Re: Object (de)serialization

From:

Richard Herring <junk@[127.0.0.1]>

Newsgroups:

comp.lang.c++

Date:

Mon, 25 Jan 2010 10:55:45 +0000

Message-ID:

<WKr9fDMxiXXLFwd7@baesystems.com>

In message <002b5892$0$30072$c3e8da3@news.astraweb.com>, Philip
Pemberton <usenet09@philpem.me.uk> writes

Hi guys,

Your subject line is wrong: try something more like "linker complains
about missing static class member" ;-)

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;

That's a declaration. Where's the corresponding definition?

std::map<std::string, Shape *> Shape::creationMap;

(hint: if this is shape.h it probably ought to be in shape.cpp)

              virtual Shape *create(string data) const =0;
              virtual string getType() const =0;
              static Shape *deserialise(string data) {
                      return creationMap[data]->create(data);
              }
};

[...]

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'

Confirmation: there's no definition of Shape::creationMap.

collect2: ld returned 1 exit status

--
Richard Herring