Unresolved External Symbol in Singleton Class - c++

Question

I've been coding for a long time now but I do not understand this error. I am writing a custom system for providing unique integer ID's to specific instances of objects (I call them tags). And I am implementing one of the classes as a Singleton.
The two classes for the Tagging system are defined as such:
#include "singleton.h"
class Tag: public bear::Singleton<Tag>
{
public:
static dUINT32 RequestTag(Tagged* requester);
static void RevokeTags(void);
private:
Tag(void);
~Tag(void);
Tagged** m_tagTable; // list of all objects with active tags
dUINT32 m_tagTable_capacity, // the maximum capacity of the tag table
m_tagIndexer; // last given tag
};
class Tagged
{
friend class Tag;
public:
inline dUINT32 GetTag(void) {return m_tag;}
private:
inline void InvalidateTag(void) {m_tag=INVALID_TAG;}
dUINT32 m_tag;
protected:
Tagged();
virtual ~Tagged();
};
The Singleton class is defined as such:
template <typename T>
class Singleton
{
public:
Singleton(void);
virtual ~Singleton(void);
inline static T& GetInstance(void) {return (*m_SingletonInstance);}
private:
// copy constructor not implemented on purpose
// this prevents copying operations any attempt to copy will yield
// a compile time error
Singleton(const Singleton<T>& copyfrom);
protected:
static T* m_SingletonInstance;
};
template <typename T>
Singleton<T>::Singleton (void)
{
ASSERT(!m_SingletonInstance);
m_SingletonInstance=static_cast<T*>(this);
}
template <typename T>
Singleton<T>::~Singleton (void)
{
if (m_SingletonInstance)
m_SingletonInstance= 0;
}
I am getting the following error upon trying to compile and link together the files:
test.obj : error LNK2001: unresolved external symbol "protected: static class util::Tag * bear::Singleton::m_SingletonInstance" (?m_SingletonInstance#?$Singleton#VTag#util###bear##1PAVTag#util##A)
1>C:...\tools\Debug\util.exe : fatal error LNK1120: 1 unresolved externals
Does anyone have any idea why I am getting this error?

Answer 1

You should provide a definition for your static data member at namespace scope (currently, you only have a declaration):
template <typename T>
class Singleton
{
// ...
protected:
static T* m_SingletonInstance; // <== DECLARATION
};
template<typename T>
T* Singleton<T>::m_SingletonInstance = nullptr; // <== DEFINITION
If you are working with C++03, you can replace nullptr with NULL or 0.