模板类中的双嵌套类正向声明

Question

I'm got "15:14: error: too few template-parameter-lists" when compiling next code using g++ 4.6.2

template <class T>
class A
{
public:
    class B
    {
    public:
        class C; //forward declaration
    };
    class D
    {
    //using B
    };

    class B::C // error here
    {
    //using D
    };
//using B::C
};

/* works fine, but required into "A"
template <class T>
class A<T>::B::C
{
public:
};
*/

How to solve the problem without changing the sequence of declarations?

Answer 1

As you discovered, you don't define forward-declared classes in an inner scope, you must do it at the corresponding outer scope. Your "works fine" code seems to compile fine with g++ 4.5. Are you saying that when you try the "works fine" way line 19 gives an error? That's because it's missing a terminating ; .