Why would a class be redeclared in another class?

Question

I came across this syntax while i was reading about Tree data structures but i couldn't figure out why it was written like that. If you understand why please help me out.

template <typename T>
class Node {
//....
};

template <typename T>
class Tree {
private:
class Node;
//....
};

Why was class Node placed in Tree instead of just a Node object?

Answer 1

~ swim ~
In that example there is a definition for class Node before the definition of class Tree. When they come in such sequence, is a forward declaration still necessary? I thought forward declaration was for the same purpose with that of function prototype (is it?)

And to clear my doubt about declaration and definition, this is how I see it, cmiiw : )

class A; // declaration

class A // definition
{
// member definition
};

Answer 2

~ swim ~
Big Thanks for clearing my doubt 👍