Object (de)serialization

Hi guys,
I'm trying to write the contents of a set of classes to a file in a
reasonably portable way. Obviously I also want to be able to read the
files back into memory later on. At this point, my serializer works fine;
I can create an object (or several objects) and save them to a file. Now
I need to get them back out of the file...

I've been reading the C++ FAQ (notably Section 36, Serialization and
Unserialization) and I've been writing small prototype apps to try and
learn how all this stuff works. I came up with this, based on the textual
description in C++ FAQ 36.8:

#include <map>
#include <string>
#include <iostream>

using namespace std;

class Shape {
    public:
        Shape() { cerr<<"ctor: Shape\n"; };
        static std::map<std::string, Shape *> creationMap;
        virtual Shape *create(string data) const =0;
        virtual string getType() const =0;
        static Shape *deserialise(string data) {
            return creationMap[data]->create(data);
        }
};

class Triangle : public Shape {
    public:
        Triangle() {
            cerr<<"ctor: Triangle\n";
            creationMap["triangle"] = new Triangle();
        }
        virtual Shape *create(string data) const {
            if (creationMap.count(data) == 0) throw -1;
            return new Triangle();
        }
        virtual string getType() const {return "triangle";}
};

int main(void)
{
    Shape *x = Shape::deserialise("triangle");

    return 0;
}

This looks fine to me, and it compiles -- but it won't link:
philpem@cougar:~/dev$ g++ -o test test.cpp && ./test
/tmp/ccDHDpxd.o: In function `Shape::deserialise(std::basic_string<char,
std::char_traits<char>, std::allocator<char> >)':
test.cpp:(.text._ZN5Shape11deserialiseESs[Shape::deserialise
(std::basic_string<char, std::char_traits<char>, std::allocator<char> >)]
+0x12): undefined reference to `Shape::creationMap'
collect2: ld returned 1 exit status

Compiler is g++ (gcc) 4.4.1-4ubuntu8, on Ubuntu Linux 9.10 (32-bit).

Can anyone see what I'm doing wrong here? I'm sure it's blatantly obvious
to an experienced developer, but this is the first time I've tried to
implement something like this, and I'm really not having much luck...

Thanks,
Phil.