模板「使用」聲明的這種用法有什麼問題

Question

模板using聲明的以下用法有什麼問題？

template <typename T1, typename T2> struct A { 
    template <typename AnotherT1> 
    using MyTemplate = A<AnotherT1, T2>; 
}; 

template<class SomeA> struct B { 
    using MyType = typename SomeA::MyTemplate<double>; 
}; 

int main() { 
    B<A<int, int>> b; (void)b; 
} 

gcc 4.8.2抱怨：

temp.cpp:17:46: error: expected ‘;’ before ‘<’ token 
    using MyType = typename SomeA::MyTemplate<double>; 
              ^
temp.cpp:17:46: error: expected unqualified-id before ‘<’ token 
temp.cpp: In instantiation of ‘struct B<A<int, int> >’: 
temp.cpp:21:20: required from here 
temp.cpp:17:46: error: ‘typename A<int, int>::MyTemplate’ names ‘template<class AnotherT1> using MyTemplate = struct A<AnotherT1, int>’, which is not a type 

Answer 1

參考這個網頁。 https://msdn.microsoft.com/en-us/library/0afb82ta.aspx

修改如下圖所示：

using MyType = typename SomeA::template MyTemplate<double>;