C++ Abstract Base Template Class Non-Void Method - c++

Question

I have an abstract base template class that contains members functions, some that are void, others returning data types ranging from primitive data types to the template parameter data type. I am still new to C++, and I know that I don't necessarily have to implement the pure virtual functions in the base class unless I wish to call them in the derived classes. This idea was fine for member functions that had no return value. For a function that returned an integer, I returned a 0. But when I got to a function that returned a reference to T, the template parameter, I did not know what to do. I tried the two following returns.
template <typename T>
T& AbstractBaseClass<T>::function(){
return T();
}
and
template <typename T>
T& AbstractBaseClass<T>::function(){
return T& tmp= T();
}
But neither seem to work, and I cannot not define these functions because I get an undefined reference error for the functions. I assume this error is because the functions are templates. What is the appropriate way to implement a non-void pure virtual function?
template <typename T>
class AbstractBaseClass{
public:
virtual ~AbstractBaseClass() = 0;
T& function();
}
template <typename T>
T& AbstractBaseClass<T>::function(){
//what must I do here when I don't have any member variable
}

Answer 1

Here is an example of a pure virtual function and a derived class that implements it (some member functions omitted for clarity):
template <class T>
class Base
{
public:
virtual T& get() = 0;
virtual ~Base() = 0;
};
template <class T>
Base<T>::~Base() {}
template <class T>
class Derived: public Base<T>
{
T t;
public:
T& get() override { return t; }
};
Note how get() in Base has no implementation, and the implementation of get in Derived uses a new data member not available in Base.
Note also that a pure virtual destructor, unlike other pure virtual functions, must have an implementation. Normally one doesn't need a pure virtual destructor if there are other pure functions in the class though. It is shown here for illustration.