Re: extern const int n; considered undefined reference?

From:

SG <s.gesemann@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 9 Jan 2014 04:02:32 CST

Message-ID:

<9c9e5231-f80c-4ec0-bbce-d5178607cf51@googlegroups.com>

On Saturday, December 28, 2013 11:13:07 AM UTC+1, Jim Michaels wrote:

C:\Users\JIM-MI~1\AppData\Local\Temp\ccQU9nKz.o:m.cpp:(.text+0x17):
undefined reference to `n'
collect2.exe: error: ld returned 1 exit status

Fri 12/27/2013 20:12:44.26|d:\prj\lib\prsinum\test|>type i.cpp

const int n=5;

Fri 12/27/2013 20:16:06.97|d:\prj\lib\prsinum\test|>type i.h

extern const int n;

Fri 12/27/2013 20:16:11.93|d:\prj\lib\prsinum\test|>type m.cpp

#include "i.h"
#include <iostream>
int main(void) {
std::cout<<n;
return 0;
}

I won't repeat the correct answers of Bo Persson and Wil Evers. But
let me just add that in case you don't want to include i.h into i.cpp
the definition in i.cpp should look like this:

  extern const int n=5;

The "extern" overrides the behaviour of the const with respect to
linkage. So, this is a _definition_ of n to have _external_ linkage.
It's a definition and not a declaration because there is an
initializer (the value 5). It has external linkage because of the
extern keyword. That's what you want. Without the extern, the default
linkage would be internal in this case because of the top-level const.

--
      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]