[cpp] 关于返回内嵌类对象的成员函数

[ling]

请教一个问题，下面代码中类Array2D的成员函数Array1D& operator[] (int index)
如果定义在类外面时（注释掉的代码），编译通不过，定义在类里面就没有问题，
不知道为什么。
编译错误：
E:\work\study\c&c++\src\m30.cpp:61: error: expected constructor, destructor, 
or type conversion before "Array2D"
E:\work\study\c&c++\src\m30.cpp:61: error: expected `;' before "Array2D"
E:\work\study\c&c++\src\m30.cpp:67: error: expected init-declarator before 
"Array2D"
E:\work\study\c&c++\src\m30.cpp:67: error: expected `;' before "Array2D"


#include <iostream>
using namespace std;

template<class T>
class Array2D {
public:
 Array2D(int x, int y);
 ~Array2D();
 class Array1D {
 public:
  Array1D(int x);
  ~Array1D() { delete [] value; }
  T& operator[] (int index);
  const T& operator[] (int index) const;
 private:
  T * value;
 };
 Array1D& operator[] (int index) { return array[index]; }
 const Array1D& operator[] (int index) const { return array[index]; }

private:
 Array1D * array;
 int xx, yy;
};

template<class T>
Array2D<T>::Array1D::Array1D(int x) : value(new T[x])
{
}

template<class T>
T& Array2D<T>::Array1D::operator[](int index)
{
 return value[index];
}
template<class T>
const T& Array2D<T>::Array1D::operator[](int index) const
{
 return value[index];
}

template<class T>
Array2D<T>::Array2D(int x, int y) : xx(x), yy(y)
{
 array = static_cast<Array1D*>(operator new(x * sizeof(Array1D)));
 for (int i = 0; i < y; ++i) {
  new(&array[i]) Array1D(y);
 }
}

template<class T>
Array2D<T>::~Array2D()
{
 for (int i = xx - 1; i >= 0; --i) {
  array[i].~Array1D();
 }
 operator delete [](array);
}

/*template<class T>
Array1D Array2D<T>::operator[](int index)
{
 return a.array[index];
}*/

/*template<class T>
const Array1D Array2D<T>::operator[] (int index) const
{
 return array[index];
}*/

int main()
{
 Array2D<int> a(10, 10);

    a[1][1] = 1;
 a[0][0] = 10;
 cout << a[1][1] << endl;
 cout << a[0][0] << endl;
}