c – 提供(空)用户定义的析构函数会导致编译错误

当我没有用户定义的析构函数时,编译完全正常的代码(在GCC 4.7.2上),即使提供了一个空的用户定义的析构函数,也会产生错误：

#include <memory>

class Test
{
   std::unique_ptr<int> val;
};

template <typename Type>
class B
{
public:
   //destructor:
   // if I comment this out,the code compiles just fine:
   ~B() { }

private:
   Test a;
};

int main()
{
   auto s = B<int>();
}

析构函数未注释时产生的错误中的突出点是：

> Test的复制构造函数不存在,并且没有隐式创建,因为它会形成错误
>有些东西试图使用unique_ptr的已删除拷贝构造函数.

对于任何感兴趣的人来说,完整的错误输出位于这篇文章的底部.

我知道unique_ptr不能被复制构造(除非参数是rvalue),因此编译器不可能为类Test生成有效的隐式复制构造函数.

我无法弄清楚为什么定义一个空的析构函数应该突然需要这些复制工具.显然,当使用unique_ptrs之类的东西时,这是不可能提供的.

这里的任何人都可以告诉我为什么会这样吗？

未解析析构函数时完成错误输出：

In file included from /usr/include/c++/4.7/list:64:0,from ../../DropBox/ProgrAMMing/C++/test/main.cpp:2:
/usr/include/c++/4.7/bits/stl_list.h: In instantiation of 'std::_List_node<_Tp>::_List_node(_Args&& ...) [with _Args = {Const Test&}; _Tp = Test]':
/usr/include/c++/4.7/ext/new_allocator.h:110:4:   required from 'void __gnu_cxx::new_allocator<_Tp>::construct(_UP*,_Args&& ...) [with _Up = std::_List_node<Test>; _Args = {Const Test&}; _Tp = std::_List_node<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:503:8:   required from 'std::list<_Tp,_Alloc>::_Node* std::list<_Tp,_Alloc>::_M_create_node(_Args&& ...) [with _Args = {Const Test&}; _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp,_Alloc>::_Node = std::_List_node<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:1533:63:   required from 'void std::list<_Tp,_Alloc>::_M_insert(std::list<_Tp,_Alloc>::iterator,_Args&& ...) [with _Args = {Const Test&}; _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp,_Alloc>::iterator = std::_List_iterator<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:997:9:   required from 'void std::list<_Tp,_Alloc>::push_BACk(const value_type&) [with _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp,_Alloc>::value_type = Test]'
/usr/include/c++/4.7/bits/stl_list.h:1466:6:   required from 'void std::list<_Tp,_Alloc>::_M_initialize_dispatch(_InputIterator,_InputIterator,std::__false_typE) [with _InputIterator = std::_List_const_iterator<Test>; _Tp = Test; _Alloc = std::allocator<Test>]'
/usr/include/c++/4.7/bits/stl_list.h:582:9:   required from 'std::list<_Tp,_Alloc>::list(const std::list<_Tp,_Alloc>&) [with _Tp = Test; _Alloc = std::allocator<Test>; std::list<_Tp,_Alloc> = std::list<Test>]'
../../DropBox/ProgrAMMing/C++/test/main.cpp:11:7:   required from here
/usr/include/c++/4.7/bits/stl_list.h:115:71: error: use of @R_489_9421@ed function 'Test::Test(const Test&)'
../../DropBox/ProgrAMMing/C++/test/main.cpp:5:7: note: 'Test::Test(const Test&)' is implicitly @R_489_9421@ed because the default deFinition would be ill-formed:
../../DropBox/ProgrAMMing/C++/test/main.cpp:5:7: error: use of @R_489_9421@ed function 'std::unique_ptr<_Tp,_Dp>::unique_ptr(const std::unique_ptr<_Tp,_Dp>&) [with _Tp = int; _Dp = std::default_@R_489_9421@e<int>; std::unique_ptr<_Tp,_Dp> = std::unique_ptr<int>]'
In file included from /usr/include/c++/4.7/memory:86:0,from ../../DropBox/ProgrAMMing/C++/test/main.cpp:3:
/usr/include/c++/4.7/bits/unique_ptr.h:262:7: error: declared here

B(B &&) = default;