C++: Using a templated class to implement an abstract class

Question

I have an interface and am trying to make my templated class implement this interface. A simple example to demonstrate the problem is:

class Base{

    virtual void do_something(int a) = 0;
    virtual ~Base();

};

template<typename T>
class Concrete : Base{

    T _t;
    Concrete(T t):_t(t){};
    virtual void do_something(int a);
};

template<typename T>
virtual void Concrete<T>::do_something(int a){
    std::cout << a << std::endl;
}

int main(int argc, char **argv) {

    Concrete<int> c;
    c.do_something(5);
}

However the compiler complains that:

error: templates may not be 'virtual'
 virtual void Concrete<T>::do_something(int a){

Is there a way to achieve this behaviour?

Answer 1

Yes, what n.m. said. Clang's error message is a little more helpful here:

so.cpp:19:1: error: 'virtual' can only be specified inside the class definition
virtual void Concrete<T>::do_something(int a){
^~~~~~~~

Remove the virtual keyword (writing it inside the class definition is enough/the only correct way).

Your code has a few other problems (~Base() is not defined, Concrete() expects a parameter), but the problem you ask about is solved by simply deleting the erroneous virtual.