Re: Undefined symbol error for static const char

From:

Michael Doubez <michael.doubez@free.fr>

Newsgroups:

comp.lang.c++

Date:

Fri, 9 Oct 2009 00:28:28 -0700 (PDT)

Message-ID:

<801703f2-4b26-408d-8500-21a9e31c5e9b@k17g2000yqb.googlegroups.com>

On 8 oct, 23:05, James Kanze <james.ka...@gmail.com> wrote:

On Oct 8, 3:20 pm, Pete Becker <p...@versatilecoding.com> wrote:

rawhm wrote:

What gives?

$ cat sc.cpp

#include <vector>
class cls {
public:
    static const char foo = 100;
};
int main(int argc, char **argv) {
    std::vector<char> vect;
    vect.resize(10, cls::foo);
    return 0;
}

$ g++ sc.cpp

Undefined symbols:
"cls::foo", referenced from:
__ZN3cls3fooE$non_lazy_ptr in ccB49b7c.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

And the linker is right: it's not defined. Roughly speaking,
if your code takes its address (and that includes passing it
by reference) you have to define it.
const char cls::foo; // definition
The declaration in the class definition is a declaration, not
a definition.

To which I might add (considering that others have mentionned
that it does compile and link successfully with certain versions
of g++): not providing the definition is undefined behavior; the
compiler doesn't have to reject the code. In fact, your results
are likely to vary depending on whether vector::resize is inline
or not, and the level of optimization.

I wondered about that. There is no mention in the standard of UB in
case of missing declaration. I assumed the code would be incorrect and
the error caught at link time.

I understand that depending on the level of optimisation, a reference
on the actual data might or might not be needed but could it really
happen that a compiler generates code but some memory location is
missing and causing UB ?

--
Michael