具有模板成员函数的C ++继承类

Question

I´m having problems while writing a Resource class:

class BaseResource {
protected:
    unsigned int size;

public:
    virtual ~BaseResource() {}
    template<class T> const T& GetValue() const;
    template<class T, class U> void GetValue(const U& rhs);

    unsigned int GetSize() {
        return this->size;
    }
    void SetSize(unsigned int size) {
        this->size = size;
    }
};

template<class T>
class Resource : public BaseResource {
    T value;

public:
    virtual ~Resource() {}      
    Resource(unsigned int size, const T& rhs) { this->size = size; this->value = rhs; }

    const T& GetValue() const {return value;}
    void SetValue(const T& rhs) {value=rhs;}  
};

I think that the classes above are defined correctly so I do not understand why the following code produce a linker error:

Test.obj : error LNK2001: unresolved external symbol ""public: char * const & __thiscall BaseResource::GetValue(void)const " (??$GetValue@PAD@BaseResource@@QBEABQADXZ)".

char* c = new char[3];
c[0] = '1';
c[1] = '2';
c[2] = '3';
BaseResource* resource = new Resource<char*>(3, c);
char* loadedResource = resource->GetValue<char*>();

In my opinion this should create an instance of Resource which holds a char* and can return it.

Can anybody tell me where I`ve done the mistake causing this?

Answer 1

Following methods are not implemented :

template<class T> const T& GetValue() const;
template<class T, class U> void GetValue(const U& rhs);

I hope you are not planning on making them virtual because that wil not work. Template methods cannot be made virtual. As they are not implemented that definitely explains the linking problem.

Answer 2

The implementation of these function should be in the same header as the class. In this case you have instantiated the template function and the concrete instantiated function is not defied. Any time you use templates you need to include the definition of the function in the translation unit that uses the function.

Edit

---

This is the basic Idea. You need to have the implantation defined so that when the class is instantiated the class is fully defined.

public:
    virtual ~BaseResource() {}
    template<class T> const T& GetValue() const
    {
       return someT;
    }
    template<class T, class U> void GetValue(const U& rhs)
    {
       return someT;
    } 

    unsigned int GetSize() {
        return this->size;
    }
    void SetSize(unsigned int size) {
        this->size = size;
    }
};