Thread: Polymorphism with templated classes

I'm trying to define a variable type at runtime based on user input, so what I did was created an abstract base class Variable and derived a templated class from it.

Code:

class Variable
{
public:
     virtual ~Variable() = 0;
};

template<class T>
class Data : public Variable
{
public:
     Data();
     virtual ~Data();
protected:
     T value;
};

This way I can get the input, determine what type to use, and then do something like

Code:

Variable* var;
var = new Data<int>;

and then add all the variables to a map<string,Variable*>. My problem is that when I try to compile the linker gives me this error:

Error LNK2019: unresolved external symbol "public: _thiscall Data<int>:ata<int>(void)" (??0?Data@H@@QAE@XZ) referenced in function "protected: void _thiscall Console::def(void)" (?def@Console@@IAEXXZ)

My constructor for Data looks like this

Code:

template<class T>
Data<T>::Data()
{
}

Why isn't the linker able to use my constructor? Is polymorphism even possible with a derived templated class?

How can I avoid linker errors with my template classes?, C++ FAQ

Actually, the correct solution here could quite possibly be to delete the constructor and its prototype. Having said that, the class currently also has a destructor that does nothing, and a member variable with no way to access it, so clearly this is not the real code.

For template code you're often better off declaring the entire method inside the declaration of the class.

Personally, with the template implementation file, I prefer to use the extention *.hpp. Depending on the build system, this can stop the compiler from needlessly building and linking it, but it maintains the separation between declaration and definition that is the usual practice with C++ classes.

(*.hpp is one of the standard C++ header extensions, along with *.h and *.hxx, that any progam that deals with C++ will recognize).