Template function calls protected methods in a class?

Question

I did a test. I have a template function with a template parameter of a class's member method pointer. When I pass a protected member method pointer there, the template function can call the protected method. Is this reasonable? I think it also works for private methods.

Thanks to the deleted comments for my code. I simplify my code and find the reason.

template<class T, void (T::*f)()>
void foo(T t)
{
    (t.*f)();
}

class A
{
public:
    void g() { foo<A, &A::f>(*this); } // ok for access in class A scope
protected:
    void f() {}
};

int main()
{
    A a;
    a.g();
    //foo<A, &A::f>(a); // error, no access to the protected method in this scope.
    return 0;
}

The compiler is smart!

Answer 1

If you deliberately hand a thing out to someone, they can access it.

This is no different from the fact that you can pass e.g. a private int member as a parameter to wherever you want, even by reference so they can modify it.

In a sense, protected/private prevents theft, not gifts.