如何創建容器持有策略類型的容器迭代器？

Question

我有以下代碼：

#include <map> 
#include <string> 

class policy1 
{ 
    public: 
    struct data 
    { 
    }; 
}; 

template<typename policy> 
class policy_user : public policy 
{ 
    typedef std::map<std::string, typename policy::data> mymap;  // good 
    typedef std::map<std::string, 
      typename policy::data >::iterator myiterator;    // bad 
    typedef mymap::iterator myseconditerator;      // also bad 
}; 

失敗有：

der.cpp:17: error: type ‘std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, typename policy::data, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, typename policy::data> > >’ is not derived from type ‘policy_user<policy>’ 
der.cpp:17: error: expected ‘;’ before ‘myiterator’ 
der.cpp:18: error: type ‘std::map<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, typename policy::data, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::basic_string<char, std::char_traits<char>, std::allocator<char> >, typename policy::data> > >’ is not derived from type ‘policy_user<policy>’ 
der.cpp:18: error: expected ‘;’ before ‘myseconditerator’ 

而且我不知道爲什麼。任何幫助我如何完成我嘗試做的事情（創建一個迭代器）？

Answer 1

您在類型定義忘記typename。

寫：

typedef typename std::map<std::string, typename policy::data >::iterator myiterator;  
typedef typename mymap::iterator myseconditerator; 

現在。沒關係。 typename是必需的，因爲iterator是一個dependent name。又見this