[C++] injected class name

Why is there an injected class name?

reference:
http://en.cppreference.com/w/cpp/language/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...

# quote from S.O
The injected class name means that X is declared as a member of X, so that name lookup inside X always finds the current class, not another X that might be declared at the same enclosing scope, e.g.

void X() { }
class X {
public:
  static X create() { return X(); }
};