如何模板類初始化靜態字段類型的內部類

Question

我有這樣的事情

template <class T> 
class Outer { 
    public: class Inner; 

    static Inner* x; 

    //... 

    class Inner { 
     //... 
    }; 
}; 

// Not working 
template <class T> 
Outer<T>::Inner* Outer<T>::x = NULL; 

錯誤我收到說：:16: error: expected constructor, destructor, or type conversion before ‘*’ token

Answer 1

template<class T> 
class Outer { 
public: 

    class Inner; 

    static Inner* x; 

    //... 

    class Inner { 
     //... 
    }; 
}; 

template<class T> 
typename Outer<T>::Inner *Outer<T>::x = NULL; 

1. 至於typename和class，請參考C++ difference of keywords 'typename' and 'class' in templates

2. 爲什麼這個請參考Trouble with dependent types in templates