when I define the template class static member, why doesn't it initialize until it appears in some implementations?

Question

H i, everyone.

I wrote a simple template Singleton class, like the following:

template<typename T>
class Singleton
{
public:
    static T& getInstance () {return ms_instance;}
    static T ms_instance;
};

template<typename T>
T Singleton<T>::ms_instance;

and I also wrote some subclass, such as:

class A
    : public Singleton<A>
{
public:
    A ()
    {
        cout << "xxx" << endl;
    }
};

What I want to do is calling A's ctor when I declear class A, but the above code doesn't work. I don't use A explictly elsewhere.

I found 2 ways to slove it. The first is explicitly defining A::ms_instance, and the second is simply code it in the Singleton's ctor, like the following:

...
Singleton ()
{
    ms_instance;
}
...

Now, I have 2 questions.

1. Why does the second way work?

2. If I have many subclass, what's the order of their initialization? In order to facilitate understanding, I'll give an example.

   class A
       : Singleton<A>
   {
   public:
       A () { cout << "A" << endl; }
   };
   
   class B
       : Singleton<B>
   {
   public:
       B () { cout << "B" << endl; }
   };
   
   class C
       : Singleton<C>
   {
   public:
       C () { cout << "C" << endl; }
   };
   ...
   //when the application runs, does it output ABC, CBA or indeterminacy?
   

Thanks in advance.