从模板化函数返回结构指针

Question

I keep receiving linker errors when I do this:

//function declaration
template<typename T>
T * EntityManager::GetComponent(EID _entity, CType _type)

//Main.cpp
Position * pos = GetComponent<Position>(eid, POSITION);

Error LNK2019 unresolved external symbol "public: struct Position * __thiscall EntityManager::GetComponent(unsigned int,enum CType)" (??$GetComponent@UPosition@@@EntityManager@@QAEPAUPosition@@IW4CType@@@Z) referenced in function _main

I believe the error resides in "struct Position * GetComponent(...)" I don't want it to return a "struct Position pointer" I want it to return a "Position pointer!" I've tried various template prefaces such as "class" and "struct"

I would like for this to be possible as it is much more concise than

Position * pos = static_cast<Position *>(GetComponent(eid, POSITION));

(which does work)

Thanks for any help!

EDIT: Here is the full source to prove that its not the function, but something to do with the template...

//EntityManager.h
template<typename T>
T * GetComponent(EID _entity, CType _type);

//EntityManager.cpp
template<typename T>
T * EntityManager::GetComponent(EID _entity, CType _type)
{
    T * component = nullptr;
    int index = GetComponentIndex(_entity, _type);

    if (index >= 0)
        component = m_entities.find(_entity)->second[index];

    return component;
}

//Main.cpp
EntityManager EM;
Position * pos = EM.GetComponent<Position>(eid, POSITION);

struct Position inherits from struct Component

As I said, the function works flawlessly if I remove the template and replace "T" with "Component" then static_cast the return value. I want to avoid having to use the static cast

EDIT EDIT...

This compiles:

//EntityManager.h
class EntityManager
{
public:
    Component * GetComponent();
};

//EntityManager.cpp
Component * EntityManager::GetComponent()
{
 return new Position;
}

//Main.cpp
EntityManager EM;
Position * pos = static_cast<Position *>(EM.GetComponent());

This does not:

//EntityManager.h
class EntityManager
{
public:
    template<typename T>
    T * GetComponent();
};

//EntityManager.cpp
template<typename T>
T * EntityManager::GetComponent()
{
 return new T;
}

//Main.cpp
EntityManager EM;
Position * pos = EM.GetComponent<Position>();

WHY? All I'm asking is what format the template should be in.

(Yes, I tested this simplified example, please don't nitpick syntax)

Answer 1

You can't separate declaration and definition in classes which uses generic templates, as you do with non generic classes.

Try to move all your EntityManager.cpp to EntityManager.h .