c++11 Pass template functor callback using in virtual function not allowed

So I am writing an interface class where on of the functions takes a function-callback as a parameter.

Usually I just write this as follows:

template <typename functor>
void some_fn(functor & callback)
{
    callback();
}

But in my interface it would be virtual:

template <typename functor>
virtual void some_fn(functor & callback) = 0;

This gives the error: template may not be virtual. So fair enough.

Now, I think I can get around this by using std::function (and therefore it does not need to be a template), but I just find std::function more of a pain in the arse. So I am wandering if there is some other method to have a virtual function that takes a callback as a parameter?