C++ 11 decltype：如何聲明指針指向的類型？

Question

我有以下代碼：

#include <memory> 

int main() 
{ 
    int* a = new int(2); 

    std::unique_ptr<decltype(*a)> p(a); 
} 

從而導致這些錯誤消息：

In file included from a.cpp:1: 
In file included from /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/memory:81: 
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/unique_ptr.h:138:14: error: '__test' declared as a pointer to a reference of type 'int &' 
      static _Tp* __test(...); 
        ^
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/unique_ptr.h:146:35: note: in instantiation of member class 'std::unique_ptr<int &, 
     std::default_delete<int &> >::_Pointer' requested here 
     typedef std::tuple<typename _Pointer::type, _Dp> __tuple_type; 
           ^
a.cpp:7:35: note: in instantiation of template class 'std::unique_ptr<int &, std::default_delete<int &> >' requested here 
    std::unique_ptr<decltype(*a)> p(a); 
           ^
In file included from a.cpp:1: 
In file included from /usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/memory:81: 
/usr/bin/../lib64/gcc/x86_64-unknown-linux-gnu/4.9.2/../../../../include/c++/4.9.2/bits/unique_ptr.h:227:33: error: 'type name' declared as a pointer to a reference of type 'int &' 
       is_convertible<_Up*, _Tp*>, is_same<_Dp, default_delete<_Tp>>>> 
            ^
a.cpp:7:35: note: in instantiation of template class 'std::unique_ptr<int &, std::default_delete<int &> >' requested here 
    std::unique_ptr<decltype(*a)> p(a); 
           ^
2 errors generated. 

我理解的原因是模板的unique_ptr預計鍵入int，但decltype(*a)給int&。如果int是一個非常長和複雜的類型，我該如何使這個代碼與decltype一起工作？

Answer 1

使用std::decay_t。這是在按值傳遞參數給函數時應用的轉換。

Answer 2

可以使用一個typedef一個模板類中，然後使用模板專業化，這樣

template<typename T> struct unref { 
    typedef T raw; 
}; 
template<typename T> struct unref<T&> { 
    typedef T raw; 
}; 

int main() { 
    int* a = new int(2); 
    std::unique_ptr<unref<decltype(*a)>::raw> p(a); 
}