In need of assistance deciphering template-metaprogramming macro

Question

I would very much like to try out this technique for myself, however I am getting a compile error on GCC and Clang:

Undefined symbols for architecture x86_64:
  "foo::_theInfo", referenced from:
      foo::info() in reflect-eLsDqz.o
ld: symbol(s) not found for architecture x86_64

Here is the code (straight out of the zip hosted on the blog), I hope Mr. Watte doesn't mind.

/* A simple C++ reflection mark-up library. The strength 
   of this library is that it is entirely static (all the 
   reflection information is built without using dynamic 
   memory allocation), and it is exremely compact, allowing
   you to declare the members of a structure right where 
   that struct is declared, leading to minimal risk of 
   version mismatch.

   This code is placed in the public domain by Jon Watte.
   http://www.enchantedage.com/cpp-reflection
   Version 2009-04-20
*/

#include <typeinfo>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>


struct TypeBase
{
  virtual void Marshal(void *, void const *) const = 0;
  virtual void Demarshal(void const *, void *) const = 0;
  virtual char const *name() const = 0;
  virtual size_t size() const = 0;
};

template<typename T> struct Type : TypeBase
{
  static Type<T> instance;
  // custom marshaling is handled by template specialization
  void Marshal(void *dst, void const *src) const { memcpy(dst, src, sizeof(T)); }
  void Demarshal(void const *src, void *dst) const { memcpy(dst, src, sizeof(T)); }
  char const *name() const { return typeid(T).name(); }
  size_t size() const { return sizeof(T); }
};

template <typename T> T& instance() {
  static T t;
  return t;
}

template<typename T, typename Q>
TypeBase *get_type(Q T::*mem) {
  return &instance<Type<Q> >();
}

struct member_t
{
  char const *name;
  TypeBase *type;
  size_t offset;
};

struct ReflectionBase
{
  void ReflectionConstruct();
  virtual size_t size() const = 0;
  virtual char const *name() const = 0;
  virtual size_t memberCount() const = 0;
  virtual member_t const *members() const = 0;
};

void ReflectionBase::ReflectionConstruct()
{
  //  todo: register myself in some table, etc
  members();
  memberCount();
  name();
  size();
}

#define MEMBER(x) \
   { #x, get_type(&T::x), (size_t)&((T*)0)->x },

#define RTTI(_type, _mems) \
  template<typename T> struct _info : ReflectionBase { \
    /* overrides used by ReflectionBase */ \
    inline size_t size() const { return sizeof(T); } \
    inline char const *name() const { return #_type; } \
    inline size_t memberCount() const { size_t cnt; get_members(cnt); return cnt; } \
    inline member_t const *members() const { size_t cnt; return get_members(cnt); } \
    _info() { ReflectionConstruct(); } \
    static inline member_t const *get_members(size_t &cnt) { \
      static member_t members[] = { _mems }; \
      cnt = sizeof(members) / sizeof(members[0]); \
      return members; \
    } \
    static inline _info<T> &info() { \
      return instance<_info<T> >(); \
    } \
  }; \
  inline static member_t const * members() { return _info<_type>::info().members(); } \
  static _info<_type> _theInfo; \
  inline static _info<_type> &info() { return _theInfo; }

struct foo {
  int x, y, z;
  RTTI(foo, MEMBER(x) MEMBER(y) MEMBER(z))
};

int main() {
  printf("type: %s\n", foo::info().name());
  printf("size: %ld\n", foo::info().size());
  for (size_t i = 0; i != foo::info().memberCount(); ++i) {
    printf("  %s: offset %ld  size %ld  type %s\n",
      foo::info().members()[i].name, foo::info().members()[i].offset,
      foo::info().members()[i].type->size(), foo::info().members()[i].type->name());
  }
  return 0;
}

So, static _info<_type> _theInfo is declared in the line above, but something isn't working. I think I'm sort of close to having an understanding of how this code works (MEMBER is a neat little macro that fetches the member's offset, that was fun), but I have never had a need to do something as scary as put a templated class definition in a macro before, so I actually don't know where to start poking at this.

Answer 1

I've found a different version that does compile.

Diffchecker is a useful tool.

inline static _info<_type> &info() \
{ \
    static _info<_type> _theInfo; \
    return _theInfo; \
}