在成员函数定义中使用typedef'ed内部类的正确C ++语法是什么？

Question

在定义成员函数时，使用内部类的typedef的正确语法是什么？从下面的代码中可以看出，我正在尝试在其ctor定义中使用内部类的typedef，但我找不到正确的语法 - 有人可以帮忙吗？

#include <iostream>

class Outer
{
public:
  Outer( int a );

  class ThisInnerClassNameIsAwfullyLong
  {
  public:
    ThisInnerClassNameIsAwfullyLong( int a );
  private:
    int a_;
  };

  typedef ThisInnerClassNameIsAwfullyLong Inner;

private:
  Inner inner_;
};

Outer::Outer( int a )
  : inner_( a )
{
}

//Outer::Inner::Inner( int a ) // This doesn't compile
//Outer::Inner::Outer::Inner( int a ) // This looks like nonsense, but I tried it anyway. It does not compile, obviously.
Outer::ThisInnerClassNameIsAwfullyLong::ThisInnerClassNameIsAwfullyLong( int a ) // This compiles.
  : a_( a )
{
}

int main( int argc, char* argv[] )
{
  Outer::Inner inner( 2 ); // Although this looks "similar to" line 27, it compiles.
  std::cout << "hello, world." << std::endl;
  return 0;
}

我还想问为什么在Outer::Inner中使用main()，与编译中断第27行类似，编译，而后者则不好。