strange link problem

[arcimboldo]

Hello,

I got a strange link problem; here are the simplest files showing the
error:

a.h::
#include <utility>

class a{
public:
typedef int perm;
static const perm PERM_NONE = 0;
a();
private:
std::pair<int, perm> aaa;
};

a.cpp::
#include "a.h"
a::a() : aaa(0,PERM_NONE) { }
main () { }

Compiling with g++ (GCC) 3.3.5 (Debian 1:3.3.5-13) I get::

$ g++ a.cpp
/tmp/ccKKUXg8.o: In function
`a::a[not-in-charge]()':a.cpp:(.text+0xa): undefined reference to
`a::PERM_NONE'
/tmp/ccKKUXg8.o: In function `a::a[in-charge]()':a.cpp:(.text+0x34):
undefined reference to `a::PERM_NONE'
collect2: ld returned 1 exit status

The file compiles correctly, the error shows up only when linking.

What could it possibly be? Thanks for any help!

Arcimboldo

[marcwentink]

> What could it possibly be?

What if you delete the 'static' keyword?
What if you construct aaa not in the list but in the
compound statement "{}" of the constructor?

My guess is that the static var is not yet initialized when a::a() :
aaa(0,PERM_NONE) { } is called.

Marc Wentink

[rmansfield]

The problem is because static data members need to be defined exactly
once in every translation unit. Please see
http://www.parashift.com/c++-faq-lit...html#faq-10.11

Regards,

Ryan Mansfield

[Dietmar Kuehl]

rm********@gmail.com wrote:

The problem is because static data members need to be defined exactly
once in every translation unit.

Nope! Static data members have to be defined exactly once in the
whole program. The only exception are constant integral expressions
whose address is never taken: these don't need any definition.
However, addresses are easily taken, e.g. by passing the value to a
function taking an 'int const&' as parameter. The best bet for a
constant integral expression which does not need a separate
definition is using an enum.
--
<mailto:di***********@yahoo.com> <http://www.dietmar-kuehl.de/>
<http://www.eai-systems.com> - Efficient Artificial Intelligence

[Fei Liu]

ar********@gmail.com wrote:

Hello,

I got a strange link problem; here are the simplest files showing the
error:

a.h::
#include <utility>

class a{
public:
typedef int perm;
static const perm PERM_NONE = 0;
a();
private:
std::pair<int, perm> aaa;
};

a.cpp::
#include "a.h"
a::a() : aaa(0,PERM_NONE) { } try a::a(): aaa(0, a::PERM_NONE){} main () { }

Compiling with g++ (GCC) 3.3.5 (Debian 1:3.3.5-13) I get::

$ g++ a.cpp
/tmp/ccKKUXg8.o: In function
`a::a[not-in-charge]()':a.cpp:(.text+0xa): undefined reference to
`a::PERM_NONE'
/tmp/ccKKUXg8.o: In function `a::a[in-charge]()':a.cpp:(.text+0x34):
undefined reference to `a::PERM_NONE'
collect2: ld returned 1 exit status

The file compiles correctly, the error shows up only when linking.

What could it possibly be? Thanks for any help!

Arcimboldo