << endl;
}

def.h:

Code:

#ifndef _def_h_
#define _def_h_
#include <iostream>
extern const std::string server_name;

#endif

def.cpp:

Code:

#ifndef _def_cpp_
#define _def_cpp_
#include <iostream>
const std::string server_name = "www.google.com";

#endif

I tried to compile them like this:
g++ main.cpp -c
g++ def.cpp -c
g++ main.o def.o

And I get this error:
main.o(.text+0x12e):main.cpp: undefined reference to `server_name'
collect2: ld returned 1 exit status

def.cpp should include def.h
def.cpp doesn't need include guards.

also def.cpp
does not need <iostream>
instead <string> should be included

guards = the #ifndef?

Originally Posted by hardi

guards = the #ifndef?

yes
guards are used only in files that are included in other files to prevent multiple includes

cpp file in most cases are not included in other file - so there is no need in the guards

But why doesn't def.cpp work without including the def.h

btw: I included def.h and it worked.

def.h is used to notify the compiler that the const std::string server_name is defined somethere else

when compiling def.cpp compiler already knows that this variable is defined and how, so including def.h is not needed

Originally Posted by vart

def.h is used to notify the compiler that the const std::string server_name is defined somethere else

when compiling def.cpp compiler already knows that this variable is defined and how, so including def.h is not needed

What you are saying contradicts with Salem's sayings:

Originally Posted by Salem

def.cpp should include def.h
def.cpp doesn't need include guards.

He tells me to include def.h - which strangely helped me to compile the program, but without including the def.h file, I got the error I posted above. (You are telling me, that def.cpp doesn't need def.h so it should not be included - yes, g++ def.cpp -c gives me no errors, but if I type g++ def.o main.o I get the linker errors. but when I include the def.h file in def.cpp, the compiler shuts up and makes an executable. So: Why does it work? What's wrong with the initial system?)

Imho def.h helps main.cpp understand, that server_name is defined elsewhere and def.cpp just defines the variables. so if I link them together, all should be happy, but unfortunately that's not the case.