Calling a callback passed from another class - c++

Question

I want to register a callback handler (method) of the one class (Y) in another (X). I can't use std::function because of possible heap allocation and I must have an access to members of a class that registers the handler. I also want to avoid static functions.
I've came up with some workaournd but got stuck on calling the callback:
template<class T>
using clbkType = void(T::*)(void);
template<class T>
class X
{
public:
void registerClbck(clbkType<T> clbk) {
callback = clbk;
}
void call() {
callback(); // ERROR C2064: term does not evaluate to a function taking 0 arguments.
//(((X<T>*)this)->X<T>::callback)(); // same error
}
private:
clbkType<T> callback;
};
class Y
{
public:
Y() {
x.registerClbck(&Y::handler);
}
// just for a test: fire a callback in class X
void fire() {
x.call();
}
int getA() { return a; }
private:
int a{ 0 };
X<Y> x{};
void handler() {
a = 5;
}
};
int main()
{
Y y;
y.fire();
return y.getA();
}
link to code: https://godbolt.org/z/PhY41xsWE
PS. I'm not sure if this is a safe solution, so please put any comment on that.
Thanks!

Answer 1

The member function pointer needs a specific class object to invoke, so you need to do this:
template<class T>
class X
{
public:
// ...
void call(T& obj) {
(obj.*callback)();
}
// ...
};
class Y
{
public:
// just for a test: fire a callback in class X
void fire() {
x.call(*this);
}
// ...
};
Demo.

Answer 2

The member function pointer need an instance to be called on.
If you want to bind the instance, here is a possible way to implement it.
template<typename T>
struct member_callback {
T* instance;
void(T::*callback)();
void operator()(){(instance->*callback)();}
};
template<typename T>
struct X{
void registerClbck(member_callback<T> clbk) { callback = clbk; }
void call() { callback(); }
member_callback<T> callback;
};
struct Y{
public:
Y() { x.registerClbck({this,&Y::handler}); }
void fire() { x.call(); }
int getA() { return a; }
private:
int a{ 0 };
X<Y> x{};
void handler(){ a = 5; }
};
int main()
{
Y y;
y.fire();
return y.getA();
}