Recently, I saw a strange C++ feature: injected class name.

class X { };
X x1;
class X::X x2; // class X::X is equal to X
class X::X::X x3; // ...and so on...

But I cannot figure out why this feature is necessary. Is there any practice that requires this feature?

And I heard this feature didn't exist in old C++. Then, when was it introduced? C++03? C++11?