Samples for non-type template argument other than integral types

In TC++PL Special Edition, template parameters section, page 332,
second paragraph said:
“A template argument can be a constant expression, the address of an
object or function with external linkage, or a non-overloaded pointer
to member.”

I have used type parameters and also constant expressions as non-type
template arguments in my programs. Now, I am experimenting with
further possibilities from the other types of template arguments. By
now, I have successfully experiment with the following:

* the address of an object
* the address of a function with external linkage,
* a non-overloaded pointer to member

I have compiled and executed these experiments:

template<void (*n)(const char*)>
class S
{
public:
	void f()
	{
		n("here");
	}
};

template<typename T, T (*n)(T)>
class V
{
public:
	void f(T t1)
	{
		T t2=n(t1);
		std::cout<<"\n"<<t2<<"\n";
	}
};

class Agent
{
public:
	void f(){std::cout<<"\nAgent::f()\n";}
	void e(){std::cout<<"\nAgent::e()\n";}
};
Agent agent;

template<void (Agent::*g)()>
class U
{
public:
	void f(Agent* a)
	{
		(a->*g)();
	}
};

template<typename T,void (T::*g)()>
class W
{
public:
	void f(T* a)
	{
		(a->*g)();
	}
};

template<Agent* a>
class Y
{
public:
	void f()
	{
		a->f();
	}
};

void g(const char* m){std::cout<<"g:"<<m;}
int g(int n){return n*2;}
double g(double n){return n*3;}
void g(Agent* a,void (Agent::*f)())
{
	(a->*f)();
}

void main()
{
	S<&g> s;
	s.f();
	V<int,&g> v1;
	v1.f(12);
	V<int,g> v11;
	v11.f(40);
	V<double,&g> v2;
	v2.f(3);

	g(&agent,&Agent::f);

	U<&Agent::f> u1;
	u1.f(&agent);

	U<&Agent::e> u2;
	u2.f(&agent);

	W<Agent,&Agent::f> w1;
	w1.f(&agent);

	W<Agent,&Agent::e> w2;
	w2.f(&agent);

	Y<&agent> y;
	y.f();
}
I am interested to discuss how others have used these other types of
non-type template arguments. Could you please point to typical uses
of?:

1- the address of an object
2- the address of a function with external linkage,
3- a non-overloaded pointer to member

Thank you very much in advance for any comment.


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