Ignore template parameter - c++

Question

Is there a way in the code below to say that I don't care what the V parameter is in void watch(event<T, _> &e)
#include <stdlib.h>
#include <stdio.h>
enum events_a {
A_1,
A_2,
};
enum events_b {
B_1,
B_2,
};
template<typename T, T V>
class event {
public:
T id(void) {
return V;
}
};
template<typename T>
class event_loop {
public:
void watch(event<T, _> &e) {
};
};
class a1 : public event<enum events_a, A_2>
{
};
int
main (int argc, char *argv[])
{
a1 a;
printf("%d\n", a.id());
return 0;
}
What I'm trying to do is that users should be forced to subclass event<T,V> and be forced to give T and V.
However the subclass of event_loop<T>, specifically the method watch, should work with all event<T> regardless of their V.
I don't want to template just the watch function as I want the user to be forced to subclass event_loop.
Bonus achievement if I can somehow make sure all subclasses of event<T> have a unique V. Shooting for the stars... :D

Answer 1

Just create a template argument for it and ignore it.
template<typename T, typename Ignore>
class event_loop {
public:
void watch(event<T, Ignore> &e) {
};
};
You could also template just the function, if you want only the function to be templated
template<typename T>
class event_loop {
public:
template<typename Ignore>
void watch(event<T, Ignore> &e) {
};
};

Answer 2

You could specify a default argument for V and static assert like the way below:
enum events_a {
A_1 = 1,
A_2,
};
enum events_b {
B_1 = 1,
B_2,
};
template<typename T, T V = {}>
class event {
static_assert(V != 0, "You should Define a V");
public:
T id() {
return V;
}
};
template<typename T>
struct event_loop {
void watch(event<T> &e) {}
};
class a1 : public event<enum events_a, A_2> { };
Live Demo