Template Factory Pattern in C++

I am trying to make a factory that will have the type passed in, rather then having it hard coded for types. However, when I attempt to add the type to the factory inside of the types .cpp file, I will get a linker error. For example, he is a linker error I am currently getting.

1>RandomClass.obj : error LNK2019: unresolved external 开发者_运维问答symbol "public: short __thiscall TemplatedFactory::AddType(char const *)" (??$AddType@VRandomClass@@@TemplatedFactory@@QAEFPBD@Z) referenced in function "void _cdecl `dynamic initializer for 'private: static short RandomClass::ID''(void)" (??_E?ID@RandomClass@@0FA@@YAXXZ)

I tried to make the test case as small as possible, though it does cross five files, they are very small

BaseClass.h : http://codepad.org/MhZMw7t0

#pragma once
class BaseClass{ };

RandomClass.h : http://codepad.org/xoObzP8G

#pragma once
#include "BaseClass.h"
class RandomClass : public BaseClass
{
private:
    static short ID;

public:
    RandomClass();
    virtual ~RandomClass();
};

TemplatedFactory.h : http://codepad.org/qkcTBw24

#pragma once
#include <map>
using std::map;
#include "BaseClass.h"

template<typename Type> BaseClass* createType() { return new Type; }

class TemplatedFactory
{
private:
    typedef BaseClass* (*ComponentFactoryFuncPtr)();
    typedef map<const char*, ComponentFactoryFuncPtr> map_type;

    map_type m_Map;

public:
    static TemplatedFactory &GetInstance();

    template<typename Type>
    short AddType(const char* componentName);
};

RandomClass.cpp : http://codepad.org/ALoF3Ysb

#include "RandomClass.h"
#include "TemplatedFactory.h"

short RandomClass::ID = TemplatedFactory::GetInstance().AddType<RandomClass>("RandomClass");

RandomClass::RandomClass() { }

RandomClass::~RandomClass() { }

TemplatedFactory.cpp : http://codepad.org/iqgNqa6H

#include "TemplatedFactory.h"

TemplatedFactory &TemplatedFactory::GetInstance()
{
    static TemplatedFactory instance;
    return instance;
}

template<typename Type>
short TemplatedFactory::AddType(const char* componentName)
{
    ComponentFactoryFuncPtr function = &createType<Type>;
    m_Map.insert(std::make_pair(componentName, function));

    return 0;
}

I can remove the the linker error if I move

short RandomClass::ID = TemplatedFactory::GetInstance().AddType<RandomClass>("RandomClass");

from RandomClass.cpp to TemplatedFactory.cpp, however, I would like to have the declaration in RandomClass.cpp. Does anyone know of a way to fix this or perhaps a better design (without the use of external libraries).

Templated functions cannot have their definition and declaration separated from each other without export, which your compiler probably doesn't support. You need to move the TemplateFactory::AddType definition to the header.

You cannot place implementation of class template in cpp-file See some technique for "cheating" on http://www.parashift.com/c++-faq-lite/templates.html#faq-35.14