C++11 decltype:如何声明指针指向的类型

C++11 decltype: How to declare the type that a pointer points to?

本文关键字：指针 类型 声明 decltype 何声明 C++11      更新时间：2023-10-16

我有以下代码：

#include <memory>
int main()
{
    int* a = new int(2);
    std::unique_ptr<decltype(*a)> p(a);
}

这导致了以下错误消息：

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.

我知道原因是unique_ptr模板需要类型int，而decltype(*a)给出的是int&。在int是一个非常长和复杂的类型的情况下，我如何使此代码与decltype一起工作？

使用std::decay_t。这是在按值向函数传递参数时应用的转换。

您可以在模板化类中使用typedef，然后使用模板专业化，如

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);
}
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